Category 6™

Matthew Vaults to Category 4 Strength; Uncertainty Clouds the Forecast

By: Bob Henson , 10:43 PM GMT on September 30, 2016

The spectacular three-day strengthening of Hurricane Matthew continued on Friday afternoon. Not yet even classified as a depression on Wednesday morning, Matthew was packing Category 4 winds of 140 mph as of the 5 pm EDT Friday advisory from the National Hurricane Center. Satellite imagery on Friday put a spotlight on Matthew’s rapid intensification, as a 15- to 20-mile-wide eye developed during the day (see Figure 1 below). Air Force Hurricane Hunter flights on Friday afternoon found peak flight-level winds of 117 knots (135 mph) around 2:20 pm EDT. Top near-surface winds, estimated by radiometer outside of rain-contaminated areas, were 111 knots (128 mph) around 3:45 pm EDT.

Hurricane Matthew is already being blamed for two deaths. One occurred on St. Vincent on Thursday, when a teenager was trapped against a house by a boulder that was dislodged by the storm. The other came on Friday in Colombia, where a drowning death in a rain-swollen river occurred in Uribia, La Guajira.


Figure 1. Enhanced infrared satellite image for Tropical Storm Matthew as of 2015Z (4:15 pm EDT) Friday, September 30, 2016. Image credit: NOAA/NESDIS.

How did Matthew get so strong so quickly?
Vertical wind shear of up to 20 knots has plagued Matthew for most of the last two days, yet the storm has not only maintained its structure but grown at a ferocious rate. Dissertations may be written on how this happened! Working in Matthew’s favor has been a steadily moistening atmosphere along its westward path, which means that the shearing winds didn’t push too much dry air into Matthew. Once it developed a central core, Matthew was able to fend off the wind shear much more effectively. In addition, water temperatures are unusually warm throughout the Caribbean (and the entire western North Atlantic), with an area of high oceanic heat content directly beneath Matthew’s path. Such deep oceanic heat allows a storm to strengthen without churning up cooler waters from below that could blunt the intensification.

Matthew’s ascent highlights the nagging challenge of predicting hurricane intensity. NHC statistics for the past few years show a steady improvement in track forecasts and much more erratic progress in intensity forecasts (see Figure 2 below). The typical 48-hour track error has been cut in half since the late 1990s, dropping from around 150 nautical miles (170 miles) to around 75 nautical miles today. Meanwhile, the 48-hour intensity forecast error has averaged about 12 knots (15 mph) in the last several years, which is not much better than the 15-knot errors that were typical in the mid-to-late 1990s. Much of that error is the result of just a few rapidly intensifying storms, such as Matthew.


Figure 2. Trends in track and intensity forecasts from the National Hurricane Center for Atlantic hurricanes through 2015. Units are nautical miles (left) and knots (right); add 15% to obtain miles and miles per hour. Image credit: NHC.

Although Matthew strengthened far more quickly than projected in the official outlook--and expected by most observers--there were signs that rapid intensification was possible, as we discussed on Wednesday afternoon. The 18Z (2:00 pm) Wednesday run of the SHIPS statistical model included a 44% chance that Matthew’s strength would increase by 55 knots in 48 hours. In fact, this is exactly how quickly Matthew intensified: from 50 knots (60 mph) at 18Z Wednesday to 105 knots (120 mph) at 2:00 pm Friday. SHIPS is only one tool used by forecasters to assess potential intensity change. Dynamical forecast models were generally less gung-ho on rapid intensification on Wednesday, and even subsequent SHIPS runs pulled back a bit.


Figure 3. Radar image of Hurricane Matthew as seen from NOAA’s P-3 Orion hurricane hunter aircraft at 1:40 pm EDT September 30, 2016. At the time, Matthew was a rapidly intensifying Category 3 hurricane with 120 mph winds. Image credit: NOAA/AOML/HRD, via tropicalatlantic.com

Figure 4. Visible satellite image of Matthew (1-kilometer resolution) at 1945Z (3:45 pm EDT) Friday, September 30, 2016. The Colombian and Venezuelan coastlines are outlined in yellow. Image credit: CSU/RAMMB/CIRA.


Matthew poses a major threat to Jamaica
Matthew is moving just south of due west at 9 mph. Its location about 75 miles north of Punta Gallinas, Colombia, puts it about as close to South America as any major hurricane is known to have gotten (even about 50 miles closer than 2004’s Hurricane Ivan).

Although Matthew’s westward track will keep it offshore of Colombia and Venezuela, a Tropical Storm Warning is in effect for the Colombian coast from the Venezuelan border west to Riohacha. The Columbian coast will remain on the less-intense left-hand side of Matthew, reducing the odds of hurricane-force winds and limiting the heaviest rains. Riohacha’s Almirante Padilla reported sustained winds of less than 20 mph on Friday afternoon.

Models agree strongly that Matthew will begin taking a fairly sharp right turn on Saturday, heading north-northwest through the central Caribbean. Conditions will be even more favorable for strengthening by this point. Wind shear is projected to drop dramatically (perhaps below 10 knots by Sunday), the deep atmosphere will moisten further (close to 80% relative humidity), and Matthew will be passing over waters with extremely high oceanic heat content. Now that Matthew is a major hurricane, we can expect ups and downs in its strength from day to day as internal processes such as eyewall replacement cycles kick in, but we can’t rule out the possibility Matthew will become a Category 5 while in the Caribbean.

The threat to the Greater Antilles from Matthew is becoming increasingly worrisome. The most immediate concern is for Jamaica, where a Hurricane Watch has been posted. The latest NHC outlook brings Matthew over the eastern tip of Jamaica on Monday afternoon. A westward shift of just 50 miles--well within the range of uncertainty at this point--would put the city of Kingston in Matthew’s dangerous right-hand side. A major hurricane striking Jamaica from the south would be a virtually unprecented event. Figure 5 shows the tracks of all major hurricanes passing over or very near Jamaica since 1851. All of the prior events involved storms tracking on a classic west-northwest path except for an unnamed 1912 hurricane that crossed the northwest tip of the island on a northeast path, then made a 180-degree turn. Among all hurricanes since 1851 (not shown), the only one to have crossed Jamaica on a primarily northward track during the last 80 years is Sandy (2012), which struck eastern Jamaica at Category 2 strength. Sandy caused an estimated $100 million in damage in Jamaica and knocked out power to most of the island. Matthew could be much stronger than Sandy, and a northward-oriented path through central Jamaica could bring a severe storm surge into the highly vulnerable Kingston area.

Next in Matthew’s sights on the NHC-predicted track would be Cuba, whose excellent history of hurricane awareness and preparation would likely reduce potential impacts. Matthew may also be weakened by any direct passage over mountainous Jamaica, although it could easily strike Cuba as a major hurricane.

If Matthew were to trend eastward rather than westward, the risk to western Haiti will rise dramatically. Model guidance has trended gradually west over time, which gives some hope that Haiti will escape the worst of Matthew. Still recovering from the devastation of the 2010 earthquake, and plagued with deforestation and poverty, Haiti would be highly vulnerable to the impacts of a major hurricane.


Figure 5. Tracks of all major hurricanes passing within the shaded circle encompassing Jamaica during the period from 1851 to 2015. Each of these was moving from right to left (east to west), except for the 1912 hurricane, which moved east-northeast and then backtracked toward the west-southwest as it weakened. Image credit: NOAA.


Figure 6. WU depiction of National Hurricane Center track and intensity forecast for Matthew as of 5:00 pm EDT Friday, September 30, 2016.

Long-term outlook for Matthew
If anything, the prospects for Matthew later next week have become more uncertain over time. Models continue to take Matthew north through The Bahamas, but then we have major divergence among our top models. As just one example, the 12Z Friday operational run of the GFS model pulls Matthew almost due north, slamming it into eastern Maine as a significant hurricane or very intense post-tropical storm by next weekend. In stark contrast, the 12Z run of the ECMWF model strands Matthew in the Bahamas, where it lingers through next weekend and into the following week as a major hurricane. The 12Z run of the UKMET, our other top track model, also stalls Matthew in the Bahamas, then angles it northwest toward the Southeast U.S. coast.

Why such profound disagreement? The simplest explanation is that track errors increase markedly over time, and there is little skill beyond about 5-7 days. In this case, there is a great deal of uncertainty over how the mid-latitude steering features over the United States and the western Atlantic will evolve over the next week. NOAA’s Gulfstream-IV jet has been flying regular missions to sample the environment around Matthew, which has likely led to improvements in the short-term track forecasts. The problem is that the upper-level trough that will be a key influence on Matthew’s track next week is still thousands of miles away--moving through the northeast Pacific, where observations are scarce. It is far too soon to know with confidence how the upper-level features will evolve next week, so we need to keep our expectations very modest for confidence in any East Coast forecast.

The bottom line:

--Matthew poses a very serious risk to the western Greater Antilles early next week.

--A trek over the mountainous terrain of Jamaica, Cuba, and/or Haiti would dramatically weaken Matthew. At least some restrengthening would be possible over the Bahamas.

--Matthew could affect any part of the U.S. East Coast from Florida to Maine at some point from the middle of next week into the weekend.

--Long-range forecasts will vary, perhaps several times each day. Because the key features that will steer Matthew are very uncertain at this point, any given model shift may not mean much until the evolution of these features becomes better defined, which could take several days.

Jeff Masters and I will be posting regular updates through the weekend, typically between 10 am and noon EDT and between 6 and 8 pm EDT. For those new to our blog, the comments section is packed with valuable insights from our many members, including meteorologists as well as dedicated laypeople. Wunderblogger Steve Gregory posted a Friday afternoon update on Matthew, MAJOR HURRICANE MATTHEW – FLORIDA TO MAINE ON ALERT (INTERIM UPDATE).

Bob Henson


Figure 7. The 70 forecasts from the 12Z Friday European (ECMWF) model ensemble (left) and 18Z GFS model ensemble (right) continued to show a wide variety of solutions for the track of Matthew that pose various threats along the U.S. East Coast. (The tracks from the ECMWF that previously targeted the Gulf Coast have almost completely disappeared.) Image credit: Climate Forecast Applications Network (CFAN).

Hurricane

Hurricane Matthew Rapidly Intensifies to Category 3 Strength

By: Jeff Masters , 3:00 PM GMT on September 30, 2016

Hurricane Matthew put on an impressive and unexpected display of rapid intensification overnight, becoming the Caribbean's first major hurricane since Sandy of 2012. An Air Force hurricane hunter aircraft was in Matthew on Friday morning, and between 7 am and 10 am EDT found that Matthew’s winds continued to rise and the pressure to fall. Surface winds measured by their stepped frequency microwave radiometer (SFMR) were as high as 114 mph, and flight-level winds at 10,000 feet hit 118 mph, putting Matthew at minimal Category 3 strength. Between 7 am and 10 am, Matthew’s central pressure fell 3 mb, to 968 mb.


Figure 1. Latest satellite image of Matthew.

Matthew’s rapid intensification was something of a surprise, as it occurred despite the presence of strong upper-level winds out of the southwest that were creating high wind shear of 20 knots. The intensity models generally failed to predict the rapid intensification, though the SHIPS model did give a 38% chance last night that we would see the observed amount of intensification that has occurred.


Figure 2. Curacao radar at 7:30 am EDT September 30, 2016 showed that Matthew had a well-formed eye and had spiral bands that were bringing heavy rains to the northern coast of South America. Plenty of lightning (black squiggles) was observed in the western eyewall and in region of heavy thunderstorms well to the east of the center. At this time, Matthew was a Category 2 storm with 105 mph winds. Image credit: Meteorological Department of Curacao.

Satellite loops on Friday morning showed an eye that was intermittently visible. At upper levels, high cirrus clouds streaming to the north of Matthew showed the presence of a powerful outflow channel, which was helping ventilate the storm and allowing it to intensify in the face of the high wind shear. There was little evidence of a second outflow channel becoming established to Matthew’s south, which one can see in the latest Upper Level Winds analysis from the University of Wisconsin CIMSS group. If this second outflow channel becomes well-established, continued intensification of Matthew becomes more likely. Aiding development today were warm ocean waters of 29°C (84°F) and 60 - 65% relative humidity at mid-levels of the atmosphere, as analyzed by the SHIPS model.


Figure 3. Enhanced infrared satellite image of Matthew as of 1345Z (9:45 am EDT) Friday, September 30, 2016. An eye at Matthew's center is partially enclosed by the satellite signal of the most intense thunderstorms.

Three-day forecast for Matthew
Matthew will continue west-southwest to west through Saturday, slowing down from a forward speed of 14 mph on Friday morning to 7 mph by Saturday morning. The core of the storm will make its closest approach to northern Venezuela and northern Colombia on Friday night. This region will be on the weak (left) side of the storm, and will likely escape receiving tropical-storm-force winds, though rains of 2 - 4” can be expected. The latest SHIPS model forecast predicts that wind shear will steadily drop during the next two days, reaching the low range, less than 10 knots, by Saturday evening. Intensification by Matthew into a strong Category 3 hurricane by Sunday may occur due to the declining shear, though none of our top three intensity models—the HWRF, LGEM, and SHIPS models—were predicting on Friday morning that Matthew would achieve Category 4 strength over the coming five days. The SHIPS model gave Matthew a 32% chance of becoming a Category 4 storm by Saturday, and a 12% chance of intensifying into a Category 5 storm by Sunday.


Figure 4. This morning’s forecasts had the advantage of assimilating data from a mission flown Thursday night by NOAA’s Gulfstream-IV jet. This aircraft, nicknamed “Gonzo”, released a series of 29 dropsondes that fell on parachutes and radioed back information on temperature, pressure, humidity, and winds as they fell from the 45,000-foot flying altitude of Gonzo. Data from these sorts of missions can improve hurricane track forecasts by as much as 20%. It’s good to have “Gonzo” back, as it was down for extended maintenance in late August and early September due to a corrosion problem, and was not available at all during Hurricane Hermine. Image credit: NOAA/AOML/Hurricane Research Division.

Longer-range forecast for Matthew: Miami in the cone of uncertainty
Major differences continue in the longer-range model forecasts for Matthew, though the differences between our two best models—the GFS and European model—have shrunk since Thursday’s runs. This may be because of dropsonde data taken by the NOAA jet on Thursday evening, which was ingested into the 00Z Friday runs of the models. A large upper-level low pressure system over east-central U.S. will begin pulling Matthew sharply to the northwest by Sunday, but the exact timing of the turn is in doubt, resulting in major model differences in Matthew’s track. An earlier turn and faster northward motion is being predicted by the GFS model, with a landfall by the storm in Jamaica on Monday morning. The European model has Matthew heading northwards about half a day later than the GFS model, with a landfall in southwest Haiti on Monday night. As one can see from the latest set of ensemble model runs (Figure 5), the long-range uncertainties in Matthew’s long-range track are high. The tight clustering of the GFS model ensembles is quite striking compared to the spaghetti-like appearance of the European model ensembles. The differences between the Euro and GFS, and the large spread within the Euro ensemble, are perhaps not too surprising, since a slower motion for Matthew means the anticipated steering currents are weak, making them more prone to random variations. However, the persistent tight clustering of the GFS ensembles is a bit suspicious, suggesting that the model may have some sort of systematic error in its forecast.





Figure 5. The 70 forecasts from the 00Z Friday European (ECMWF) model ensemble (top) and GFS model ensemble (bottom) continued to show a wide variety of solutions for the track of Matthew. The operational (deterministic) versions of the models, run at higher resolution, are shown in red lines. The two models have grown closer together in their solutions compared to Thursday, but the European model still shows a considerably slower track for Matthew than the GFS model.

Matthew’s anticipated landfall over Jamaica/Cuba/Haiti on Monday will weaken the storm, due to the high mountains it will interact with. This process may completely disrupt the inner core of Matthew, reducing the storm to Category 1 or 2 strength for several days, as it traverses The Bahamas. The storm may be able to re-intensify to major hurricane status in 2 - 3 days, though, over the exceptionally warm waters surrounding The Bahamas. If the GFS model is correct, Matthew will not punish The Bahamas for multiple days, but will instead march northwards just offshore the U.S. East Coast on Tuesday and Wednesday. However, the 00Z Friday run of the UKMET model suggests that the trough of low pressure pulling Matthew to the north will be too weak to continue doing this next week; high pressure will build in on Tuesday, forcing Matthew on a more northwesterly track through the heart of The Bahamas to a point perilously close to South Florida by Thursday. The five members of the European model ensemble that most closely match the operational run over the first 72 hours (Figure 6) have two members that go along with this idea. NHC has put the 5-day cone of uncertainty for Matthew very close to Miami, and it appears likely at this point that South Florida will experience at least the fringes of Matthew, with some heavy rains, if not a direct hit. In addition, the GFS and European ensembles suggest that Matthew could move quite close to the Outer Banks of North Carolina late next week.


Figure 6. Track forecasts from the five European model ensemble members [gray lines] that most closely match the operational run [red line] during the first 72 hours, starting at 00Z Friday, September 30, 2016. The red line is a version of the 00Z Friday operational model track that has been adjusted and calibrated using a proprietary technique to account for systemic model errors. Image credit: Climate Forecast Applications Network (CFAN).

We will be back this afternoon with an update on Matthew. Wunderblogger Steve Gregory posted a Friday morning update on Matthew, MATTHEW NEARS CAT 3 - EAST COAST THREAT CONTINUES.

Jeff Masters

Hurricane

Hurricane Matthew Is Born in Caribbean; Uncertainty Reigns in Long-Term Outlook

By: Bob Henson , 10:06 PM GMT on September 29, 2016

The 13th named storm of the 2016 Atlantic season became Hurricane Matthew at 2:00 pm EDT Thursday. After detecting a small area of hurricane-force winds earlier in the day, an Air Force Hurricane Hunter flight found more consistent evidence of surface winds up to the 75-mph hurricane threshold, thus prompting the upgrade. This is the first Matthew to reach hurricane strength since the name was introduced in 2004, and it’s also the fifth hurricane of the 2016 Atlantic season. Most of this year’s storms have been on the weak side, together producing only about 70% of the usual amount of accumulated cyclone energy for this point in the season. Matthew could boost that percentage considerably over the next week or more.


Figure 1. Satellite image for Matthew as of 5:07 pm EDT Thursday, September 26, 2016.

Fighting and surviving headwinds
Tropical cyclones often weaken or fail to develop in the “hurricane graveyard” of the eastern Caribbean Sea. Trade winds typically accelerate through the region in a way that leads to sinking air and enhanced vertical wind shear. (See details in this 2010 paper from the Bulletin of the American Meteorological Society). Indeed, Matthew has been fighting vertical wind shear of close to 20 knots that was predicted by some but not all models. Early Thursday, Matthew’s low-level center decoupled from its central convection (showers and thunderstorms) and was clearly visible on satellite. A new batch of convection erupted by midday Thursday atop the low-level center, and the old convection has morphed into a banding feature feeding into Matthew. These elements should help sustain and nourish Matthew’s growth. At the same time, Matthew is plowing into fairly dry air at middle levels of the atmosphere, as revealed in visible satellite imagery that shows low-level outflow features to the west of the center. Matthew’s sustained winds remained at 75 mph in the 5 pm EDT update from the National Hurricane Center. Radiometer observations from the midday Hurricane Hunter flight detected peak surface winds of up to 67 knots (77 mph) around 17Z (1:00 pm EDT).

A Tropical Storm Watch is in effect for the southeast Caribbean islands of Bonaire, Curacao, and Aruba as Matthew passes to the north. In addition, a rare Tropical Storm Watch is now in effect for the coast of Colombia, extending from the Venezuala border west to Riohacha. Matthew is not expected to make landfall in South America, but its large circulation could bring gales and heavy rains near the coast.


Figure 2. Vertical wind shear across the Caribbean as of 18Z (2:00 pm EDT) Thursday, September 29, 2016. Higher shear (unfavorable for tropical cyclones) is shown in red. Matthew has reached and maintained hurricane strength despite wind shear of around 20 knots. The shear may continue for another couple of days as Matthew enters the central Caribbean, but then is predicted to lessen. Image credit: CIMSS/University of Wisconsin/SSEC.


Figure 3. MODIS satellite image of Matthew taken at 11:30 am EDT September 29, 2016. Image credit: NASA.

Short-term forecast for Matthew
The outlook for Matthew is fairly straightforward over the next couple of days. Late Wednesday, Matthew was moving due west at 17 mph, and a very gradual, well-predicted bend to a track just south of due west appears to be in progress. Along the way, conditions will favor at least some strengthening, as the atmosphere around Matthew moistens (relative humidity will rise from around 55-60% Wednesday to around 65% by Friday) and wind shear relaxes to around 10 knots by Saturday. There is fairly strong model agreement that Matthew will be positioned in the central Caribbean north of Colombia and south of Hispaniola by Saturday, perhaps as a Category 2 hurricane.

The outlook grows much more complex from this weekend onward. Models continue to agree that an upper low cut off from the jet stream over the eastern U.S. will extend into the Gulf of Mexico by the weekend, helping to urge Matthew northward. One big question is how far west Matthew will get before that sharp right-hand turn occurs. The longitude of the turn will help determine the westward extent of Matthew’s subsequent track, which in turn will shape whether Matthew threatens Cuba, Jamaica, and/or Hispaniola by early next week. The NOAA Gulfstream-IV aircraft that samples the environment around hurricanes is again in service after a multiweek outage, and data from the G-IV flights will be incorporated in model runs starting at 00Z Friday, which should help nail down Matthew’s track.

Conditions may turn even more favorable for Matthew to intensify from around Saturday to Tuesday as it approaches the Greater Antilles. The official NHC outlook brings Matthew to the high end of Category 2 strength by Monday, when it is forecasted to be approaching eastern Cuba. There is only limited skill in predicting hurricane intensity five days out, and we cannot rule out the possibility that Matthew will intensify even more, or will fail to intensify very much. Rapid intensification is a distinct possibility, given the very large heat content in the northern Caribbean waters. Interests in the Greater Antilles, especially from eastern Cuba to the Dominican Republic, should pay especially close attention to Matthew’s progress. Matthew is a large storm and could lead to torrential rains and life-threatening floods and mudslides near its path in the Greater Antilles.


Figure 4. National Hurricane Center five-day outlook for Matthew as of 5 pm EDT Thursday, September 29, 2016.

Long-range forecast for Matthew
It appears increasingly likely that Matthew will move north from the Caribbean into The Bahamas and avoid the Gulf of Mexico, although residents along the Gulf Coast would be prudent to keep an eye on it. On Wednesday and Wednesday night, the European model included a minority of ensemble members that took Matthew into the Gulf later next week. In its 12Z Thursday run, the European ensemble became somewhat more unified around the idea that Matthew will move north into The Bahamas, then perhaps angle northwest from that point. The 12Z Thursday operational run of the UKMET model tracked along the same general lines as the Euro ensemble. Meanwhile, the GFS model and its ensemble members have been resolute for more than a day in taking Matthew through The Bahamas early next week on a steady northward track, with uncertainty growing as it approaches the Southeast and Mid-Atlantic coast.

Beyond The Bahamas, the GFS and European solutions leave open the possibility that Matthew could approach the U.S. East Coast anywhere from Florida to Maine, but the timing and location of that track remain very uncertain. Steering currents will hinge on the evolution of this weekend’s cut-off low in the eastern U.S. and on a new upper-level low that will be sweeping into the western U.S. It is far too soon to know exactly how these features will evolve. Next week’s upper trough could arrive in time to steer Matthew out to sea late next week, as suggested by the 12Z Thursday runs of the GFS and ECMWF operational models. However, any slowdown in that trough’s arrival, or any change in its configuration, could lead to a vastly different solution for Matthew—including a track angling inland. It is quite rare for such a trough to be perfectly predicted a week in advance.

The bottom line: Matthew will pose a significant threat to the Greater Antilles in the 3-to-5-day time frame, and a potential threat to the U.S. East Coast in the 6-to-10-day time frame. Future model runs will allow us to be more specific about the areas that will be most at risk and when that might be. As always, the five-day outlooks from the National Hurricane Center and the associated local bulletins are the place to turn for official guidance.

Jeff Masters will be back with our next update on Matthew by late Friday morning.

Bob Henson


Figure 5. Track forecasts from (left) the five European model ensemble members that have performed best with Matthew thus far [gray lines], effective 12Z Thursday, September 29, 2016; and (right) the full 20-member GFS ensemble, effective 18Z Thursday.

Hurricane

Hurricane Hunters Find Hurricane-Force Winds in Matthew; Wind Shear Attacking Storm

By: Jeff Masters , 2:10 PM GMT on September 29, 2016

Tropical Storm Watches are posted for the ABC islands of the Netherlands Antilles—Aruba, Bonaire, and Curacao—as Tropical Storm Matthew advances westwards at 16 mph across the eastern Caribbean. An Air Force hurricane hunter aircraft was in Matthew on Thursday morning, and at 9 am EDT found hurricane-force winds of 75 mph in a small region to the northeast of the storm’s center. Surface winds were a bit lower in data from the airborne stepped frequency microwave radiometer (SFMR), and based on this, the National Hurricane Center held Matthew's top sustained winds at 70 mph, just shy of hurricane strength, in its 11 am EDT advisory. Satellite loops on Thursday morning showed that Matthew was showing the classic signs of a storm struggling with high wind shear—the center was fully exposed to view, and Matthew’s heavy thunderstorms were limited to the northeast side, thanks to strong upper-level winds out of the southwest that were creating high wind shear of 20 - 25 knots.

Matthew has grown to impressive size, with its heavy thunderstorms extending from the northern coast of South America to the northern Lesser Antilles Islands, and if it weren’t for the high wind shear affecting it, Matthew would fill the entire eastern Caribbean. At upper levels, high cirrus clouds streaming to the north of Matthew show the presence of a powerful outflow channel, which is helping ventilate the storm and allowing it to hold its own in the face of the high wind shear. There is also evidence of a second outflow channel becoming established to Matthew’s south, which one can see in the latest Upper Level Winds analysis from the University of Wisconsin CIMSS group. If this second outflow channel becomes well-established, rapid intensification of Matthew becomes more likely if the wind shear affecting the storm relaxes. Aiding development today were warm ocean waters of 29.5°C (85°F). The 8 am EDT Thursday SHIPS model output analyzed 50 - 55% relative humidity at mid-levels of the atmosphere over Matthew, which is lower than optimal for tropical cyclone formation, and water vapor satellite loops showed Matthew was butting into a region of dry air.


Figure 1. Latest satellite image of Matthew.

Three-day forecast for Matthew
Matthew will continue west through Friday, slowing down from a forward speed of 15 mph on Thursday morning to 5 - 10 mph by Friday night. The core of the storm will make its closest approach to the ABC islands of the Netherlands Antilles—Aruba, Bonaire, and Curacao—on Friday. These islands will be on the weak (left) side of the storm, and may escape receiving tropical-storm-force winds, though rains of 1 - 2” can be expected. Given the high wind shear currently affecting Matthew, no intensification is likely the remainder of Thursday, and we may even see a weakening of the storm’s winds. The latest SHIPS model forecast predicts that the wind shear will drop to the low range, less than 10 knots, by Saturday. If this forecast verifies, we should see some gradual intensification on Friday, followed by more rapid strengthening on Saturday and Sunday.





Figure 2. The 70 forecasts from the 00Z Thursday European (ECMWF) model ensemble (top) and GFS model ensemble (bottom) showed a wide variety of solutions for the track of Matthew. The operational (deterministic) versions of the models, run at higher resolution, are shown in red lines. The two models have grown closer together in their solutions compared to Wednesday, but the European model still shows a considerably slower and more westerly track for Matthew than the GFS model.

Longer-range forecast for Matthew
Huge differences continue in the longer-range model forecasts for Matthew. A large upper-level low pressure system has separated from the jet stream and will meander over east-central U.S. during the remainder of this week. The steering currents associated with this low are expected to be strong enough to pull Matthew sharply to the north by the weekend. This sharp turn is expected to occur on Friday night or on Saturday, and the exact timing of the turn has major implications for who experiences the peak wrath of the storm. An earlier turn is being predicted by the GFS model, with a landfall by the storm in eastern Cuba on Monday morning. Matthew is then predicted to move through the central Bahamas on Tuesday. The European model has Matthew heading northwards more than a day later than the GFS model, with a landfall in Haiti on Tuesday morning. As one can see from the latest set of ensemble model runs (Figure 2), the long-range uncertainties in Matthew’s long-range track are high. Hopefully, these forecast discrepancies will be resolved by Friday morning, when data from the NOAA Gulfstream-IV jet on the environment surrounding Matthew will be available. This aircraft (nicknamed “Gonzo”), whose dropsondes can improve track forecasts by as much as 20%, will make its first flight Thursday evening. It’s good to have “Gonzo” back, as it was down for extended maintenance in late August and early September due to a corrosion problem, and was not available at all during Hurricane Hermine.

Matthew is expected to have favorable conditions for intensification this weekend as it heads north, with low wind shear, very warm ocean waters, and a very moist atmosphere. The models are quite bullish on this storm being a hurricane when it makes its landfall early next week in the islands, and residents of Jamaica, Haiti, and eastern Cuba should anticipate the possibility of a Category 2 hurricane--possibly stronger--affecting them early next week.


Figure 3. How strong could Matthew get? On rare occasions, when wind shear is low and dry air is absent, a hurricane will become a “perfect storm”, reaching the maximum potential intensity (MPI) that physics will allow. This maximum is a function of how warm the ocean is and how unstable the atmosphere is. Last year’s Hurricane Patricia, which intensified to 215 mph sustained winds over record-warm waters of 31°C (88°F) off the Pacific coast of Mexico, was one example of such a rare “perfect storm.” Hurricane scientist Dr. Kerry Emanuel of MIT has developed a relatively simple set of equations which will determine the approximate MPI of a hurricane, and real-time plots of this are available at wxmaps.org. In the case of Matthew, its MPI if it was in the region between Jamaica and Haiti today would be approximately 150 knots (170 mph), seen in the dark blue colors. If Matthew were in the northeast Bahamas, where ocean temperatures are warmer and the atmosphere more unstable, the MPI is higher, near 165 knots (190 mph), seen in black colors. These MPI numbers will change slightly when and if Matthew reaches these areas several days from now. Keep in mind that “prefect storm” conditions are difficult to achieve, and it is quite unlikely that Matthew will reach its MPI.

We will be back this afternoon with an update on Matthew.

Jeff Masters

Hurricane

Matthew Rolls Through Lesser Antilles; Huge Rains for D.C. This Week

By: Bob Henson , 10:37 PM GMT on September 28, 2016

Not even classified as a tropical depression early Wednesday morning, Tropical Storm Matthew rolled through the Lesser Antilles as a mid-strength tropical storm on Wednesday afternoon. As of 5 pm EDT, the National Hurricane Center placed Matthew about 65 miles west of St. Lucia, moving west at 18 mph with top sustained winds of 60 mph. Some observers may have done double takes when they saw Matthew debut as a 60-mph tropical storm in its very first advisory (11 am EDT Tuesday). NHC typically relies on Hurricane Hunter data before upgrading a system like Matthew, and we had a gap between flights on Tuesday afternoon and Wednesday morning. Hurricane hunter flights are now scheduled every 12 hours for the next couple of days. Moreover, in their first pass through Matthew on Wednesday morning, the Hurricane Hunters did not find a closed circulation, so the storm had 60-mph winds without being a tropical storm with a closed circulation. The last “instant tropical storm” of this magnitude was Tropical Storm Karen in 2013, whose first advisory was also as a 60-mph tropical storm. Update: According to Weather Channel tropical expert Michael Lowry, Karen's initial top winds were reduced to 45 knots (50 mph] in NHC's best-track reanalysis, so the only system aside from Matthew that has debuted as a 60-mph tropical storm was the very unorthodox Hurricane Debbie (1961).

Satellite imagery of Matthew on Wednesday afternoon revealed a very healthy tropical storm, with excellent outflow ventilating the upper reaches of the storm and an expanding area of intense thunderstorms at its core.


Figure 1. Satellite image for Tropical Storm Matthew as of 5:36 pm EDT Wednesday, September 26, 2016.


Figure 2. Radar imagery from weather.com for the Lesser Antilles as of 4:49 pm EDT Wednesday, September 26, 2016.

Heavy rains and gusty squalls were sweeping across the Lesser Antilles as Matthew moved through on Wednesday afternoon, with some of the strongest activity east of Matthew’s center still to reach the islands. Winds at Dominica’s Melville Hall Airport gusted to 53 mph at 10 am EDT Wednesday, and Martinique’s Le Lamentin Airport reported a sustained wind of 40 mph with gusts to 60 mph at 5:00 pm EDT.

Wunderground member java162 posted this report at 1834Z (2:34 pm EDT): “It has been pretty blustery here in Dominica. So far we have had just over 2 in. of rain. The wind is the real issue. Constant gusts between 60 and 90 kph [37 - 56 mph]. What I have been hearing is of a number of trees down throughout the island and quite a few areas are without electricity.”

Outlook for Matthew through the weekend
Long before it poses any possible threat to the United States, Matthew could mean big trouble for parts of the Greater Antilles. Computer models agree that Matthew will continue on a general westerly track for the next couple of days. Because Matthew’s center is a bit further north than earlier expected, the storm may be able to avoid too much interaction with the land mass of South America. By this weekend, a large upper-level low over the Mid-Atlantic (see below) should provide a pathway for Matthew to take a sharp right turn. While quite unusual, such a sharp turn is hardly unprecedented, as we noted yesterday. Tropical expert Brian McNoldy (University of Miami/RSMAS) delves into one powerful analog--Hurricane Hazel (1954)--in a very timely post today at Capital Weather Gang.

Should Matthew take its expected turn to the north, it would have a very hard time avoiding landfall somewhere between Cuba and the Dominican Republic. It’s too soon to pin down the longitude of that northward turn, but models have been gradually converging on a path that could bring Matthew somewhere near the Windward Passage between eastern Cuba and Haiti, perhaps as soon as Sunday. Residents of eastern Cuba, Jamaica, Haiti, and the Dominican Republic should keep especially close tabs on Matthew over the next several days.


Figure 3. NHC track and intensity forecast for Matthew as of 5:00 pm EDT Wednesday, September 26, 2016.

How quickly will Matthew strengthen?
Adding to the concern for the Caribbean is the potential for Matthew to strengthen quickly. Through Friday, the relative humidity in the lowest few miles of the atmosphere will be only modestly supportive of development (50-60 percent), but wind shear will be very low (5 - 10 knots) and sea-surface temperatures quite high (around 29°C or 84°F). In addition, Matthew will be entering a region with high oceanic heat content, between 50 and 100 kilojoules per square centimeter (see Figure 4 below). Even higher values of oceanic heat content lie further downstream, south of Cuba and Haiti. CSU/CIRA/RAMMB notes: “For tropical cyclones in favorable environmental conditions for intensification (i.e., vertical wind shear less than 15 kt, mid-level relative humidity > 50 %, and warm SSTs [i.e., > 28.5°C]) and with intensities less than 80 knots, values of ocean heat content greater than 50 kJ/cm^2 have been shown to promote greater rates of intensity change.”


Figure 4. Tropical cyclone heat potential, an index of the amount of heat in the upper ocean, for the Caribbean as of September 27, 2016. Image credit: NOAA/AOML.

The 12Z and 18Z Wednesday runs of the SHIPS statistical model give unusually high odds for rapid intensification of Matthew over the next 24 to 48 hours. Based on the starting-point conditions (50 knots, or 60 mph, at 2:00 pm EDT Wednesday), the 18Z SHIPS run projects the following likelihoods of a rapid increase in Matthew’s peak winds:

70% odds of a 25-knot increase in 24 hours
50% odds of a 35-knot increase in 24 hours
42% odds of a 45-knot increase in 36 hours
44% odds of a 55-knot increase in 48 hours

It should be stressed that these numbers are based on a set of statistical predictors that compare the environmental conditions now present with Matthew to those in play during past tropical storms and hurricanes. (See this PowerPoint for more details on the index.) This index has shown some modest skill in predicting rapid strengthening. Other tools used by SHIPS give Matthew considerably lower odds of rapid intensification. Forecasters at NHC also take into account satellite imagery and the results of dynamical models (such as the GFS, Euro, and UKMET) before issuing official predictions. The NHC forecast issued at 5:00 pm EDT Wednesday makes Matthew a hurricane by Thursday night and a Category 2 hurricane by Sunday. We cannot say with any confidence that Matthew will undergo rapid intensification beyond the official forecast, but the possibility is there.

Long-term outlook for Matthew
There remains huge uncertainty in Matthew’s fate beyond the weekend. A large minority of the members of the European ensemble model run from 12Z Wednesday take Matthew back westward toward the Gulf of Mexico as it is approaching Cuba and Haiti, while members of the 12Z Wednesday GFS ensemble are in unanimous agreement that Matthew will continue northward. We cannot yet discount the possibilities in the Euro ensemble, but assuming that Matthew moves into The Bahamas by early next week--as indicated by the 12Z Wednesday operational runs of the GFS, European, and UKMET models--Matthew’s subsequent path will hinge on the state of the upper-level low parking over the Mid-Atlantic into the weekend, as well as another upper-level trough that will be plowing eastward across the United States next week. The upper-level flow across North America and the North Atlantic will include several blocking features late this week into early next week, and these are notoriously difficult to predict. The most we can say at this point is that Matthew has the potential to make landfall somewhere along the Gulf or Atlantic U.S. coasts by later next week.


Figure 5. Forecasts from the 12Z Wednesday European (ECMWF) model ensemble (left) show a wide variety of potential tracks for Tropical Storm Matthew after it reaches the western Caribbean, while the members of the 12Z GFS model ensemble (right) are much more tightly clustered.


Figure 5. Predicted 3-day rainfall totals from 8:00 pm EDT Wednesday, September 28, 2016, through 8:00 pm EDT Saturday, October 1. Image credit: NOAA/NWS/WPC.

Epic multi-day rains for Mid-Atlantic
Even without the help of a tropical storm, the Mid-Atlantic low will bring a period of extremely heavy rain over the next several days, especially in the Washington, D.C., area. The 3-day precipitation outlook from NOAA’s Weather Prediction Center calls for widespread 3” - 6” rains from northern Virginia to southern Pennsylvania, with an 8” - 10” maximum possible close to the District of Columbia (see Figure 5 above). “The D.C. area should prepare for the possibility of the heaviest rain event in at least five years and possibly longer,” noted Jason Samenow (Capital Weather Gang). Given the drought conditions that emerged during the parched summer of 2016, much of this rain will be welcome and beneficial, but at least some areas will get too much, leading to the potential for “severe flooding,” according to the NWS/DC office.

Such heavy rain is very rare for D.C. outside of tropical cyclones. The city’s top 3-day rainfall of 10.34” (June 25-27, 2006, part of a billion-dollar flood event) was associated with slow-moving mid-latitude weather features together with an unclassified tropical low off the North Carolina coast. All but one of the runner-up totals--in 1955, 1928, 1933, 1972, and 2005--were directly associated with hurricanes or tropical storms. Deep moisture will be flowing from the Caribbean into the mid-Atlantic as the rains unfold. This flow will pass over a large section of the Northwest Atlantic that experienced record-warm sea-surface temperatures (SSTs) in August. SSTs continue to run 1-2°C above average throughout the Northwest Atlantic, and these record or near-record values will enhance the flow of water vapor heading into the Mid-Atlantic deluge. Should Matthew move north into this region, it could also benefit from the unusually warm waters, and any possible rain from Matthew in the D.C. area would fall on saturated ground.

We’ll be back with our next update on Matthew by late Thursday morning.

Bob Henson

Hurricane Flood

Tropical Storm Matthew Forms in the Lesser Antilles Islands

By: Jeff Masters , 3:23 PM GMT on September 28, 2016

Tropical Storm Warnings are flying in the Lesser Antilles Islands thanks to newly-formed Tropical Storm Matthew. An Air Force hurricane hunter aircraft found on Wednesday morning that Invest 97L had finally developed a closed circulation, and had surface winds near 60 mph in a powerful cluster of thunderstorms that was located about 50 miles east of Martinique at 9:22 am EDT. These strong winds will move over the islands of Martinique and Dominica early this afternoon, given Matthew’s westerly motion at 20 mph. At 11 am EDT, Dominica reported sustained winds of 33 mph, gusting to 53 mph, and Martinique reported sustained winds of 28 mph, gusting to 40 mph. Radar imagery out of Martinique and Barbados on Wednesday morning showed plenty of rotation to the storm’s echoes, and an increase in their intensity and areal coverage. Satellite loops showed that Matthew was developing a well-defined surface circulation, and had an increasing amount of heavy thunderstorm activity that was growing more organized. Aiding development was moderate wind shear of 10 - 20 knots and warm ocean waters of 29.5°C (85°F). The 8 am EDT Tuesday SHIPS model output analyzed 50 - 55% relative humidity at mid-levels of the atmosphere over Matthew, which is lower than optimal for tropical cyclone formation, and water vapor satellite loops showed Matthew was butting into a region of dry air that lay just west of the Lesser Antilles Islands. Lack of spin from being too close to the equator was less of a problem for Matthew than before, as the system had worked its way northwards to a latitude of 13°N. This is far enough from the equator for the storm to be able to leverage the Earth’s spin and acquire more spin of its own.


Figure 1. Barbados radar at 10:20 am EDT September 28, 2016 showed a large region of heavy rains from Matthew beginning to move into the Lesser Antilles Islands. The Hurricane Hunters found surface winds of 60 mph under the cell just east of Martinique. An "X" marks the center of Matthew. Image credit: Barbados Met Service.


Figure 2. MODIS satellite image of Matthew taken at 10:30 am EDT September 28, 2016. Image credit: NASA.

Two-day forecast for Matthew
Matthew will average a westerly motion at about 15 mph through Thursday morning. The core of the storm will pass through the Lesser Antilles Islands on Wednesday afternoon, with the storm’s strongest winds and heaviest rains of 4 - 8” affecting the islands just north of the center, including St. Lucia, Martinique, Dominica, and Guadaloupe. The storm will continue westwards on Thursday, and make its closest approach to the ABC islands of the Netherlands Antilles—Aruba, Bonaire, and Curacao—on Thursday night and Friday morning. These islands will be on the weak (left) side of the storm, and will likely escape receiving tropical storm winds, though rains of 1 - 2” can be expected. As Matthew passes through the southeastern Caribbean, it will be in an environment somewhat unfavorable for development. Tropical cyclones passing near the coast of South America often suck in dry continental air from the land areas to the south, and the latest SHIPS model forecast predicts that Matthew will have to contend with moderate wind shear and dry air through Friday. The last hurricane to pass through the southeastern Caribbean, Hurricane Tomas of 2010, degraded from a Category 1 hurricane to a tropical depression due to high wind shear and dry air as it moved across the region. Expect only slow intensification of Matthew on Thursday and Friday.





Figure 3. Forecasts from the 00Z Wednesday European (ECMWF) model ensemble (top) and GFS model ensemble (bottom) had a number of their 70 members predicting a hurricane for late in the week in the Caribbean (light blue dots.) The operational versions of the models, run at higher resolution (red lines), also showed the storm becoming a hurricane by four days into the future. The two models have grown closer together in their solutions compared to Tuesday, but the European model still shows a considerably slower and more westerly track for Matthew than the GFS model.

Longer-range forecast for Matthew
Matthew is being steered by a ridge of high pressure that extends only as far west as the ABC islands. Matthew will slow down to a forward speed of 5 - 10 mph by Friday as it reaches the edge of this ridge, and the storm will maintain that slow forward speed though the weekend. A large upper-level low pressure system has separated from the jet stream and will settle over east-central U.S. late this week, and the steering currents associated with this low are expected to be strong enough to pull Matthew sharply to the north by the weekend, according to the Wednesday morning runs of the models. This sharp turn is expected to occur on Friday night or on Saturday, and the exact timing of the turn has huge implications for who experiences the peak wrath of the storm. An earlier turn is being predicted by the GFS model, with a landfall by the storm in eastern Cuba on Monday morning. Matthew is then predicted to move through the central Bahamas on Tuesday. The European model forecasts a later turn, with a landfall in Jamaica on Monday night, and then in eastern Cuba on Tuesday night. The 00Z Wednesday run of the UKMET model brings Matthew northward across Haiti on Monday and into the southeastern Bahamas by Tuesday. As one can see from the latest set of ensemble model runs (Figure 3), the long-range uncertainties in Matthew’s long-range track are high. Now that Matthew has finally established a coherent center of circulation, expect the forecast uncertainty to improve in this evening’s model runs. Matthew is expected to have favorable conditions for intensification this weekend as it heads north, with low wind shear, very warm ocean waters, and a very moist atmosphere. The models are quite bullish on this storm being a hurricane when it makes its landfall early next week in the islands, and residents of Jamaica, Haiti, and eastern Cuba should anticipate the possibility of a hurricane affecting them early next week.

We will be back this afternoon with an update on Matthew.

Jeff Masters

Hurricane

97L Approaches Tropical Storm Strength

By: Bob Henson and Jeff Masters , 10:42 PM GMT on September 27, 2016

Air Force Hurricane Hunters were unable to find a closed circulation in the midst of Invest 97L, but the system is still on the verge of becoming a tropical storm. Flight-level winds at 2034Z (4:34 pm EDT) were sustained at up to 36 knots (41 mph) on the northwest side of 97L. This translates to ground-level sustained winds of around 32 knots (37 mph), or just below the 39-mph threshold for tropical storm formation. However, a tropical wave cannot be classified by NHC as a tropical depression or tropical storm until it has a closed circulation. The lack of a fully closed circulation was manifested in much weaker winds on the south side of 97L. Southwest winds at flight level were only about 5-6 knots, and no significant westerly winds were found.


Figure 1. Enhanced infrared image of Invest 97L at 2115Z (5:15 pm EDT) Tuesday, September 27, 2016. Image credit: NASA/MSFC Earth Science Office.

It shouldn’t take too much longer for 97L to close off a circulation and intensify enough to become Tropical Storm Matthew. Satellite loops showed that showers and thunderstorms were increasing in coverage and strength in the core of 97L. The system’s upper reaches are well ventilated, with excellent outflow supporting the continued growth of thunderstorms. The usual nighttime maximum in shower and thunderstorm activity could be enough to give 97L a name. In a special outlook issued at 4:20 pm EDT Tuesday, the National Hurricane Center gave 97L a 90% chance of becoming at least a tropical depression by Thursday afternoon.

The outlook for 97L
As we discussed this morning, 97L has a long road ahead of it, with many twists and turns possible over the next week or more. Models continue to support the idea that fast-moving 97L will sweep through the Lesser Antilles on Wednesday, possibly as a strong tropical storm. The strongest winds and heaviest rains of 4 - 8”, with locally higher amounts, can be expected over the islands just north of where the center of 97L is, including the islands of St. Lucia, Martinique, Dominica, and Guadaloupe. The storm will continue westwards on Thursday, and make its closest approach to the ABC islands of the Netherlands Antilles--Aruba, Bonaire, and Curacao--on Thursday night and Friday morning. These islands will be on the weak (left) side of the storm, and may escape receiving tropical storm winds. However, heavy rains of 2 - 4” can be expected, as predicted by the 12Z (2 am EDT) Tuesday run of the HWRF model (see Figure 2).


Figure 2. Cumulative rainfall projected by the 12Z Tuesday run of the HWRF model through Sunday afternoon, October 2. Image credit: NOAA/NWS/NCEP.

As it moves into the southeast Caribbean--a challenging place for tropical cyclones, due to persistent sinking air--97L may struggle. Models are in fairly close agreement that 97L’s track will bend slightly toward the west or even southwest in the Caribbean, which could bring the center fairly close to the northern coast of Venezuela, leading to interaction with land that would hinder its growth. Toward the weekend, it appears that an upper-level low over the eastern U.S. will provide an opening for 97L to make a very sharp northward turn. As it enters the central Caribbean, 97L may have a better opportunity to intensify, and residents of the Greater Antilles--including Cuba, Haiti, Jamaica, and the Dominican Republic--will need to keep a close eye on it. While a sharp right-hand turn might seem implausible, there are a number of cases of storms taking such a northward bend, especially in October, with the classic example being 1954’s Hurricane Hazel (see Figure 3 below).

The 12Z runs of the GFS, UKMET, and European models--our best three long-range track models--all support the idea of 97L moving north from the Caribbean into the Bahamas, as soon as Monday of next week (GFS) or as late as Wednesday (UKMET and Euro). Whether or not the U.S. East Coast is threatened by 97L will depend on how soon the expected northward turn occurs and on the evolution of this weekend’s upper-level trough. These factors are too far out to be predicted with any confidence. The same is true for 97L’s eventual peak intensity, although the waters of the Caribbean and northwest Atlantic are at record or near-record warmth, more than capable of supporting a powerful hurricane if the atmospheric preconditions fall into place.


Figure 3. A comparision of the track of 1954’s Hurricane Hazel (left) with the GEFS ensemble model tracks for 97L produced at 18Z Tuesday (left). These ensemble members are produced by running the same model for the same period, each time varying the starting-point conditions in order to simulate the uncertainty in our observations of the atmosphere. The idea is not that 97L will mimic Hazel but that the sharp right-turn depicted by the GEFS does have a precedent.

In the Central and Eastern Pacific: Ulika and Roslyn
In the Central Pacific, Tropical Storm Ulika formed on Tuesday morning. Ulika is not expected to threaten any land areas, and is forecast to pass several hundred miles to the southeast of Hawaii early next week as a tropical depression. Further to the east, about 500 miles west-southwest of Cabo San Lucas, Mexico, Tropical Storm Roslyn is hanging on by a thread amid strong vertical wind shear that will push progressively drier air into its core. Rosyln should be a remnant low by Wednesday afternoon if not sooner.


Figure 4. Infrared satellite image of Tropical Storms Uliki (left) and Rosyln (right) as of 2045Z (4:45 pm EDT) Tuesday, September 27, 2016. Image credit: CIMSS/University of Wisconsin/SSEC.

Megi makes its second landfall
The Northwest Pacific was quiet on Tuesday afternoon apart from deadly Typhoon Megi, which slammed into Taiwan on Tuesday local time, causing at least 4 deaths and hundreds of injuries (see our Tuesday morning post for more details). According to the Joint Typhoon Warning Center, Megi swept onto the southeast coast of China just north of Quanzhou around 18Z Tuesday (2:00 am Wednesday local time). This landfall is about 50 miles northeast of where devastating Typhoon Meranti slammed ashore only a few days ago. Torrential rains, flooding, and landslides are once again possible as a rapidly weakening Megi moves inland and pushes moisture against the area’s rugged terrain.

We’ll be back by late Wednesday morning with our next update.

Bob Henson and Jeff Masters

Hurricane

97L Close to Tropical Depression Status; Category 4 Megi Hammers Taiwian

By: Jeff Masters and Bob Henson , 4:08 PM GMT on September 27, 2016

A tropical wave located about 400 miles east-southeast of Barbados in the Lesser Antilles Islands late on Tuesday morning (Invest 97L) was headed west to west-northwest at 15 - 20 mph, and appears likely to develop into a tropical depression or tropical storm later on Tuesday. Satellite loops late Tuesday morning showed 97L did not yet have a well-defined surface circulation, though it appeared close to establishing one. The amount of heavy thunderstorm activity was modest at best, but upper-level outflow was very well established to 97L’s north. Aiding development was moderate wind shear of 10 - 20 knots and warm ocean waters of 29.5°C (85°F). Significant negatives for development included the storm’s forward speed of 15 - 20 mph, which was too fast for the storm to get itself properly aligned in the vertical, plus dry air. The 8 am EDT Tuesday SHIPS model output analyzed 55% relative humidity at mid-levels of the atmosphere over 97L, which is lower than optimal for tropical cyclone formation. The 12Z (8 am EDT) Tuesday balloon sounding from Trinidad, 500 miles to the west of 97L, showed several bands of dry air with humidities below 40% located between 618 mb and 300 mb in altitude. Water vapor satellite loops showed 97L was butting into a region of dry air that lay just east of the Lesser Antilles Islands. Lack of spin from being too close to the equator was less of a problem for 97L than before, as the system had worked its way northwards to a latitude of 12°N. This is far enough from the equator for the storm to be able to leverage the Earth’s spin and acquire more spin of its own. The outermost spiral rainband of 97L was bringing rain showers to Barbados late Tuesday morning, as seen on Barbados radar.


Figure 1. Latest satellite image of 97L.

Forecast for 97L
Model support for development of 97L over the next day or so remains high. Our top three models for predicting hurricane genesis—the GFS, UKMET and European models—all predicted in their 00Z Tuesday runs that 97L would develop into a tropical depression or tropical storm between Tuesday and Wednesday. In their 8 am EDT Monday Tropical Weather Outlook, NHC gave 97L 2-day and 5-day development odds of 90%. The Hurricane Hunters are in the air, and will investigate the storm on Tuesday afternoon. The next name on the Atlantic list of storm names is Matthew.

Invest 97L will continue west to west-northwest at 15 - 20 mph through Thursday, then slow down to a forward speed of 5 - 10 mph for the remainder of the week. The outer spiral bands of 97L will begin spreading over the Lesser Antilles Tuesday night, bringing high winds and heavy rains. The core of the storm will pass through the islands on Wednesday afternoon. It is unlikely that 97L will have time to intensify into a hurricane by then, though a strong tropical storm with 55 - 65 mph winds is quite possible. The strongest winds and heaviest rains of 4 - 8” can be expected over the islands just north of where the center of 97L is, including the islands of St. Lucia, Martinique, Dominica, and Guadaloupe. The storm will continue westwards on Thursday, and make its closest approach to the ABC islands of the Netherlands Antilles--Aruba, Bonaire, and Curacao--on Thursday night and Friday morning. These islands will be on the weak (left) side of the storm, and may escape receiving tropical storm winds. However, heavy rains of 2 - 4” can be expected, as predicted by the 06Z (2 am EDT) Tuesday run of the HWRF model.

As 97L passes through the southeastern Caribbean, it will be in an environment somewhat unfavorable for development. The southeast Caribbean is a well-known tropical cyclone graveyard, where scores of healthy-looking storms have died or suffered severe degradation. This is often due to the fact that the southeastern Caribbean is a place where the surface trade winds tend to accelerate, due to the geography and meteorology of the area. A region of accelerating flow at the surface means that air must come from above to replace the air that is being sucked away at the surface. Sinking air from above warms and dries as it descends, creating high pressure and conditions unfavorable for tropical cyclones. In addition, tropical cyclones passing near the coast of South America often suck in dry continental air from the land areas to the south. The last hurricane to pass through the southeastern Caribbean, Hurricane Tomas of 2010, degraded from a Category 1 hurricane to a tropical depression due to high wind shear and dry air as it moved across the region. Recent runs of the SHIPS model are showing moderate wind shear of 10 - 15 knots and some dry air for Thursday and Friday along 97L’s path, and these conditions may interfere with development. In addition, the Tuesday morning 00Z runs of the European and UKMET models predicted that 97L would move slightly south of due west and pass very close to the coast of South America on Friday, which would also inhibit development.





Figure 2. Forecasts from the 00Z Tuesday European (ECMWF) model ensemble (top) and GFS model ensemble (bottom) had a number of their 70 members predicting a hurricane for late in the week in the Caribbean (light blue dots.) The operational versions of the models, run at higher resolution (red lines), also showed the storm becoming a hurricane by ten days into the future. The European model showed a more westerly track for 97L, with a long-range threat to the Gulf of Mexico, while the GFS model predicted more of a threat to the U.S. East Coast.

Long range forecast for 97L
A large upper-level low pressure system is expected to separate from the jet stream and settle over the Mid-Atlantic states late this week, and the steering currents associated with this low are expected to be strong enough to pull 97L sharply to the north by the weekend, according to a majority of the Tuesday morning runs of the models. While the sharp right-hand turn in Figure 2 might look implausible, there are a number of cases of storms taking such a northward bend, especially in October, with the classic example being Hurricane Hazel (1954). This sharp turn is expected to occur on Friday night or on Saturday, and the exact timing of the turn has huge implications for who experiences the peak wrath of the storm. An earlier turn is being predicted by the GFS model, which would put the Dominican Republic and Haiti in the cross hairs for a direct hit early next week, with a long-range threat to the Bahamas and the U.S. East Coast later in the week. The European model and UKMET model forecast a later turn, resulting in Jamaica and Cuba being more at risk of a direct strike early next week, and Florida and the U.S. Gulf Coast being at risk later in the week. As one can see from the latest set of ensemble model runs (Figure 2), the uncertainties are high. 97L is expected to have favorable conditions for intensification this weekend as it heads north towards the islands, with low wind shear, very warm ocean waters, and a very moist atmosphere. The models are quite bullish on this storm being a hurricane when it makes its landfall early next week in the islands.

Ulika forms in the Central Pacific
In the Central Pacific, Tropical Storm Ulika formed on Tuesday morning. Ulika is not expected to threaten any land areas, and is forecast to pass several hundred miles to the southeast of Hawaii early next week as a tropical depression.


Figure 3. Churning waters in the Jhihtan Dam in Xindian district, New Taipei City, as Typhoon Megi hit eastern Taiwan on September 27, 2016. Taiwan went into shutdown on September 27 as the island faced its third typhoon in two weeks, with thousands evacuated, schools and offices closed across the island and hundreds of flights disrupted. Image credit: Sam Yeh/AFP/Getty Images.

Deadly Typhoon Megi slams into Taiwan
Bolting to Category 4 status just before landfall, Typhoon Magi crashed into central Taiwan at full force on Tuesday evening local time. CNN reported least four deaths and more than 300 injuries by late Tuesday, with close to 2.7 million homes without power. Megi’s top winds were assessed at 115 knots (132 mph) by the Joint Typhoon Warning Center at 06Z Tuesday (2:00 am EDT and 2:00 pm local time). Meg plowed ashore near Hualien City, where Josh Morgerman (@iCyclone) reported calm winds in the large eye of the typhoon at 1:55 pm local time. See the photo embedded at bottom for a spectacular view of waves smashing into the seawall at Hualien City.

Megi completed an eyewall replacement cycle just before reaching Taiwan, which allowed for a burst of intensification even as it approached the island. At 12Z Tuesday, the JTWC placed Megi’s center in the Taiwan Strait, midway between Taiwan’s west coast and the southeast coast of China (about 50 miles from each). Top sustained winds at 12Z were down to 100 knots (115 mph), making Megi a fast-weakening Category 3. Not surprisingly, Megi’s quick journey across the rugged mountains of Taiwan took a major toll on the integrity of its structure, but Megi will still pack some punch as it sweeps into China as a strong tropical storm or minimal typhoon. Mega is the third major typhoon to affect Taiwan in the last two weeks, following Super Typhoon Meranti (which passed just southwest of the island) and Typhoon Malakas (which passed just to the northeast). Meg’s landfall in southeast China will be close to the area where Meranti claimed at least 29 lives and caused at least $2.6 billion in damage.


Figure 4. Aqua/MODIS satellite image of Typhoon Megi at 0525Z (1:25 am EDT and 1:25 pm local time) Tuesday, September 27, 2016, just as Megi was making landfall in east-central Taiwan. Image credit: NASA.


Figure 5. Rainfall across Taiwan for the calendar day Tuesday, September 27, 2016, up through 10:30 pm local time (10:30 am EDT Tuesday). The scale at right is in millimeters; 100 mm = 3.94 inches. Image credit: Central Weather Bureau, Taiwan.

It was still too soon on Tuesday morning EDT to assess the full impact of Megi’s assault on Taiwan. Wind gusts above 90 mph were widespread, as reported by weather.com. Sustained winds reached 71 mph with gusts to 99 mph at 2:30 pm local time at Taipei’s Taiyuan International Airport, with a gust to 124 mph recorded at Wuqi on Taiwan’s west coast, according to Taiwan’s Central Weather Bureau. Wind damage may be widespread over northeast Taiwan in Yilan County, home to about half a million people. Meg’s path put coastal Yilan County just north of Megi’s large eyewall, and the typhoon’s fierce winds pushed upslope to produce torrential rains in the county’s mountainous western section. A total of 945 mm (37.20”) fell at Taipingshan in western Yilan County from midnight through 10:30 pm local time Tuesday, according to Taiwan’s Central Weather Bureau. By late Tuesday, the heaviest rains were wrapping into southwest Taiwan on the south side of Megi’s circulation, where 10” amounts were widespread.


Figure 6. Rainfall amounts topping 10” are possible along and near the coast of southeast China and in far South Korea over the next several days as Megi and its remnants move into China and eventually recurve toward the northeast. Shown above is cumulative precipitation for the six-day period starting at 06Z (2:00 am) Tuesday, September 27, 2016, from the GFS model. The scale at right is in millimeters; 100 mm = 3.94 inches. Image credit: tropicaltidbits.com

We will be back this afternoon with an update on 97L and the Pacific storms.

Jeff Masters and Bob Henson



Figure 7. James Reynolds says: “Hualien port sea wall taking a beating earlier today as #typhoon #Megi hit #Taiwan.” See the associated video. Image credit: James Reynolds, @EarthUncutTV.


Hurricane

Thousands Urged to Evacuate Iowa Floods; Megi’s Threat to Taiwan Escalates

By: Bob Henson and Jeff Masters , 10:12 PM GMT on September 26, 2016

Water from the Cedar River will overspread a large swath of Cedar Rapids, Iowa’s second-largest city, on Tuesday. Fed by massive rains over northeast Iowa during the past week, the flood crest of 23 feet projected on Monday afternoon for Tuesday afternoon would be the second-worst in city history--topped only by the 31 feet during the catastrophic flood of June 2008, and 3 feet higher than all other crests. Officials have strongly encouraged residents of about 5000 homes in central Cedar Rapids to evacuate ahead of the flood crest (see map at bottom of this post).


Figure 1. Iowa National Guard members walk past a local business covered in sandbags, Monday, Sept. 26, 2016, in Cedar Rapids, Iowa. Residents are waiting anxiously as a quickly rising Cedar River threatens to inundate their city with devastating floodwaters for the second time in eight years. Image credit: AP Photo/Charlie Neibergall.


The floods of 2008 eclipsed anything on record across eastern Iowa, particularly in Cedar Rapids and Iowa City. The estimated cost of that flood reached $10 billion, with more than 5000 homes and nearly 1000 business damaged. This time, the heaviest rains (more than 10” in 24 hours last Wednesday--see Figure 3 below) have been focused across the Cedar River basin north of Cedar Rapids, which is helping to keep the most serious flooding within that corridor, including Cedar Falls and Waterloo. Another plus: the flood crest heading toward Cedar Rapids in 2008 was boosted by an intense storm just one day before it peaked downtown. “That’s why the intensity of the flooding caught people by surprise. Nothing like that is in the cards [this time]” Witold Krajewski, director of the Iowa Flood Center, told me. In fact, the flood crest should arrive on Tuesday beneath sunny skies. “Because this event is well predicted, the city is well prepared,” said Krajewski.


Figure 2. An aerial image shows flood-affected areas on June 13, 2008 in Cedar Rapids, Iowa. Image credit: David Greedy/Getty Images.

Cedar Rapids since 2008: Progress and problems
In the wake of the 2008 floods, Cedar Rapids and surrounding Linn County took big strides toward preparing for future disaster. The city and county are among three pilot communities nationwide in the Resilient America project, organized by the National Academies to help communities build resilience and reduce the physical and economic toll of disasters. In Cedar Rapids, roughly 1400 flood-damaged homes were acquired by the city and removed over the last eight years. That action has already cut back on the destruction that this week’s flood might wreak. The city has also adopted a $625 million flood protection plan, including a series of new levees and floodwalls. However, these structures have yet to be built: federal funding has been authorized, but has not yet been provided. As a result, some of the areas restored in the wake of the 2008 flood are imperiled once again.

Citizen-volunteers have been pitching in this week, helping to put more than 250,000 sandbags in place to help stem the flood crest. A Facebook page devoted to the unfolding flood is helping connect people to the most pressing needs. About 100 volunteers spent Sunday removing almost 1000 seats from the historic Paramount Theatre in downtown Cedar Rapids. After being heavily damaged in 2008, the theatre was restored with federal, state, and local funds totalling $35 million.


Figure 3. A swath of 8” - 10” of rain fell on Wednesday, September 21, 2016, atop the northwest-to-southeast Cedar River basin. Enhanced by other rains over the past week, this flood crest has moved southeast past Waterloo and Cedar Falls and is now approaching Cedar Rapids (southeast of map domain). Image credit: Iowa Flood Information System/Iowa Flood Center.

A very out-of-season flood for eastern Iowa
September is not when Iowans are usually keeping an eye on flood risk. In fact, this is the first time on record that a flood of any magnitude (at least 12 feet) has been measured on the Cedar River at Cedar Rapids. Heavy rains are far more common during spring and summer across eastern Iowa. Climate change has been shown to exacerbate the intensity of the heaviest rainfall events in the United States and many other parts of the world, as warmer temperatures allow more moisture to evaporate into the atmosphere. Research led by Eugene Takle (Iowa State University) found that eastern Iowa has already seen a measurable rise in the number of days with rainfall that is typically too heavy for soils to absorb (1.25” in a 24-hour period). “We can logically conclude that enhanced streamflow and the potential for flooding are also rising,” Takle wrote. His research has also found a shift away from spring floods (as warmer spring temperatures allow for earlier snowmelt) and toward late-summer events. Storms in late summer tend to be oriented along northwest-to-southeast trajectories, which aligns them with river basins in eastern Iowa. “This seems to be the new norm for seasonal flood occurrence in the state,” noted Takle.


Figure 4. Enhanced infrared satellite image of Typhoon Megi as of 2100Z (5:00 pm EDT) Monday, September 26, 2016. Image credit: RAMMB/CIRA/CSU.

Megi intensifying as it nears Taiwan
Typhoon Megi vaulted to Category 3 strength on Monday afternoon, heightening the risk to Taiwan. As of 18Z (2 pm EDT), Megi’s top sustained winds (averaged over 1 minute) were estimated at 115 mph by the Joint Typhoon Warning Center. Satellite imagery suggests that Megi has almost completed an eyewall replacement cycle, with its small eye from last weekend now superseded by a much larger but still-ragged eye, about 35 miles in diameter. The timing of this replacement cycle is bad news for Taiwan, as Megi has had just enough time to begin consolidating very intense, widespread thunderstorms around this newly formed eye. The 21Z Monday (5 pm EDT) forecast from JTWC brings Taiwan onshore with peak winds of at least 110 mph.


Figure 5. As of 18Z (2:00 pm EDT) Monday, September 26, the center of Typhoon Megi is projected to pass through central Taiwan between 06Z and 18Z Tuesday (2:00 pm Tuesday to 2:00 am Wednesday local time). Image credit: Central Weather Bureau, Taiwan.


Although Taiwan is experienced with typhoons—it gets three to four in a typical year--Megi will be striking the island head on, with its center passing through the heart of Taiwan. This will put the northern half of the island, which includes the capital of Taipei, in the more dangerous right-hand part of Megi. All signs point to Megi being an expensive and potentially deadly disaster for Taiwan, with wind damage, massive rainfall, and landslides a good bet across mountainous eastern Taiwan and major disruption in the capital city of Taipei, at the island’s north end. As of 2:20 am local time (2:20 pm EDT), 24-hour rains had already topped 200 mm (7.87”) at Taoyuan City, just west of Taiwan. Exacerbating the flood risk will be the saturated ground produced by heavy rain from Super Typhoon Meranti and Typhoon Malakas over the last two weeks.

On its predicted course, Megi would make a second landfall as a tropical storm along the coast of southeast China, not far south of where Meranti claimed at least 29 lives and caused at least $2.6 billion in damage.

Taiwan's typhoon history
According to EM-DAT, the international disaster database, the most damaging typhoon in Taiwan’s history was Typhoon Herb of 1996, which hit the island as a Category 4 super typhoon with 150 mph winds. The eye passed directly over the capital of Taipei, and the storm unleashed rains as high as 78.23”. Damage was estimated at $1.7 billion (2016 dollars), and the storm killed 73 people. The only other billion-dollar typhoon for the island was Typhoon Nari of 2001, which hit the island as a Category 2 storm with 100 mph winds, killing 104 and causing $1.1 billion in damage (2016 dollars.) Two typhoons have hit Taiwan at Category 5 strength. The first was Super Typhoon Joan, which made landfall in 1959 with 185 mph winds. Joan was Taiwan’s deadliest typhoon in recorded history, with 1046 deaths. The other Cat 5 was Super Typhoon Bilis, which intensified from a Category 2 storm with 105 mph winds to a Category 5 storm with 160 mph winds in the 30 hours before making landfall on the island on August 22, 2000. Bilis killed 14 people and did $134 million in damage to Taiwan. The island was hit by one major typhoon this year—Category 4 Super Typhoon Neparak—and by two major typhoons last year: Category 4 Typhoon Dujuan and Category 3 Typhoon Soudelor. The Central Weather Bureau of Taiwan has a list (in Chinese) of all the typhoons that have affected Taiwan.

Megi Links
Brian McNoldy has a continuously updating radar loop of Megi. It will be fascinating to watch as the storm makes landfall and gets deflected and distorted by the high mountains of Taiwan.

Stunning zoomed-in visible animation of Megi from September 26, 2016, from NOAA/RAMMB/Colorado State.

Storm chaser James Reynolds is in Taiwan and will be making live updates on Megi’s landfall on his Twitter feed.


Figure X. Satellite image of Invest 97L from Monday afternoon, September 26, 2016.

97L slowly organizing over tropical Atlantic
As it continues hustling westward at about 20 mph, the tropical wave dubbed Invest 97L is gaining latitude and organization. At 2:00 pm EDT Monday, 97L was located about 950 miles east-southeast of the Windward Islands. Considerable spin was already evident in a large swath of scattered showers and thunderstorms on 97L’s north side. The National Hurricane Center has upped the odds of 97L’s developing into at least a tropical depression to 70% by Wednesday and 90% by Saturday. The large envelope of showers and thunderstorms around 97L will make it slower to develop, but potentially more powerful once it does.

A Hurricane Hunter flight is scheduled for Tuesday afternoon into 97L, and it would not surprising if it finds that 97L is already Tropical Storm Matthew by then. Models agree that 97L will track into the southeast Caribbean, and tropical storm conditions may spread across parts of the Lesser Antilles as it does so. As noted in our morning update, there remains huge uncertainty as to 97L’s future beyond that point. The 12Z Monday operational runs of the GFS, ECMWF, and UKMET models differ on how closely 97L might come to the north coast of South America, but they agree on the general idea of a sharp northward turn through the central Caribbean by the weekend or early next week. Should this turn materialize, a landfall on the East Coast of the U.S. or Canada would be a possibility, or 97L could even head out to sea. All this will hinge on how quickly one upper-level storm system moves through the eastern United States and how quickly the next one approaches. It is far too soon to know how those midlatitude storms will evolve, and we still can’t rule out the possibility that 97L will continue across the Caribbean much farther to the southwest. One thing we do know is that 97L will have at least some of the ingredients needed to become a powerful and dangerous hurricane, including very deep warm water through much of the Caribbean as well as sea surface temperatures at or near record-warm levels across the western Atlantic.

We’ll be back with our next update on the Atlantic and Pacific tropics by late morning Tuesday.

Bob Henson and Jeff Masters


Hurricane Flood

97L Growing More Organized as it Approaches the Lesser Antilles Islands

By: Jeff Masters , 3:11 PM GMT on September 26, 2016

A tropical wave located about 1000 miles east-southeast of Barbados in the Lesser Antilles Islands late Monday morning (Invest 97L) was headed west at 15 - 20 mph, and has the potential to become a dangerous storm in the Caribbean later this week. Satellite loops on Monday morning showed 97L was growing considerably more organized, with more curvature to the cloud pattern, an increasing amount of heavy thunderstorm activity, low-level spiral bands that were getting more defined, and upper-level outflow that was becoming better established to 97L’s north. The storm’s organization was being aided by low wind shear of 5 knots, a very moist atmosphere (relative humidities at mid-levels of the atmosphere near 70%) and warm ocean waters of 30°C (86°F). Significant negatives for development included the storm’s forward speed of 15 - 20 mph, which was too fast for the storm to get itself properly aligned in the vertical, plus 97L’s nearness to the equator. The system was centered near 9.5°N, which was too far south to be able to leverage the Earth’s spin and acquire much spin of its own.


Figure 1. Latest satellite image of 97L.

Forecast for 97L
Invest 97L will continue west to west-northwest at 15 - 20 mph through Tuesday, reaching a latitude of about 12°N by Tuesday afternoon. This is far enough away from the equator to give 97L an extra boost of spin that may allow it to become a tropical depression on Tuesday. With the SHIPS model predicting wind shear remaining low, mid-level moisture staying high at 65 - 70%, SSTs remaining a very warm 29 - 30°C (84 - 86°F), and 97L slowing its forward speed to about 10 - 15 mph, conditions will be ripe on Tuesday for 97L to become a tropical depression or tropical storm before it reaches the Lesser Antilles Islands. By Tuesday night, the outer spiral bands of 97L will begin spreading over the Lesser Antilles, bringing high winds and heavy rains. The core of the storm will pass through the islands on Wednesday afternoon. It is unlikely that 97L will have time to intensify into a hurricane by then, though a strong tropical storm with 60 - 70 mph winds is quite possible.

Invest 97L may pass very close to the coast of South America on Thursday and Friday, which would interfere with development. In addition, the southeastern Caribbean is a well-known tropical cyclone graveyard, where scores of healthy-looking storms have died or suffered severe degradation. This is often due to the fact that the southeastern Caribbean is a place where the surface trade winds tend to accelerate, due to the geography and meteorology of the area. A region of accelerating flow at the surface means that air must come from above to replace the air that is being sucked away at the surface. Sinking air from above warms and dries as it descends, creating high pressure and conditions unfavorable for tropical cyclones. In addition, tropical cyclones passing near the coast of South America often suck in dry continental air from the land areas to the south. The last hurricane to pass through the southeastern Caribbean, Hurricane Tomas of 2010, degraded from a Category 1 hurricane to a tropical depression due to high wind shear and dry air as it moved across the region. Recent runs of the SHIPS model have been predicting that 97L will increase its forward speed to 25 mph on Thursday in response to the acceleration of the trade winds over the southeastern Caribbean, and this will likely interfere with development. The model is also indicating that 97L will draw in dry air from northern South America, further slowing intensification. Once 97L manages to separate itself from the coast of South America early next week, more significant intensification can occur.

Model support for development of 97L continues to remain high. Our top three models for predicting hurricane genesis—the GFS, UKMET and European models—all predicted in their 00Z Monday runs that 97L would develop into a tropical depression or tropical storm between Tuesday and Thursday. About 80% of the 20 forecasts from the members of the 00Z Monday GFS ensemble showed development into a tropical storm, with 50% predicting a hurricane. The European model ensemble was less aggressive developing the storm, probably because of a predicted track too close to the coast of South America—about 60% of its 50 ensemble members predicted a tropical storm in the Caribbean, with 30% predicting a hurricane. In their 8 am EDT Monday Tropical Weather Outlook, NHC gave 97L 2-day and 5-day development odds of 60% and 90%, respectively. The Hurricane Hunters are scheduled to make their first flight into the storm on Tuesday afternoon. The next name on the Atlantic list of storm names is Matthew.





Figure 2. Forecasts from the 00Z Monday European (ECMWF) model ensemble (top) and GFS model ensemble (bottom) had a number of their 70 members predicting a hurricane for late in the week in the Caribbean (light blue dots.) The operational versions of the models, run at higher resolution (red lines), also showed the storm becoming a hurricane by ten days into the future. The European model showed a more westerly track for 97L, with a long-range threat to the Gulf of Mexico, while the GFS model predicted more of a threat to the U.S. East Coast.

Long-range forecast for 97L
The steering currents for 97L will be complex over the coming ten days, and it is very difficult at this point to narrow down where the storm will be 5 - 10 days into the future. A large upper-level low pressure system is expected to separate from the jet stream and settle over the Mid-Atlantic states late this week, and the steering currents associated with this low are expected to be strong enough to pull 97L more to the northwest by the weekend, according to a majority of the Monday morning runs of the models. In this scenario, Haiti, the Dominican Republic, Jamaica, or eastern Cuba would be at risk from a direct strike by 97L on Sunday or Monday. Early next week, this upper level low is expected to lift out to the northeast as a strong trough of low pressure passes its north and captures it. This trough may be strong enough to pull 97L to the northeast with it, if the storm is far enough to the north. Otherwise, 97L will likely continue on a west-northwesterly path. As one can see from the latest set of ensemble model runs (Figure 2), 97L could eventually make landfall anywhere from Nicaragua to Newfoundland, Canada, so we really can’t narrow things down much at this point. If 97L ends up consolidating its center at a latitude significantly different from what these models are expecting, or on a day different from what is expected, the forecast tracks may change dramatically. Making an accurate long-range track forecast from a tropical wave in the process of transitioning into a tropical depression is notoriously difficult.

Roslyn forms off the Pacific coast of Mexico
In the Eastern Pacific, Tropical Storm Roslyn, the seventeenth named storm of this very busy 2016 Eastern Pacific hurricane season, formed on Monday morning. An average season in the Eastern Pacific has just fifteen named storms during the entire year. Roslyn is expected to head north into an area of high wind shear and dry air, and will dissipate late this week without affecting any land areas.


Figure 3. Radar image of Typhoon Megi taken at 10:40 am EDT September 26, 2016 (22:40 local time.) Image credit: Taiwan Central Weather Bureau.

Typhoon Megi takes aim at Taiwan
Category 2 Typhoon Megi was holding at 105 mph sustained winds late Monday morning as it headed west-northwest at 10 mph towards Taiwan. Satellite images and radar images indicated that Megi was undergoing an eyewall replacement cycle late Monday morning, where the inner eyewall collapses and is replaced by a larger-diameter outer eyewall, and this process will likely keep Megi from intensifying before the storm makes landfall in Taiwan near midnight Monday (U.S. EDT.) Megi is the fourth significant typhoon to affect Taiwan this year. Super Typhoon Nepartak hit Taiwan on July 7 as a Category 4 storm with top sustained winds of 150 mph. Earlier in September, Super Typhoon Meranti passed just to Taiwan's southwest, killing two and leaving nearly a million without power, and Typhoon Malakas passed just to Taiwan's northeast a few days later. Taiwan averages 3 to 4 typhoon strikes per year, according to the Central Weather Bureau. On its predicted course, Megi would make a second landfall as a tropical storm along the coast of southeast China, not far south of where Meranti claimed at least 29 lives and caused at least $2.6 billion in damage. Storm chaser James Reynolds is in Taiwan and will be making live updates on Megi’s landfall on his Twitter feed.

Bob Henson will be back this afternoon with an update on major flooding occurring on the Cedar River and Mississippi River in Iowa.

Jeff Masters

Hurricane

97L Potentially a Dangerous Storm for the Caribbean

By: Jeff Masters , 5:38 PM GMT on September 25, 2016

A tropical wave located midway between the Lesser Antilles Islands and the coast of Africa on Sunday afternoon was headed west at 15 - 20 mph, and has the potential to become a dangerous storm in the Caribbean later this week. NHC designated this system Invest 97L on Sunday morning. After looking remarkably unimpressive on satellite loops for the previous few days, 97L was turning that situation around on Sunday. The system had a large circulation at middle levels of the atmosphere, with an increasing amount of heavy thunderstorm activity. Some low-level spiral bands were beginning to develop, and upper-level outflow was becoming established to 97L’s north. The storm’s organization was being aided by low wind shear of 5 - 10 knots, a very moist atmosphere (relative humidities at mid-levels of the atmosphere near 75%) and warm ocean waters of 29°C (84°F). Significant negatives for development included the storm’s forward speed of 15 - 20 mph, which was too fast for the storm to get itself vertically aligned, plus 97L’s nearness to the equator. The system was centered near 8°N, which was too far south to be able to leverage the Earth’s spin and acquire much spin.


Figure 1. Latest satellite image of 97L.

Forecast for 97L
Invest 97L will continue west to west-northwest at 15 - 20 mph through Tuesday, reaching a latitude of about 12°N by Tuesday. This is far enough away from the equator to give 97L an extra boost of spin that may allow it to become a tropical depression on Tuesday. With the SHIPS model predicting wind shear remaining low, mid-level moisture staying high at 70 - 75%, SSTs remaining a very warm 29°C (84°F), and 97L slowing its forward speed to about 15 mph, conditions will be ripe on Tuesday for 97L to become a tropical depression or tropical storm before it reaches the Lesser Antilles Islands. By Tuesday night, the outer spiral bands of 97L will begin spreading over the Lesser Antilles, bringing high winds and heavy rains. The core of the storm will pass through the islands on Wednesday afternoon.

Invest 97L may pass very close to the coast of South America, which would interfere with development. In addition, the southeastern Caribbean is a well-known tropical cyclone graveyard, where scores of healthy-looking storms have died or suffered severe degradation. This is primarily due to the fact that the southeastern Caribbean is a place where the surface trade winds tend to accelerate, due to the geography and meteorology of the area. A region of accelerating flow at the surface means that air must come from above to replace the air that is being sucked away at the surface. Sinking air from above warms and dries as it descends, creating high pressure and conditions unfavorable for tropical cyclones.

Model support for development of 97L continues to remain high. Our top three models for predicting hurricane genesis—the GFS, UKMET and European models—all predicted in their 00Z Sunday runs that 97L would develop into a tropical depression or tropical storm between Monday and Wednesday. About 70% of the 20 forecasts from the members of the 00Z Sunday GFS ensemble showed development into a tropical storm, with 40% predicting a hurricane. The European model ensemble was less aggressive developing the storm, probably because of a predicted track too close to the coast of South America—about 40% of its 50 ensemble members predicted a tropical storm in the Caribbean, with 30% predicting a hurricane. In their 2 pm EDT Sunday Tropical Weather Outlook, NHC gave 97L 2-day and 5-day development odds of 30% and 90%, respectively. The Hurricane Hunters are scheduled to make their first flight into the storm on Tuesday afternoon. The next name on the Atlantic list of storm names is Matthew.





Figure 2. Forecasts out to ten days from the 00Z Sunday European model ensemble (top) and GFS model ensemble (bottom) had a number of their 70 members predicting a hurricane for late in the week in the Caribbean (light blue dots.) The operational versions of the models, run at higher resolution (red lines), also showed the storm becoming a hurricane by ten days into the future.

Will 97L threaten the U.S.?
Forecasts of what might happen to 97L beyond five days from now are speculative, but let’s go ahead and speculate. A large upper-level low pressure system is expected to form over the Mid-Atlantic states late this week, and the steering currents associated with this low are expected to be strong enough to pull 97L more to the northwest by the weekend, according to a majority of the Sunday morning runs of the models. In this scenario, Haiti, Jamaica, Cuba, the Bahamas, and Florida would be at greatest risk for a strike by 97L. According to the Sunday morning Extended Forecast Discussion from the NWS (thanks to WU member nrtiwlnvragn for posting this link), the models are in substantial disagreement on the evolution of this upper-level low, with the GFS model being judged to have the best handle on it. If this analysis is correct, the long-range forecasts from the GFS model may be better than the European model’s. However, you can throw all these forecasts out the window if 97L ends up consolidating its center at a latitude significantly different from what these models are expecting, or on a day different from what is expected. Making an accurate long-range track forecast from a tropical wave in the process of transitioning into a tropical depression is notoriously difficult.

Lisa and Karl die
Tropical Storm Karl became post-tropical without ever reaching hurricane strength, and was speeding to the northeast out to sea at 49 mph on Sunday morning. Karl brushed Bermuda on Saturday, bringing rains of about 4” to the airport, but no tropical storm-force winds. Tropical Storm Lisa is also no more, done in by high wind shear and dry air on Saturday night.

Invest 94E off the Pacific coast of Mexico may develop
In the Eastern Pacific, satellite loops on Sunday morning showed that an area of low pressure located about 825 miles southwest of the southern tip of the Baja California Peninsula on Sunday morning (Invest 94E) was well-organized with plenty of heavy thunderstorm activity. Our top three models for predicting hurricane genesis—the GFS, European, and UKMET models—predicted in their 00Z Sunday runs that 94E would develop into a tropical storm or tropical depression by Tuesday. The future track of the storm was uncertain; the European and UKMET models predicted that 94E would remain offshore of Mexico’s Baja Peninsula though the next five days, while the GFS model showed landfall in the central Baja Peninsula on Wednesday. In their 8 am EDT Sunday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 90%. Update: NHC upgraded 94E to Tropical Depression 18E at 2:00 pm EDT Sunday.

Typhoon Megi takes aim at Taiwan
Category 2 Typhoon Megi continues to intensify as it heads west-northwest at 12 mph towards Taiwan. Megi should reach Taiwan by late Tuesday local time. With unusually warm sea-surface temperatures of 29-30°C (84-86°F), a moist atmosphere, and low wind shear of 5 - 15 knots expected along Megi’s path over the next couple of days, Megi should continue to strengthen. The Joint Typhoon Warning Center projects that Megi will approach Taiwan as a Category 3 typhoon, with peak winds of 125 mph. Earlier in September, Super Typhoon Meranti passed just to Taiwan's southwest, killing two and leaving nearly a million without power. Also, Typhoon Malakas passed just to Taiwan's northeast a few days later, and Super Typhoon Nepartak hit Taiwan on July 7 as a Category 4 storm with top sustained winds of 150 mph. Taiwan averages 3 to 4 typhoon strikes per year, according to the Central Weather Bureau. On its predicted course, Megi would make a second landfall along the coast of southeast China, not far south of where Meranti claimed at least 29 lives and caused at least $2.6 billion in damage.

NOAA/RAMMB has a nice hi-resolution animation of Megi over the past day.

We’ll be back with our next update on Monday morning.

Jeff Masters

Hurricane

Potential Trouble in Tropical Atlantic; Karl Brushes Bermuda; Megi Eyes Taiwan

By: Jeff Masters and Bob Henson , 5:23 PM GMT on September 24, 2016

A tropical wave located a few hundred miles south-southwest of the Cabo Verde Islands on Saturday morning was poorly organized, with only a limited amount of heavy thunderstorm activity and spin. This wave was under low to moderate wind shear of 5 - 15 knots and was over warm ocean waters near 29°C (84°F), but was too close to the equator (near 9°N) to be able to leverage the Earth’s spin and acquire enough spin of its own to develop into a tropical depression. However, the tropical wave may move far enough from the equator to be able to develop by Monday or Tuesday, when it will be approaching the Lesser Antilles Islands. Working against development, at least in the next five days, will be the fast forward speed of the system. The storm will be driven west at 20 - 25 mph by the trade winds associated with an unusually strong Bermuda-Azores High; tropical waves moving at 20 mph or faster usually have trouble achieving the vertical alignment needed to intensify. However, the storm does not have as much dry air to contend with compared to other storms we have seen this year, and it would not be a surprise to see this system be close to tropical depression or tropical storm status when it begins moving into the Lesser Antilles Islands on Tuesday night or Wednesday morning. Once the storm enters the eastern Caribbean, long-range model runs suggest that the system might be very close to the coast of South America, which would interfere with development.


Figure 1. Satellite image of the tropical wave (left) located southwest of the Cabo Verde Islands at 1445Z (10:45 am EDT) Saturday, September 24, 2016. Image credit: NASA/MSFC Earth Science Office.

There was increased model support for development of this tropical wave in the Saturday morning runs of the models compared to their Friday runs. Our top three models for predicting hurricane genesis—the GFS, UKMET and European models—all predicted in their 00Z Saturday runs that this tropical wave would develop into a tropical depression or tropical storm between Monday and Thursday. About 80% of the 20 forecasts from the members of the 00Z Saturday GFS ensemble showed development into a tropical storm, with 40% predicting a hurricane. The European model ensemble was less aggressive developing the storm, probably because of a predicted track too close to the coast of South America—about 30% of its 50 ensemble members predicted a tropical storm in the Caribbean. In their 8 am EDT Saturday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 10% and 50%, respectively. The Hurricane Hunters are scheduled to make their first flight into the storm on Tuesday afternoon.





Figure 2. Forecasts out to ten days from the 00Z Saturday European model ensemble (top) and GFS model ensemble (bottom) had a number of their 70 members predicting a hurricane for late in the week in the Caribbean (light blue dots). The operational versions of the models, run at higher resolution (red lines), also showed the storm becoming a hurricane by ten days into the future.


Figure 3. Enhanced infrared image of Tropical Storm Karl as of 1545Z (11:45 am EDT) Saturday, September 24, 2016. Image credit: NOAA/NESDIS.

Karl brushes Bermuda
Tropical Storm Warnings were lifted for Bermuda on Saturday morning after Tropical Storm Karl brought a night of heavy rains and squalls to the island. At 11 am EDT, Karl was located about 125 miles east-northeast of Bermuda, packing top sustained winds of 65 mph and moving northeast at 18 mph. Earlier on Saturday morning, a Hurricane Hunter flight measured top flight-level winds of 63 knots (73 mph) and detected peak near-surface winds of 47 knots (54 mph) with the SFMR instrument (stepped frequency microwave radiometer).

Karl’s core of showers and thunderstorms, already east of Bermuda, reintensified late Saturday morning after some fragmentation earlier in the day. Strong upper-level winds associated with a large upper-level low in the North Atlantic were accelerating Karl to the northeast. Karl could become the fifth hurricane of the 2016 Atlantic season on Saturday night or Sunday morning before it evolves into a powerful post-tropical storm in the North Atlantic, eventually getting swept up into the massive upper low east of the Canadian Maritimes.

Karl’s recurving path brough the storm’s weaker left-hand side to within about 50 miles of Bermuda. Sustained winds topped out at a mere 29 mph Saturday morning at Bermuda International Airport, with gusts reaching 43 mph. Heavy rainbands doused the island, though, as the airport racked up more than 4” from Friday afternoon to Saturday morning.

Lisa is a tropical storm again
After decaying to tropical depression status late Friday, Tropical Storm Lisa got a new lease on life Saturday morning, regaining its tropical storm strength thanks to a burst of convection that developed atop its low-level center. Those storms have now weakened and blown eastward in strong wind shear of 30-40 knots, leaving the center again exposed (see satellite loop at bottom], so Lisa's resurgence will be brief. The NHC outlook brings Lisa's peak winds back below the tropical storm threshold by late Saturday afternoon, and Lisa should be a post-tropical low by Sunday or Monday.

Invest 94E off the Pacific coast of Mexico may develop
In the Eastern Pacific, satellite loops on Saturday morning showed that an area of low pressure located about 900 miles southwest of the southern tip of the Baja California Peninsula on Saturday morning (Invest 94E) was well-organized with plenty of heavy thunderstorm activity. One of our top three models for predicting hurricane genesis—the GFS model—predicted in its 00Z Saturday run that 94E would develop into a tropical storm or tropical depression over the weekend, and remain offshore of Mexico’s Baja Peninsula though the next five days. In their 8 am EDT Saturday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 80% and 90%, respectively.


Figure 4. Enhanced satellite image of Typhoon Megi as of 1641Z (12:41 pm EDT) Saturday, September 24, 2016.

Typhoon Megi taking aim on Taiwan
The last thing Taiwan needs is another approaching typhoon, but that’s exactly what is on the table with intensifying Typhoon Megi. With sustained winds at 65 mph as of 12Z (8:00 am EDT) Saturday, Megi was already a large and well-structured typhoon, with excellent outflow at upper levels helping to nourish its growth. Located about 900 miles east-southeast of Taipei, Megi was heading west-northwest at about 16 mph.

Models are in unusually close agreement on Megi’s continuing a remarkably straight west-northwest course, which would bring it to the vicinity of Taiwan by late Tuesday local time. That will give Megi plenty of time to strengthen, with unusually warm sea-surface temperatures of 29-30°C (84-86°F), a moist atmosphere (relative humidities around 70-75%) and low wind shear of 5 - 15 knots expected along Megi’s path over the next couple of days. The Joint Typhoon Warning Center projects that Megi will approach Taiwan as a Category 3 typhoon, with peak winds of 120 mph. There is a distinct possibility that Megi could strengthen more than predicted. The 00Z Saturday runs of the GFS, European, and UKMET models all bring Megi across southern or central Taiwan. The island has already dealt with the close approaches of Super Typhoon Meranti just to its southwest (which killed two residents and left nearly a million without power) and Typhoon Malakas just to its northeast. Only a few weeks earlier, Typhoon Nepartak hit Taiwan on July 7 as a Category 4 super typhoon with top sustained winds of 150 mph. Taiwan averages 3 to 4 typhoon strikes per year, according to the Central Weather Bureau.

On its predicted course, Megi would make a second landfall along the coast of southeast China, not far south of where Meranti claimed at least 29 lives and caused at least $2.6 billion in damage.

We’ll be back with our next update by early Sunday afternoon.

Jeff Masters and Bob Henson


Figure 5. WU depiction of Joint Typhoon Warning Center track forecast for Tropical Storm Megi issued on Saturday morning, September 24, 2016.



Hurricane

Karl Approaches Bermuda; Trouble in the Caribbean Next Week?

By: Bob Henson and Jeff Masters , 4:14 PM GMT on September 23, 2016

After nearly a week as a lackluster system, Tropical Storm Karl is finally gaining strength as it heads toward a close encounter with Bermuda. As of the 11 am EDT advisory, Karl was located about 250 miles south of Bermuda, moving north at 12 mph. Karl’s top sustained winds were holding at 60 mph, its peak intensity thus far. Karl is continuing its multi-day struggle with vertical wind shear that’s tended to push its showers and thunderstorms (convection) east of its center. On Thursday night, the storm managed to consolidate a healthy core of convection around its center, but Karl remains somewhat asymmetric, with a comma-shaped structure and a large band of convection well to its east.


Figure 1. Latest satellite image for Tropical Storm Karl.

Outlook for Karl
A Hurricane Watch and Tropical Storm Warning are now in effect for Bermuda, and it is not out of the question that Karl will pass near the island as a minimal hurricane. Karl’s motion slowed dramatically on Thursday as the storm began rounding the west edge of a large subtropical upper-level high. The overall track forecast is straightforward: Karl will move north on Friday and begin arcing toward the north-northeast by early Saturday, bringing it near Bermuda by midday Saturday. Karl will then rapidly accelerate northeastward through the rest of the weekend and into early next week. The longitude of Karl’s center on Friday morning was only a few miles west of Hamilton, Bermuda, so it would be difficult for Karl’s center to pass directly over the island if it were to gain any eastward component to its motion as it approaches Bermuda. The 06Z Friday run of the HWRF model is a western outlier, bringing the center of Karl very close to the island on Saturday morning. The 06Z Friday GFS run is somewhat faster and keeps Karl about 50-100 miles southeast of Bermuda, as do the 00Z Friday runs of the UKMET and European models.


Figure 2. WU depiction of National Hurricane Center forecast for Tropical Storm Karl issued Friday morning, September 23, 2016. Bermuda is depicted as the tiny dot just north of the “8 PM Fri” label.

Models agree that Karl’s strength will peak later in the weekend, when it begins to merge with a midlatitude storm system over the North Atlantic while racing northeastward. Karl will most likely be strengthening on Friday night into early Saturday as it nears Bermuda, and NHC takes Karl to minimal hurricane strength by 12Z (8:00 am EDT) Saturday. The worst-case scenario would be for Karl to approach or reach minimal hurricane strength and pass very near Bermuda. The island has experienced many hurricanes over the years and is well equipped to handle a storm of that strength. A more likely scenario, and the one favored by NHC, is that Karl will pass about 50 to 100 miles southeast of Bermuda, keeping the island on the weaker left-hand side of the storm. That would still be close enough to bring tropical storm-force conditions, so Bermuda could experience a brief period late tonight or early Saturday with sustained winds of 40 - 60 mph, including higher gusts, along with very heavy rain that could total 3” - 5”. The island will also be buffeted by high surf and large swells. Karl’s accelerating motion will limit the main period of impact to just a few hours. Karl was already bringing heavy rain showers to Bermuda on Friday morning, as seen on Bermuda radar.

An African tropical wave that could be trouble
A  tropical wave located a few hundred miles west of the coast of Africa and about 350 miles southeast of the Cabo Verde Islands on Friday morning was poorly organized, with only a limited amount of heavy thunderstorm activity and spin. This wave is currently too close to the equator (near 8°N) to be able to leverage the Earth’s spin and acquire enough spin of its own to develop into a tropical depression, and is not likely to develop through this weekend as it heads rapidly west at 20 - 25 mph. However, the tropical wave may move far enough from the equator to be able to develop by early next week, when it reaches a point about halfway between the Lesser Antilles Islands and the coast of Africa. There was increased model support for development of this tropical wave in the Friday morning runs of the models compared to their Thursday morning runs. Our top three models for predicting hurricane genesis—the GFS, UKMET and European models—all predicted in their 00Z Friday runs that this tropical wave would develop into a tropical depression or tropical storm between Monday and Thursday next week. About 60% of the 20 forecasts from the members of the 00Z Friday GFS ensemble showed development, and about 30% of the 50 members of the European model ensemble did so. Troublingly, a considerable number of the ensemble model runs showed this storm becoming a hurricane in the Caribbean. Working against development, at least in the next five days, will be the fast forward speed of the system—tropical waves moving at 20 mph or faster usually have trouble getting organized. However, the storm does not have as much dry air to contend with compared to other storms we have seen this year, and it would not be a surprise to see this system be close to tropical depression or tropical storm status when it begins moving into the Lesser Antilles Islands on Tuesday night.  In their 8 am EDT Friday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 0% and 20%, respectively.





Figure 3. Forecasts out to ten days from the 00Z Friday European model ensemble (top) and GFS model ensemble (bottom) had a number of their 70 members predicting a hurricane for late next week in the Caribbean (light blue dots.) The operational versions of the models, run at higher resolution (red lines), also showed the storm becoming a hurricane by ten days into the future.

Tail-end development in the Gulf of Mexico looks unlikely
A cold front will move into Texas on Monday, and potentially stall just offshore of Texas on Tuesday. We’ll need to watch the tail end of this cold front for tropical development if it lingers over the Gulf of Mexico for a few days. However, there is less model support today than yesterday for this occurring, with fewer than 5% of the members of the 00Z Friday GFS and European model ensemble forecasts showing potential tropical development over the Gulf of Mexico next week.

Invest 94E off the Pacific coast of Mexico may develop
In the Eastern Pacific, satellite loops on Friday morning showed that an area of low pressure located about 800 miles south-southwest of the southern tip of the Baja California Peninsula on Friday morning (Invest 94E) was well-organized with plenty of heavy thunderstorm activity, and our top three models for predicting hurricane genesis—the GFS, UKMET and European models—all predicted in their 00Z Friday runs that 94E would develop into a tropical storm or tropical depression over the weekend. The GFS model predicted that this storm would hit the southern portion of the Baja Peninsula on Wednesday, while the other two models predicted that the storm would stay well offshore of Mexico’s Baja Peninsula though the next five days. In their 8 am EDT Friday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 70% and 90%, respectively.


Figure 4. Tropical Storm Megi (center) is gathering strength more than 1000 miles west-southwest of Taiwan (upper left), as shown in this enhanced infrared satellite image collected by Japan’s Himiwari-8 satellite at 1330Z (9:30 am EDT) Friday, September 23, 2016. Image credit: CIMMS/SSEC/University of Wisconsin.

Yet another typhoon threat for Taiwan
Tropical Storm Megi is gaining strength in the Northwest Pacific and will be approaching Taiwan as an intensifying typhoon by early next week. The island has already dealt with the close approaches of Super Typhoon Meranti to its southwest (which killed two residents and left nearly a million without power) and Typhoon Malakas to its northeast, and it’s quite possible that Megi will strike the island head on. As of 15Z Friday (11:00 am EDT), Megi was located about 1100 miles east-southeast of Taipei, Taiwan, moving west-northwest at about 16 mph. Top sustained winds were just 40 mph, but Megi has the potential to become a powerful typhoon. Sea surface temperatures of around 29-30°C (84-86°F) along Megi’s near-term path are at near-record highs--“exceptionally warm,” according to JTWC’s Friday morning update. Wind shear will be very light over the next several days, below 10 knots, and the atmosphere will be fairly moist, with relative humidity of around 70-75% at middle levels of the atmosphere.

Our best track models are in close agreement on an unusually direct path for Megi, continuing west-northwest over the next several days and arriving on or near Taiwan by early Tuesday local time. The 00Z Friday runs of the ECMWF and UKMET models bring a formidable Megi toward south Taiwan, while the 06Z GFS solution is the outlier, aiming a less intense typhoon toward north Taiwan. These are minor differences considering that Megi is still about four days from the island. The Friday morning prediction from JTWC has Megi approaching central Taiwan on Monday night with top sustained winds of at least 105 mph. With a moderate amount of oceanic heat beneath the storm’s path, a period of rapid intensification that would lead to an even stronger Megi can’t be ruled out.

We'll be back with a fresh update by early Saturday afternoon.

Bob Henson and Jeff Masters


Figure 5. WU depiction of Joint Typhoon Warning Center track forecast for Tropical Storm Megi issued on Friday morning, September 23, 2016.

Hurricane

TD Karl Headed Towards Bermuda; Record Rains in Norfolk From Julia's Remnants

By: Jeff Masters and Bob Henson , 3:42 PM GMT on September 22, 2016

A Tropical Storm Watch is up for the island of Bermuda, as Tropical Depression Karl steams northwest towards the island at 17 mph. Karl continues to remain disorganized due to moderate wind shear of 15 knots, but the shear is forecast to fall to the low range, 5 - 10 knots, on Friday. This may allow Karl to strengthen enough to spread tropical storm-force winds to Bermuda before the storm turns to the northeast and moves away from the island on Saturday.

In the eastern Atlantic, there is not much new to say about Tropical Storm Lisa. Lisa is headed to the northwest into an area of high wind shear and dry air, which should be able to destroy this unimpressive 50-mph tropical storm by Sunday.


Figure 1. Latest satellite image of Karl.

Two Atlantic threat areas to watch next week
A medium-sized tropical wave that emerged from the coast of Africa on Thursday morning had a modest amount of heavy thunderstorm activity, but not very much spin. This wave is too close to the equator (near 8°N) to be able to leverage the Earth’s spin and acquire enough spin of its own to develop into a tropical depression, and is not likely to develop through this weekend as it heads west to west-northwest at 15 - 20 mph across the tropical Atlantic. However, the tropical wave may move far enough from the equator to be able to develop by early next week, when it reaches a point about halfway between the Lesser Antilles Islands and the coast of Africa. About 30% of the 20 forecasts from the members of the 00Z Thursday GFS ensemble showed this wave developing between Monday and Wednesday, and about 10% of the 50 members of the European model ensemble did so. This system is likely to move through the Lesser Antilles Islands as early as Wednesday or as late as Friday of next week.

A cold front will move into Texas on Monday, and potentially stall offshore of Texas on Tuesday. A tropical depression could spin up along the tail end of this cold front over the Bay of Campeche, a few hundred miles south-southeast of the Texas/Mexico border, by the middle of next week. However, fewer than 10% of the members of the 00Z Thursday GFS and European model ensemble forecasts showed this occurring.

Invest 94E off the Pacific coast of Mexico may develop
In the Eastern Pacific, satellite loops on Thursday morning showed that an area of low pressure about 850 miles south of the southern tip of the Baja California Peninsula (Invest 94E) was increasing in organization and heavy thunderstorm activity, and this storm has the potential to develop into tropical depression this weekend. Our top three models for predicting hurricane genesis—the GFS, UKMET and European models—predicted in their 00Z Thursday runs that 94E would develop into a tropical storm or tropical depression, but that this storm would stay well offshore of Mexico’s Baja Peninsula though the next five days. In their 8 am EDT Thursday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 70% and 90%, respectively.


Figure 2. Precipitation analyzed for the 7-day period ending at 12Z (8:00 am EDT) Thursday, September 22, 2016. Widespread 10-15” totals have occurred over southeast VA and northeast NC since Monday in association with the remnants of Tropical Storm Julia. Image credit: NOAA/NWS Advanced Hydrologic Prediction Service.

Julia’s remnants feed massive rains across Hampton Roads area
A slow-moving slug of atmospheric moisture associated with former Tropical Storm Julia took up residence this week across the Mid-Atlantic. Together with a lingering front, the moisture has fueled several days of heavy rainfall in southeast Virginia and northeast North Carolina. Preliminary rainfall totals for the period from Sunday through Wednesday night included 13.73” near the Kempsville area of Virginia Beach, VA. By and large, the rains have been prolonged but not extremely intense, producing mainly widespread road closures.

The hardest-hit metropolitan area is Hampton Roads, VA, including Norfolk and Portsmouth. In the 72 hours ending at midnight Wednesday night, Norfolk International Airport picked up 9.35” of rain. That includes three consecutive days of daily-record precipitation: 3.04” on Monday, 2.38” on Tuesday, and 3.93” on Wednesday. As noted by Capital Weather Gang, this is the only instance of three consecutive daily precipitation records in Norfolk data going back to 1874, according to local weathercaster Tim Pandajis. The city’s wettest September--13.80”--occurred in 1979, when Norfolk experienced deluges from Hurricane David as well as a tropical depression later that month. As of this morning, Norfolk’s total for this September was up to around 13”. With showers still in the area today, the September 1979 record is in striking distance.

Jeff Masters and Bob Henson

Hurricane

Karl and Lisa Fail to Impress; Paine’s Moisture Spritzes the U.S. Southwest

By: Bob Henson , 4:49 PM GMT on September 21, 2016

The possibility of two hurricanes--or even one--in the Atlantic this week is diminishing, as the two tropical storms far out at sea are falling short of model projections and forecaster expectations. Meanwhile, two new systems in the Pacific could end up being significant, especially one in the Northwest Pacific that could threaten China and/or Taiwan as a typhoon next week.

Karl now a depression
Downgraded on Wednesday morning after six days as a tropical storm, Tropical Depression Karl is now clinging to life in the central Atlantic, still with some hope of strengthening. Located about 265 miles northeast of the Leeward Islands, Karl’s top sustained winds were just 35 mph as of the 11 am EDT Wednesday advisory from the National Hurricane Center.


Figure 1. Visible satellite image of Tropical Storm Karl as of 1545Z (11:45 am EDT) Wednesday, September 21, 2016.

To say Karl has been unimpressive is putting it mildly. Karl’s peak winds since becoming a tropical storm on Thursday have yet to top 45 mph. Phil Klotzbach pointed out on Tuesday that Karl was tied with Tropical Storm Dennis (1981) for having generated the least amount of accumulated cyclone energy (ACE) in its first five days for any Atlantic storm since satellite monitoring began in 1966. As we noted yesterday, Karl has been plagued by moderately strong wind shear and relatively dry air at middle levels of the atmosphere. Together, these have kept Karl’s showers and thunderstorms shunted well east of its low-level center, thus keeping the storm from gathering strength. For much of Karl’s life, wind shear had been stronger than expected--a frequent occurrence in the Atlantic this season. As NHC forecaster Lixion Avila put it in a discussion this morning: “Global models have totally failed so far in forecasting the upper-level winds surrounding Karl. The upper-low near Karl which unanimously all models have been forecasting to weaken is still strong and producing shear over the cyclone.”

Karl is still expected to recurve toward the northeast around Friday, eventually accelerating toward an upper-level low in the North Atlantic. Our best track models now agree on keeping Karl southeast of Bermuda, most likely well to the southeast, where it may finally have a chance to intensify. Sea surface temperatures at that location are 1°C to 1.5°C above average (1.8-2.7°F), and Karl will have left behind the tenacious upper low that’s hindered its growth. The official NHC forecast brings Karl to hurricane strength for only a day, on Sunday, before it shoots northeast and becomes an extratropical storm. Less than half of the members of the 00Z Wednesday GFS and European model ensemble members bring Karl to hurricane strength, and I won’t be at all surprised if Karl never reaches that threshold.


Figure 2. Infrared satellite image of Tropical Storm Lisa as of 1445Z (10:45 am EDT) Wednesday, September 21, 2016. Image credit: NASA/MSFC Earth Science Office.

Lisa continues rolling through eastern Atlantic
Sprawling yet still fairly disorganized, Tropical Storm Lisa is only slowly gaining strength in the eastern Atlantic, about 580 miles west-northwest of the Cabo Verde Islands as of 11 am EDT Wednesday. Lisa’s top sustained winds are now 50 mph, with showers and thunderstorms scattered widely around its somewhat ill-defined center. Lisa continues on a well-predicted northwest track that will keep it safely away from land areas. Only a tiny fraction of ensemble members from the 00Z Wednesday GFS and European model runs bring Lisa to hurricane strength, and the official NHC forecast has Lisa peaking as a mid-strength tropical storm. Lisa’s location and satellite appearance on Tuesday bore a striking similarity to our last Tropical Storm Lisa, from 2010 (see embedded image at bottom).


Figure 3. Typical areas of tropical development in the Atlantic during October. Image credit: NOAA/NHC.


Elsewhere in the Atlantic: Quiet for now
There are no other areas of immediate concern in the tropical Atlantic. We’re now on the downswing of the Cape Verde season, so we can expect fewer systems to be traversing the region from Africa to the Caribbean. Recent runs of the GFS model have suggested that a low-latitude tropical wave could make this trek next week and strengthen in the Caribbean more than a week from now. As we head toward October, we’ll need to watch the Caribbean more closely, as this becomes a more favored area for development (see Figure 3 above).


Figure 4. Moisture from Post-Tropical Cyclone Paine (located just off the coast of Baja California) continues to funnel into southern California and Arizona, as shown in this visible satellite image from 1615Z (12:15 am EDT) Wednesday, September 21, 2016. Image credit: NASA/MSFC Earth Science Office.

Paine’s moisture heads into Southwest U.S.
With the headline “Paine goes away,” NHC discontinued advisories on former Hurricane Paine on Tuesday night, as the rapidly decaying system continued on its northward path west of Baja California, Mexico. As of 11 am EDT Wednesday, Post-Tropical Cyclone Paine was located near the central coast of Baja California, still bearing 35 mph sustained winds but lacking the showers and thunderstorms (convection) that would qualify it as a tropical depression.

Some of Paine’s moisture is feeding into the Southwest U.S., with additional moisture being shuttled into the area ahead of an upper-level trough. At 12Z (8:00 am EDT) Wednesday, the atmospheric sounding from San Diego, CA, showed 2.12” of precipitable water (the amount of water in a column of air over a particular point). This puts Wednesday morning in a tie for the largest amount of atmospheric moisture recorded in September at San Diego since soundings began in 1948. Welcome rains of 1” - 2” have fallen in parts of Southern California’s high country since Monday. Even San Diego’s Lindbergh Field got in on the action, with 0.31” recorded in the 48 hours through 8:00 am MDT Wednesday. It’s the heaviest rain observed there since May 5-6.


Figure 5. Infrared image of Invest 94E at 1600Z (noon EDT) Wednesday, September 21, 2016. Image credit: NOAA/NESDIS.

Next up in the East Pacific: 94E
The next in the seemingly endless string of systems in the East Pacific is Invest 94E, located a few hundred miles southwest of Acapulco, Mexico. On its expected westward path over the next several days, 94E will benefit from warm SSTs (28-29°C) and a moist atmosphere (mid-level relative humidity of 70-75%), with light to moderate wind shear of 10 - 20 knots. As of Wednesday morning, 94E featured a large envelope of moisture and scattered convection but not much organization yet. Like Paine, 94E is likely to be pulled northward toward Baja California after a few days of westward motion.

Another potential threat to East and Southeast Asia next week
Global models are suggesting that a tropical cyclone expected to form in the next day or two several hundred miles east-southeast of Guam has a good shot at intensifying into a significant typhoon. This system is still a few days away from the Northern Philippines, Taiwan, and the South China coast. Any of these could be eventual targets, according to the 00Z runs of the UKMET, European, and GFS models. Further north, Japan was picking up the pieces after the departure of Typhoon Malakas, which killed 2 people and triggered widespread flooding and landslides across southern Japan.

We’ll be back on Thursday morning with our next update.

Bob Henson



Figure 6. Philippe Papin (University of Albany, State University of New York) says:
“#Lisa (2016) is a mirror image of Lisa (2010) occurring exactly 6 years ago in nearly the exact same location. Incredible. #tropics #dejavu”. Image credit: @pppapin.

Hurricane

August Extends an Exceptional String of Record-Warm Global Months

By: Jeff Masters and Bob Henson , 9:24 PM GMT on September 20, 2016

August 2016 was Earth's warmest August since record keeping began in 1880, said NOAA's National Centers for Environmental Information (NCEI) on Tuesday. In the NOAA database, August 2016 came in 0.92°C (1.66°F) warmer than the 20th-century average for August, beating the previous record for August, set in 2015, by 0.05°C. NASA also reported the warmest August in its database, as well as a tie with July 2016 for the warmest absolute temperature recorded in any month. Because most of the world’s land area is in the Northern Hemisphere, absolute global temperatures are warmest in northern summer--about 3-4°C (5-7°F) higher than in northern winter. This is why monthly global anomalies (departures from the monthly average) are commonly used to assess the relative warmth or coolness of a given month.


Figure 1. The departure from average (compared to temperatures from 1980 - 2015) of Earth’s surface temperature from 1880 to 2016, with the seasonal cycle left in. July and August 2016 were Earth’s hottest months on record in absolute terms, while February 2016 had the largest departure from average (in relative terms) from average of any month in the historical record. Image credit: Gavin Schmidt, director of NASA’s Goddard Institute of Space Studies.


Figure 2. Departure of temperature from average for August 2016, the warmest August for the globe since record keeping began in 1880. Pockets of record warmth were observed across every major ocean basin, including the northwest Atlantic, and over a few land areas. Image credit: National Centers for Environmental Information (NCEI).

A year-plus streak of global records
August 2016 marked the 16th consecutive month that NOAA’s global monthly temperature record was broken, which is the longest such streak since global temperature records began in 1880. Ocean-only temperatures were 0.02°C (0.04°F) cooler than the record warmth of August 2015, while land-only temperatures were a substantial 0.19°C (0.34°F) above the previous land-only record from August 2015. For the lowest 8 km of the atmosphere, global satellite-measured temperatures in August 2016 were the second warmest for any August in the 38-year record, behind only 1998, according to the University of Alabama in Huntsville (UAH).

With the powerful 2015-16 El Niño event now over, the impressive global warmth in recent months can mostly be attributed to the steady build-up of heat-trapping greenhouse gases due to human activities. NOAA’s global surface temperature for the year so far (January-August 2016) is an eye-opening 1.01°C (1.82°F) above the 20th-century average and a remarkable 0.16°C (0.29°F) warmer than the previous January-to-August record, set in 2015 (see Figure 3 below).

Following the 1997-98 “super” El Niño, monthly global temperature records were set through August 1998. The departure of the equally strong 2015-16 El Niño and the possible arrival of La Niña late this year should allow temperatures to drop slightly, perhaps breaking our string of record-warm months sometime in the near future. However, temperatures would have to truly plummet between now and December in order to keep 2016 from becoming the warmest year in global record-keeping. Gavin Schmidt, director of NASA’s Goddard Institute of Space Studies, maintains that we have a better-than-99 percent chance of 2016 ending up as Earth’s third consecutive hottest year on record. Last week, in an essay for fivethirtyeight.com, Schmidt explained how he and his colleagues gained early confidence on that 2016 would be a record-warm year, based largely on the presence of the strong El Niño late last year. “Some key climate statistics are easily predictable far beyond the scales at which weather forecasts are skillful,” Schmidt wrote. “Those predictions clearly suggest an annual global temperature record in 2016 and a (relative) cooling in 2017, all while the long-term upward trends continue.”


Figure 3. Departure from the 20th-century average for the global January-through-August temperature for the years 1880 - 2016. This year has seen by far the warmest temperatures on record for the year-to-date period. Image credit: NOAA/National Centers for Environmental Information (NCEI).

NOAA no longer expects a La Niña event
Sea surface temperatures in the Niño3.4 monitoring region of the eastern Pacific have been hovering near the threshold for a weak La Niña over the last couple of months. However, the atmospheric conditions that normally accompany La Niña have not fully evolved, and models suggest they may continue to lag. For this reason, NOAA has dropped the La Niña Watch that was in place for several months. According to the September ENSO forecast from NOAA’s Climate Prediction Center, neutral conditions are favored to persist through the Northern Hemisphere fall and into the winter (55 - 60% chance), with La Niña given about a 40% chance. This is a marked shift from NOAA’s August forecast, which called for a 55 - 60% chance of a La Niña event. Other agencies around the world are somewhat more bullish on La Niña, as noted by Climate Central’s John Upton last week. Australia’s Bureau of Meteorology retained its La Niña watch in its biweekly update on September 13, “Some climate models indicate a late and weak La Niña is possible,” the update noted. (Australia’s oceanic threshold for La Niña and El Niño is higher than NOAA’s: the Niño3.4 region must be at least 0.8°C warmer or cooler than average, rather than 0.5°C, though Australia doesn’t require those temperatures to persist for months as NOAA does.) The Japan Meteorological Agency has gone further: “It is considered that La Niña conditions are present in the equatorial Pacific,” stated the agency in its monthly update on September 9. The JMA uses the Niño3 region, which overlaps with the Niño3.4 region but extends further east.

Arctic sea ice hits its fourth lowest August extent on record
The rate of August sea ice loss was below average last month, due to cool and stormy conditions in the Arctic. As a result, sea ice extent in August 2016 was just the fourth lowest in the 38-year satellite record, according to the National Snow and Ice Data Center (NSIDC). So far, March, July and August have been the only months in 2016 that did not set a new record low for Arctic-wide sea ice extent (March 2016 was second lowest, July was third lowest). As we reported here last week, the annual minimum in sea ice occurred last week, and was statistically tied for the second lowest extent on record.

Three billion-dollar weather disasters for August 2016: Louisiana floods, China drought, U.S. severe weather
According to the August 2016 Catastrophe Report from insurance broker Aon Benfield, two billion-dollar weather-related disaster hit the planet in August: a disastrous $10 - $15 billion flood in Louisiana, and a drought in China that cost $1.6 billion. Additionally, a severe weather outbreak in the Plains and Rockies on July 28 - 29 accumulated enough damage claims to be rated a billion-dollar disaster by the end of August. Between January - August 2016, there were 24 billion-dollar weather disasters globally--four fewer than occurred during January - August 2013, the year that ended up with the most billion-dollar weather disasters on record: 41. Here is the tally of billion-dollar weather disasters for January - August 2016:

1) Flooding, Yangtze Basin, China, 5/1 - 8/1, $28.0 billion, 475 killed
2) Flooding, Louisiana (U.S.), 8/9 - 8/16, $10 - $15 Billion, 13 killed
3) Flooding, Germany, France, Austria, Poland, 5/26 - 6/6, $5.5 billion, 17 killed
4) Drought, India, 1/1 - 6/30, $5.0 billion, 0 killed
5) Flooding, Northeast China 7/16 - 7/24, $5.0 billion, 289 killed
6) Wildfire, Fort McMurray, Canada, 5/2- 6/1, $5.0 billion, 0 killed
7) Severe Weather, Plains-Southeast U.S., 4/10 - 4/13, $3.75 billion, 1 killed
8) Severe Weather, Rockies-Plains-Southeast-Midwest U.S., 3/22 - 3/25, $2.5 billion, 0 killed
9) Flooding, China, 6/18 - 6/23, $2.3 billion, 68 killed
10) Winter Weather, East Asia, 1/20 - 1/26, $2.0 billion, 116 killed
11) Tropical Cyclone Roanu, Sri Lanka, India, Bangladesh, Myanmar, China, 5/14 - 5/21, $1.7 billion, 135 killed
12) Severe Weather, Plains-Midwest U.S., 4/29 - 5/3, $1.6 billion, 6 killed
13) Drought, China, 1/1 - 3/1, $1.6 billion, 0 killed
14) Drought, Zimbabwe, 6/1 - 8/10, $1.6 billion, 0 killed
15) Severe Weather, Plains-Midwest-Southeast-Northeast U.S., 3/4 - 3/12, $1.5 billion, 6 killed
16) Typhoon Nepartak, Philippines, Taiwan, China, 7/8 - 7/9, $1.5 billion, 111 killed
17) Severe Weather, Plains-Southeast U.S., 3/17 - 3/18, $1.4 billion, 0 killed
18) Flooding, Argentina and Uruguay, 4/4 - 4/10, $1.3 billion, 0 killed
19) Severe Weather, Plains-Midwest-Southeast-Northeast U.S., 2/22 - 2/25, $1.2 billion, 10 killed
20) Severe Weather, Plains-Midwest U.S., 5/21 - 5/28, $1.1 billion, 1 killed
21) Severe Weather, Netherlands, 6/23 - 6/24, $1.1 billion, 0 killed
22) Severe Weather, Plains-Rockies U.S., 7/28 - 7/29, $1.0 billion, 0 killed
23) Tropical Cyclone Winston, Fiji, 2/16 - 2/22, $1.0 billion, 44 killed
24) Winter Weather, Eastern U.S., 1/21 - 1/24, $1.0 billion, 58 killed


And here are the three disasters from August 2016 in more detail:


Disaster 1. Torrential rains of 20 - 30” fell over portions of Louisiana August 9 - 16 from a tropical depression-like storm that meandered over the southern U.S. for a week. Catastrophic flooding killed thirteen people, and damaged as many 110,000 homes and 100,000 vehicles. Damage was estimated at $10 - $15 billion, which will likely make it the second most expensive non-hurricane related flood in U.S. history, behind the $35 billion in damage from the summer 1993 flooding in the Midwest. In this image, we see an aerial view of flooding in Hammond, Louisiana on August 13, 2016. AP Photo/Max Becherer.


Disaster 2. Severe drought began in June across northeastern China in the Inner Mongolia, Jilin and Heilongjiang provinces, and intensified during August. The Ministry of Civil Affairs (MCA) reported well-above normal temperatures and reduced rainfall that damaged more than 3.1 million hectares (7.6 million acres), with total economic losses at $1.6 billion. In this image, we see drought conditions in China as of September 1, 2016. Image credit: Beijing Climate Center.


Disaster 3. Severe thunderstorms swept across parts of the Rockies and Plains on July 28 - 29, causing $1 billion in damage. Hardest hit was Colorado, where golf-ball-and-larger-sized hail struck the Colorado Springs metro area. Hail accumulations up to one feet (0.3 meters) fell in some areas, and torrential rains led to flash flooding. Heavy losses were also reported in Wyoming, Kansas, Nebraska, and South Dakota. In this image, we see an intense thunderstorm building over Boulder, Colorado on July 29, 2016. Image credit: wunderphotographer austncitylimits.

Below-average monsoon rains cause deadly flooding in India
India, whose $5 billion drought has been Earth's fourth most expensive weather-related natural disaster of 2016, is getting a better monsoon after two straight years of poor rains, but the moisture delivered to date is still below average. According to the India Meteorological Department, monsoon rains during the period June 1 - September 19, 2016 were about 5% below average. Even a below-average Indian monsoon can still wreak havoc through flooding. Through the end of August, monsoon floods had killed at least 510 people in India and caused at least $150 million in damage, with the Ganges River reaching the highest levels ever recorded at four locations in northern India. Worst affected were the states of Rajasthan, Maharashtra, Assam, Bihar, Uttar Pradesh, West Bengal, and Jharkhand.

Notable global heat and cold marks set in August 2016
Hottest temperature in the Northern Hemisphere: 52.7°C (126.9°F) at Mitribah, Kuwait, 2 August
Coldest temperature in the Northern Hemisphere: -32.4°C (-26.3°F) at Geo Summit, Greenland, 15 August
Hottest temperature in the Southern Hemisphere: 41.7°C (107.1°F) at Palmas, Brazil, 18 August
Coldest temperature in the Southern Hemisphere: -78.1°C (-108.6°F) at Vostok, Antarctica, 31 August
(Courtesy of Maximiliano Herrera.)

Major weather stations that set (not tied) new all-time heat or cold records in August 2016 (Courtesy of Maximiliano Herrera)
Erdeni (Mongolia) max. 40.5°C, 1 August
Bayandelger (Mongolia) max. 39.4°C, 2 August
Choibalsan (Mongolia) max. 41.9°C, 3 August
Khalkh Gol (Mongolia) max. 41.7°C, 3 August
Dashbalbar (Mongolia) max. 41.4°C, 3 August
Matad (Mongolia) max.  40.7°C,  3 August
Xin Barag Youqi (China) max. 44.1°C, 3 August
Hailar (China) max. 41.7°C, 3 August
Uliastai (China) max. 40.6°C, 3 August; increased to 42.5 on 4 August
Arxan (China) max. 37.6°C, 3 August; increased to 39.1 on 4 August
Kajlastuj (Russia) max. 41.6°C, 3 August
Dubai Airport (United Arab Emirates) max. 48.9°C, 3 August
Xilin Hot (China) max. 39.8°C, 4 August
Hyesan (North Korea) max. 39.7°C, 5 August
Samjiyon (North Korea) max. 32.2°C, 5 August
Vigo (Spain) max. 40.8°C, 7 August
Braga (Portugal) max. 42.2°C, 7 August
Porto City (Portugal) max. 40.9°C,  7 August
Porto Airport (Portugal) max. 38.6°C, 7 August
Mora (Portugal) max. 44.8°C,   7 August
Kyowa (Japan) max. 33.6°C, 7 August
Mishima (Japan) max. 37.2°C, 8 August; increased to 37.4°C on 9 August
Waki (Japan) max. 37.9°C, 8 August
Kiriishi  (Japan) max. 39.2°C, 9 August
Nanbu  (Japan) max. 38.9°C, 9 August
Gotemba  (Japan) max. 35.3°C, 9 August
Kikukawa Makinohara  (Japan) max.  37.0°C, 9 August
Angra do Heroismo (Azores, Portugal) max. 29.3°C, 9 August
Omuta (Japan) max. 37.5°C, 11 August
Imari (Japan) max. 36.9°C, 11 August
Aso Otohime  (Japan) max. 34.9°C, 11 August
Aso (Japan) max. 29.8°C, 11 August
Haenam (South Korea) max. 37.1°C, 11 August
Chizu (Japan) max. 37.0°C, 12 August
Ureshino (Japan) max. 38.5°C, 12 August
Izuhara (Japan) max. 36.8°C, 13 August
Gyeongju (South Korea) max. 39.4°C, 12 August
Youngcheon (South Korea) max. 39.6°C, 13 August
Yeongdeok (South Korea) max. 38.6°C, 13 August
Pohang (South Korea) max. 39.3°C, 13 August
Busan (South Korea) max. 37.3°C, 14 August
Hinatuan (Philippines) max. 37.2°C, 19 August
Owen Int. Airport (Cayman Islands, United Kingdom) max. 34.9°C, 21 August *
Ikuchishima (Japan) max. 36.3°C, 21 August
Aki (Japan) max. 36.5°C, 21 August
Sendai (Japan) max. 37.2°C, 21 August
Kiinagashima (Japan) max. 37.9°C, 22 August  
Shingu (Japan) max. 38.4°C, 22 August
Kagoshima (Japan) max. 37.4°C, 22 August
Kiire (Japan) max. 37.4°C, 22 August
Kimotsuki Maeda (Japan) max. 36.7°C, 22 August  
Makurazaki (Japan) max. 36.7°C, 22 August
Koniya (Japan) max. 34.4°C, 23 August
Fengjie (China) max. 42.2°C, 24 August
Utirik Atoll (Marshall Islands) max. 35.6°C, 24 August **
Neijiang (China) max. 40.1°C, 25 August
North Lakimpur (India) max. 39.9°C, 25 August
Cape Arkona (Germany) max. 32.2°C, 26 August


Notes from Maximiliano Herrera:
* ties the territorial record for the highest temperature ever recorded in the Cayman Islands
** breaks the territorial record for the highest temperature ever recorded in the Marshall Islands

Two all-time national heat records set or tied in August 2016
Two nations or territories--the Cayman Islands and the Marshall Islands--set or tied records in August 2016 for their all-time hottest temperature on record. Update (25 September]: From January through September 10, 2016, a total of 21 nations or territories tied or set all-time records for their hottest temperature in recorded history, including the British Virgin Islands record just added below. This breaks the record of eighteen all-time heat records set in 2010 for the greatest number of such records set in one year. Also, one all-time cold temperature record has been set so far in 2016 (in Hong Kong.) "All-time" record here refers to the warmest or coldest temperature ever reliably reported in a nation or territory. The period of record varies from country to country and station to station, but it is typically a few decades to a century or more. Most nations do not maintain official databases of extreme temperature records, so the national temperature records reported here are in many cases not official. Our data source is international weather records researcher Maximiliano Herrera, one of the world's top climatologists, who maintains a comprehensive list of extreme temperature records for every nation in the world on his website. If you reproduce this list of extremes, please cite Maximiliano Herrera as the primary source of the weather records. Here are 2016's all-time heat and cold records as of September  10:

French Guiana tied its all-time hottest record on September 10, 2016, when the mercury hit 37.9°C (100.2°F) at Saint Laurent do Moroni.

The Marshall Islands set its all-time hottest record on August 24, 2016, when the mercury hit 35.6°C (96.1°F) at Utirik Atoll.

The Cayman Islands (United Kingdom territory) tied its all-time hottest record on August 21, 2016, when the mercury hit 34.9°C (94.8°F) at Owen International Airport.

The British Virgin Islands [United Kingdom territory] set its all-time hottest record on July 22, 2016, when the mercury hit 35.0°C (95.0°F] at Terrance B. Lettsome International Airport.

Iraq set its all-time hottest record on July 22, 2016, when the mercury hit 53.9°C (129.0°F) at Basrah.

Iran tied its all-time hottest record on July 22, 2016, when the mercury hit 53.0°C (127.4°F) at Delhoran.

Kuwait set its all-time hottest record on July 21, 2016, when the mercury hit 54.0°C (129.2°F) at Mitribah.

Guernsey (United Kingdom territory) tied its all-time hottest record on July 19, 2016, when the mercury hit 35.0°C (95°F) at the small island of Alderney.

Hong Kong Territory (China) tied its all-time hottest record on July 9, 2016, when the mercury hit 37.9°C (100.2°F) at Happy Valley.

Niger set its all-time hottest record on June 8, 2016, when the mercury hit 49.0°C (120.2°F) at Bilma.

Palau tied its all-time hottest record on June 8, 2016, when the mercury hit 34.4°C (93.9°F) at Koror AWS.

India set its all-time hottest record on May 19, 2016, when the mercury hit 51.0°C (123.8°F) at Phalodi.

Maldives set its all-time hottest record on April 30, 2016, when the mercury hit 35.0°C (95.0°F) at Hanimaadhoo.

Thailand set its all-time hottest record on April 28, 2016, when the mercury hit 44.6°C (112.3°F) at Mae Hong Son.

Cambodia set its all-time hottest record on April 15, 2016, when the mercury hit 42.6°C (108.7°F) at Preah Vihea.

Burkina Faso set its all-time hottest record on April 13, 2016, when the mercury hit 47.5°C (117.5°F) at Dori.

Laos set its all-time hottest record on April 12, 2016, when the mercury hit 42.3°C (108.1°F) at Seno.

Vanuatu in the South Pacific set its all-time hottest record on February 8, 2016, when the mercury hit 36.2°C (97.2°F) at Lamap Malekula.

Tonga set its all-time hottest record on February 1, 2016, when the mercury hit 35.5°C (95.9°F) at Niuafoou.

Wallis and Futuna Territory (France) set a new territorial heat record with 35.8°C (96.4°F) on January 10, 2016 at Futuna Airport. This is the second year in a row that Wallis and Futuna has beaten its all-time heat mark; the previous record was a 35.5°C (95.9°F) reading on January 19, 2015 at the Futuna Airport.

Botswana set its all-time hottest record on January 7, 2016, when the mercury hit 43.8°C (110.8°F) at Maun.

Hong Kong Territory (China) set its all-time coldest mark on January 24, 2016, when the mercury dipped to -6.0°C (21.2°F) at Tai Mo Shan.

We'll be back on Thursday with the latest on tropical activity in the Atlantic and Pacific (see also our update from Wednesday morning].

Jeff Masters and Bob Henson

Climate Summaries

Karl and Lisa Spinning Across Atlantic; Malakas Leaves Trail of Damage in Japan

By: Bob Henson , 4:42 PM GMT on September 20, 2016

Tropical activity was ramping down in one ocean basin and ramping up in another on Tuesday morning. There were two decaying tropical storms in the Pacific--one soon to leave Japan, the other heading for Mexico’s Baja California coast--while in the Atlantic, we have two tropical storms expected to gather steam, including one just christened. The National Hurricane Center upgraded a depression in the eastern Atlantic to Tropical Storm Lisa in its 11 am EDT Tuesday advisory. Lisa is the Atlantic’s 12th tropical storm of the year, which puts us ahead of the typical number of named storms recorded in an entire Atlantic season (11.3 per year for the period 1966-2009). Though they’ve been numerous, this year’s Atlantic storms haven’t been especially strong--the seasonal accumulated cyclone energy is only about 70-75% of average for the date--or particularly long-lived. The average duration of a named storm in the Atlantic is 5 days, according to Phil Klotzbach (CSU), and Karl is only the second of this year’s 11 named storms to last even that long.


Figure 1. Infrared satellite image of Tropical Storm Lisa as of 1445Z (10:45 am EDT) Tuesday, September 20, 2016. Image credit: NOAA/NESDIS.

Outlook for Lisa
Located about 430 miles west of the Cabo Verde Islands, Lisa should be a large but fairly innocuous storm. On its northwestward path, Lisa will be rolling over modestly warm waters for development (sea surface temperatures of 27-28°C, or 81-82°F). Light wind shear today and Wednesday should allow Lisa to gather some strength. However, wind shear will be increasing sharply from Thursday onward, as shown in 12Z Tuesday output from the SHIPS statistical model. It appears unlikely Lisa will make it to hurricane strength before its surroundings turn hostile. In any event, it should continue on a northwest bearing that will allow it to recurve long before it approaches North America.


Figure 2. Latest satellite image of Tropical Storm Karl.

Karl still expected to gain strength, could threaten Bermuda
Tropical Storm Karl continues to struggle in the central Atlantic, with top sustained winds remaining a minimal 40 mph as of the 11 am EDT Tuesday advisory from NHC. Wind shear associated with a weak upper low west of Karl has been relentless in pushing shower and thunderstorm activity to the northeast, leaving Karl’s low-level center largely exposed for most of the last couple of days. Such pockets of wind shear--which can be hard to fully detect across the open Atlantic, and tough for models to portray accurately—have been quite prevalent across the Atlantic this season. It’s been one factor helping to keep storms such as eventual Hurricane Hermine from developing as rapidly as expected.

Models agree there will be enough of a weakness in the subtropical ridge so that Karl’s current westward motion will angle toward the northwest on a gradually recurving track that stays well north of the Caribbean. On this course, Karl should encounter somewhat lighter wind shear (around 10 knots), a gradually moister atmosphere (relative humidity at middle levels of the atmosphere rising from 50-55% to around 60% by late Wednesday), and progressively warmer sea surface temperatures climbing to 29-30°C (84-86°F). These ingredients will give Karl a fighting chance at gaining hurricane strength by week’s end, as predicted by NHC. Computer models are converging on a sharp turn to the northeast that would occur just in time to keep Karl from Bermuda, but it’s too soon to rule out possible impacts there. If this year’s tendency holds, Karl could achieve its maximum strength at a fairly high latitude around 30-35°N--another reason for Bermuda to pay close attention to Karl.


Figure 3. WU depiction of official NHC forecast track for Tropical Storm Karl as of 1500Z (11:00 am EDT) Tuesday, September 20, 2016.The official track takes Karl just southeast of Bermuda, but the island remains within the “cone of uncertainty.”


Figure 4. Vehicles drive through a flooded street as Typhoon Malakas moves across the city of Tokushima, located on the east end of Japan’s Shikoku Island, on Tuesday, September 20, 2016. Image credit: Jiji Press/AFP/Getty Images.

Malakas sweeps across Japan’s south coast
At least two people are missing and more than 30 were injured by the arrival of Typhoon Malakas in Japan. Malakas slammed ashore in Kyushu, the westernmost of Japan’s main islands, before churning northeast along the south coast of Shikoku and Honshu islands. Widespread torrential rain across southern Japan triggered landslides, with some wind damage reported as well. Winds gusted to 90-105 mph across much of Kyushu: Tosu City reported a gust to 43 meters per second (96 mph) just before midnight Monday night. On the east side of Kyushu, Nobeoka reported 446 mm (17.56”) of rain in 24 hours, including 407 mm (16.02”) in just 12 hours.

As of 1500Z Tuesday (midnight Tuesday night local time, or 11:00 am EDT), Malakas was located about 70 miles west of Tokyo. Now a tropical storm, Malakas is east-northeast at about 28 mph with top sustained winds down to 45 mph. It should be out to sea as a tropical depression by midday Wednesday local time. Malakas is the sixth typhoon to make landfall in Japan this year, according to JMA. That puts 2016 in a tie with 1990 and 1993 for the second-highest number of landfalling typhoons in Japan in records going back to 1951. All three years are well behind the record of 10 landfalls set in 2004.


Figure 5. Tropical Storm Paine as of 1545Z (9:45 am EDT) Tuesday, September 20, 2016. Image credit: NASA/MSFC Earth Science Office.

Paine relief: Tropical storm is decaying fast
Before long, we’ll have no more Paine to deal with (and hopefully no more Paine-ful puns). Tropical Storm Paine is weakening quickly as it moves over progressively cooler waters amid high wind shear. Paine’s top sustained winds were down to 45 mph as of the 11 am EDT Tuesday advisory, and the storm was rapidly becoming disorganized. Paine is now expected to make landfall along the northern coast of Mexico’s Baja California on Wednesday afternoon as a tropical depression. The rich moisture associated with Paine could drop several inches of rain on northern Baja California. Some of this moisture is already being entrained into a large-scale storm system moving into the western U.S., which may enhance showers and thunderstorms from the Desert Southwest on Wednesday into the central Rockies on Thursday. The first accumulating snows of the season could fall above 8000 feet across northwest Colorado and northeast Utah.

Jeff Masters and I will be back later this afternoon with a summary of global climate for August and for meteorological summer.

Bob Henson

Hurricane

Lackadaisical TS Karl May Strengthen; Paine in the Pacific; Malakas Hits Japan

By: Bob Henson , 4:43 PM GMT on September 19, 2016

The Northern Hemisphere tropics remain quite active during this final week of astronomical summer: we have a new hurricane in the East Pacific heading toward Mexico, plus a major typhoon in the Northwest Pacific plowing into western Japan. Meanwhile, hurricane watchers in the U.S. and Caribbean are casting their eyes toward Tropical Storm Karl, which has the potential to become a significant hurricane--though likely one that’ll remain a safe distance from land.

Now in its third day as a tropical storm, Karl is in no hurry to strengthen. Karl’s top sustained winds were estimated at a minimal 40 mph in the 11 am EDT advisory from the National Hurricane Center. Located in the central tropical Atlantic, about 900 miles east of the Leeward Islands, Karl continues to track almost due west at 13 mph. As was the case through the weekend, Karl’s low-level circulation was largely exposed on Monday morning, with most showers and thunderstorms (convection) swept to the northeast of the center. Vertical wind shear is fairly modest, at 10-15 knots, but that shear combined with a quite-dry middle atmosphere (relative humidity of 45-50%) has been enough to keep convection from gathering around Karl’s center.


Figure 1. Visible satellite image of Tropical Storm Karl as of 1515Z (11:15 am EDT) Monday, September 19, 2016. Image credit: NASA/MSFC Earth Science Office.

Outlook for Karl
Karl is likely to start flexing its muscles later this week. Sea surface temperatures along Karl’s path will be rising from around 28°C (82°F) on Monday to 29-30°C (84-86°F) by midweek, and the mid-level humidity will rise to around 60-65%, according to output from the 12Z Monday run of the SHIPS statistical model. Wind shear will drop to below 10 knots, giving Karl another big boost. The official NHC outlook brings Karl to hurricane strength by Friday. Models suggest that the bulk of this intensification may not occur until late in the week, despite the favorable conditions soon to evolve.

The upper-level pattern across North America and the North Atlantic will become more amplified late this week, with strong upper-level lows in the western U.S. and north-central Atlantic and ridges in the central U.S. and eastern Atlantic. Karl’s path should begin gradually angling west-northwest this week, enough to avoid the Caribbean, and models suggest that the North Atlantic low will extend far enough south late this week to pull Karl sharply northeastward by the time the storm gets to around 65°W to 70°W longitude. Less than 10% of the 50 European ensemble runs from 00Z Monday bring Karl far enough west to affect the U.S. All 20 GFS ensemble members from 00Z Monday keep Karl well to the east of the States, as does the 00Z Monday run of the UKMET model. These track results have been quite consistent over the last couple of days, which lends added support to the idea that Karl is unlikely to pose a U.S. threat.

Karl could easily affect Bermuda, depending on how sharply it recurves. If this year’s tendency holds, Karl could achieve its maximum strength at a fairly high latitude, around 30-35°N, another reason for Bermuda to pay close attention to Karl.


Figure 2. Tracks from the 20 members of the GFS model ensemble run (GEFS) from 06Z Monday, September 19, 2016, updated with data from 12Z Monday. Ensemble runs are produced by introducing small variations in the starting-point conditions to mimic uncertainty about the state of the atmosphere.

Adios, Julia
After five days, the National Hurricane Center has pulled the plug on irritating Tropical Storm Julia. NHC discontinued advisories on Julia on Sunday night, classifying the storm as a remnant low. Julia’s biggest impact may be posthumous: some of its moisture is now being pulled into a weak upper-level low and associated front over the Mid-Atlantic coast. Rainfall amounts of 2” - 4” will be widespread from eastern North Carolina to southeast Massachusetts, with higher localized amounts, especially toward the NC Outer Banks.


Figure 3. Enhanced infrared image of Invest 96L as of 1445Z (10:45 am EDT) Monday, September 19, 2016. Image credit: NOAA/NESDIS.

Strong tropical wave may develop in eastern Atlantic
The next solid chance at a named storm in the Atlantic is Invest 96L, located a couple hundred miles southwest of the Cabo Verde Islands. This wave has much more convection than Karl, and data from the ASCAT scatterometer suggests that a weak closed circulation has already formed. 96L’s thunderstorms are being nourished by a moist middle atmosphere (relative humidity around 65%). Wind shear will be fairly light (5 - 15 knots) for the next couple of days, and SSTs of 28-29°C are more than adequate for development. In its 8 am EDT Monday tropical weather outlook, NHC gives 96L a 70% chance of becoming a tropical depression by Wednesday and an 80% chance by Saturday. It’s quite possible that 96L will be Tropical Storm Lisa by midweek, if not sooner, but hurricane status may not be in the cards. The SHIPS model shows wind shear ramping into the 25 - 35 mph range by late this week as 96L heads northwest over progressively cooler waters. GFS and Euro ensembles indicate that the most likely outcome is for 96L to peak at tropical storm strength. We can expect 96L to remain far from any threat to land.


Figure 4. Infrared image from Japan’s Himiwari-8 satellite of Typhoon Malakas as of 1520Z (11:20 am EDT) Monday, September 19, 2016. Image credit: NOAA/CIRA/RAMMB/Colorado State University.

Major Typhoon Malakas heads into Japan
Significant impacts are likely as Typhoon Malakas grinds its way northeastward along the south coast of Japan’s main islands. Malakas passed through Japan’s far-flung Yaeyama Islands as a Category 4 typhoon over the weekend: the eye passed directly over the westernmost island, Yonaguni, where the Yonagunijima airport clocked wind gusts up to 100 mph. As of 12Z (8:00 am EDT) Monday, the Joint Typhoon Warning Center rated Malakas as a minimal Category 3 storm, with top sustained (1-minute) winds of 115 mph. Malakas was slamming into the island of Kyushu early Tuesday local time. At 12:05 am JST Tuesday (11:05 am EDT Monday), Kanoya Air Field reported a wind gust to 106 mph, with surface air pressure down to 973 millibars.

Malakas will weaken dramatically as it continues northeast along the coast. By the time it passes near Tokyo early Wednesday, Malakas should be down to weak tropical storm strength. However, torrential rains of 5” - 10” and resultant landslides will be a major concern across large sections of southern Japan, including much of Honshu. According to weather.com, the city of Takanabe reported 4.33” of rain in just one hour on Monday night local time.


Figure 5. Radar image of Typhoon Malakas, centered on the south end of Japan’s Kyushu island as of 0115 JST Tuesday, September 20, 2016 (12:15 pm EDT Monday). Rainfall rates at lower right are shown in millimeters per hour: 80 mm = 3.15”. Image credit: Japan Meteorological Administration.


Figure 6. Visible satellite image of Hurricane Paine as of 1559Z (11:59 am EDT) Monday, September 19, 2016.

Tropical Storm Watch for Baja California as Hurricane Paine spins up
The 11th hurricane of the year in the East Pacific is a Paine--literally. Now located about 350 miles west of Cabo San Lucas, Mexico, Hurricane Paine has intensified rapidly, with top sustained winds rocketing from just 40 mph at 11 AM EDT Sunday to 85 mph as of the 11 am EDT Monday advisory from NHC. Paine is likely at its peak strength, as the hurricane will soon be moving northwest over cooler waters and encountering greater wind shear. By the time Paine heads into the northern coast of Baja California on Wednesday, it should be little more than a weak tropical storm or depression. Since Paine’s recurving path could bring tropical storm-force winds east of its center onto land, the Mexican government has issued a Tropical Storm Watch from Punta Eugenia to Cabo San Quintin.

Some moisture from Paine (and a nearby upper low) will filter into the Southwest U.S. by midweek, although not as much as was delivered by Tropical Storm Newton several weeks ago. A few showers have already filtered into far Southern California, where any rainfall is welcome in the midst of a five-year-long drought.

For the years 1971-2009, the average number of hurricanes per year in the East Pacific was 8. Given that we’re now on #11 for 2016, this is a busy East Pacific season indeed! The record number of hurricanes in a single East Pacific season is 16, set in 1990, 1992, and 2014.

Bob Henson


Figure 8. Projected track for Hurricane Paine from the National Hurricane Center as of 15Z (11:00 am EDT) Monday, September 19, 2016.

Hurricane

Little Change to Julia and Karl in the Atlantic

By: Jeff Masters , 3:28 PM GMT on September 18, 2016

Like some annoying insect that keeps buzzing around and won’t go away despite repeated attempts to swat it, Tropical Depression Julia continues to spin away a few hundred miles off the coast of South Carolina. Satellite images on Sunday morning showed that Julia continued to struggle with high wind shear, with the low-level center of the storm completely exposed to view and the heaviest thunderstorms several hundred miles from the center. No offshore buoys were reporting sustained winds over 15 mph on Sunday morning.


Figure 1. Latest satellite image of Julia.

Forecast for Julia
Julia is embedded in an atmosphere with very dry air (50% relative humidity at mid-levels of the atmosphere) and wind shear is expected to stay high, 20 - 30 knots, through Sunday evening. These conditions should cause the storm to continue to struggle as it meanders off the coast of South Carolina in an atmosphere with weak steering currents. Wind shear is expected to weaken to the moderate range, 10 - 20 knots on Monday, which may give Julia a window of opportunity to intensify into a weak tropical storm once more. An Air Force hurricane hunter aircraft is on call to investigate Julia on Monday afternoon, if necessary. Wind shear is expected to increase again to the high range, 20 - 30 knots, by Monday night, which would likely stop any further attempts at intensification.


Figure 2. Latest satellite image of Karl.

Tropical Storm Karl continues west with little change
Tropical Storm Karl was headed west at 12 mph in the central tropical Atlantic late Sunday morning, and does not pose a threat to any land areas for at least the next five days. Satellite images on Sunday morning showed much the same picture as the previous three days: Karl had a large circulation and plenty of low-level spiral bands, but the center was nearly completely exposed to view due to moderate wind shear of 15 knots, with the storm’s heavy thunderstorms limited to the northeast side of the center. Karl had marginal conditions for development, with relative humidity at mid-levels of the atmosphere 45 - 50% and sea surface temperatures (SSTs) near 27.5°C (82°F). By Monday, SSTs will increase to 28°C and wind shear is expected to drop to the low range—less than 10 knots. These conditions favor strengthening. However, the atmosphere surrounding Karl will be quite dry through Tuesday, which should slow development.

The track forecast for Karl for the next five days is fairly straightforward, with the steering currents expected to take the storm west, then west-northwest. Karl should clear the northern Lesser Antilles Islands by several hundred miles at the time of its closest approach on Wednesday, and wind up at a point midway between the Bahamas and Bermuda on Friday. At that point, Karl is likely to be a hurricane, and will start generating large swells which will begin impacting the U.S. East Coast and the Bahamas early next week. The big question is—will Karl make landfall on the U.S. East Coast early next week? Hurricane forecasts more than five days in advance are quite unreliable, but residents along the U.S. East Coast can take heart from the fact that a strong trough of low pressure is predicted to pass to the north of Karl early next week. This would result in a recurving path for Karl to the north and then northeast, with the storm missing the U.S. East Coast. This was the prediction from the 12Z Saturday and 00Z Sunday runs of our two top models for predicting hurricane tracks, the GFS and European models. Moreover, more than 90% of the 50 members of the 00Z Sunday run of the European ensemble forecast and all 20 members of the 00Z Sunday GFS ensemble forecast predicted that Karl would miss the U.S. Given these forecasts, it currently appears that Bermuda and the Maritime Provinces of Canada are the only land areas at significant risk from a direct hit from Karl next week. Still, long range forecasts like this are unreliable, and we will have to watch the evolution of the forecast of the upper air pattern in the coming days to see if a landfall along the U.S. East Coast early next week might be in the cards.

96L off the coast of Africa may develop
A large tropical wave with plenty of spin and heavy thunderstorm activity (Invest 96L) was located between Africa and the Cabo Verde Islands on Sunday morning. The tropical wave has favorable conditions for development through Tuesday as it heads west-northwest at 10 - 15 mph through the Cabo Verde Islands. The latest 00Z Sunday runs of our three top models for forecasting tropical cyclone genesis--the GFS, European and UKMET models—all showed development of the system over the next three days. 96L is headed into a region of ocean where very few storms ever threaten any land areas. In their 8 am EDT Sunday Tropical Weather Outlook, NHC kept their 2-day and 5-day development odds at 40% and 70%, respectively.

Jeff Masters

Hurricane

Julia Clinging to Life; Karl Continues West With Little Change

By: Jeff Masters , 3:06 PM GMT on September 17, 2016

Tropical Depression Julia continues to cling to life as it spins a few hundred miles off the coast of South Carolina. Satellite images on Saturday morning showed that Julia continued to struggle with high wind shear, with the low-level center of the storm completely exposed to view, and the heaviest thunderstorms all to the east of the center. The top winds observed at any offshore buoys on Saturday morning were 19 mph, gusting to 23 mph, at buoy 41002, 225 nm south of Cape Hatteras, North Carolina, at 7:50 am EDT.


Figure 1. Latest satellite image of Julia.

Forecast for Julia
Julia is embedded in an atmosphere with very dry air (45 - 50% relative humidity at mid-levels of the atmosphere) and wind shear is expected to stay high, 20 - 30 knots, through Sunday evening. These conditions should cause the storm to gradually weaken as it meanders off the coast of South Carolina in an atmosphere with weak steering currents. Two of our top models, the GFS and European, predict that high wind shear and dry air should combine to allow Julia to spin down into a remnant low by Monday. However, the UKMET model disagrees, predicting that Julia will survive the hostile conditions this weekend, and re-intensify into a strong tropical storm early in the week when the wind shear finally relents. The UKMET model keeps Julia wandering off the coast of South Carolina/North Carolina until at least Thursday. At this point, I don't see any reason to disagree with the official NHC forecast of dissipation of Julia by Sunday.


Figure 2. Latest satellite image of Karl.

Tropical Storm Karl continues west with little change
Tropical Storm Karl was headed west at 13 mph in the central tropical Atlantic late Saturday morning, and does not pose a threat to any land areas for at least the next five days. Satellite images on Saturday morning showed much the same picture as on Thursday and Friday: Karl was well-organized, with a large circulation and plenty of low-level spiral bands, but the center was nearly completely exposed to view due to moderate wind shear of 15 - 20 knots, with the storm’s heavy thunderstorms limited to the northeast side of the center. Karl has marginal conditions for development, with relative humidity at mid-levels of the atmosphere near 50% and sea surface temperatures (SSTs) near 27°C (81°F). By Sunday, SSTs will increase to 28°C and wind shear is expected to drop to the low range—less than 10 knots. These conditions favor strengthening. However, the atmosphere surrounding Karl will be quite dry through Tuesday, which should slow development.

The track forecast for Karl for the next five days is fairly straightforward, with the steering currents expected to take the storm slightly south of due west, then west-northwest. Karl should clear the northern Lesser Antilles Islands by several hundred miles at the time of its closest approach on Wednesday. As usual, the picture gets pretty murky more than five days into the future. The most probable track painted by the models for Karl a week or more into the future is for the storm to get caught in the steering flow of a strong trough of low pressure passing to its north late in the week. This would result in a recurving path for Karl to the north and then northeast, with the storm missing the U.S. East Coast. This is the prediction from about 90% of the 50 members of the 00Z Saturday run of the European ensemble forecast, and all 20 members of the 00Z Saturday GFS ensemble forecast. In this situation, Bermuda and the Maritime Provinces of Canada might still be at risk from a direct hit, though. The other possibility is that the trough passing to the north of Karl late in the week will not be able to capture the storm, and a ridge of high pressure will build in over Karl, forcing it the west or northwest, potentially bringing Karl to a landfall along the U.S. East Coast about nine days from now.

96L off the coast of Africa may develop
A large tropical wave with plenty of spin and heavy thunderstorm activity was located between Africa and the Cabo Verde Islands on Saturday morning. This system was designated Invest 96L by NHC on Saturday morning. The tropical wave has favorable conditions for development through Monday as it heads west-northwest at 10 - 15 mph through the Cabo Verde Islands. The latest 00Z Saturday runs of our three top models for forecasting tropical cyclone genesis--the GFS, European and UKMET models—all showed development of the system over the next three days. 96L is headed into a region of ocean where very few storms ever threaten any land areas. In their 8 am EDT Saturday Tropical Weather Outlook, NHC increased their 2-day and 5-day development odds to 40% and 70%, respectively.

Invest 93E off the Pacific coast of Mexico may develop
In the Eastern Pacific, satellite loops show that an area of low pressure about 400 miles south of Manzanillo, Mexico (Invest 93E) is close to tropical depression status. Our top three models for predicting hurricane genesis, the GFS, UKMET and European models, predicted in their 00Z Saturday runs that 93E would develop into a tropical storm or tropical depression, but that this storm would stay well offshore of Mexico’s Baja Peninsula, and not make landfall. In their 8 am EDT Saturday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 80% and 90%, respectively.


Figure 3. Radar image of Typhoon Malakas as it brushed Taiwan at 8:40 am local time on September 17, 2016. Image credit: Taiwan Central Weather Bureau.

Category 3 Malakas brushes Taiwan en route to Japan
Just as Super Typhoon Meranti narrowly avoided a direct landfall on the southern tip of Taiwan, Typhoon Malakas did the same on the island’s northeast corner on Saturday morning. Malakas peaked as a Category 4 storm with 135 mph winds at 18 UTC Friday, and its brush with Taiwan disrupted the storm’s inner core, reducing the typhoon to a low-end Category 3 storm with 115 mph winds by 12 UTC Saturday. Malakas brought up to 283 mm (11.14”) of rain to Taiwan in a 48-hour period. The typhoon will brush Japan’s Yaeyama Islands, then turn to the northeast toward Japan’s main island, Honshu. Malakas could arrive as a Category 1 typhoon, merging with the remnants of Meranti and bringing torrential rain that could easily top 10” in some locations.

There is still no word from the Philippines island of Itbayat, four days after it received a direct hit from Super Typhoon Meranti at Category 5 strength with 185 mph winds.

Jeff Masters

Hurricane

Degraded and Disheveled, Arctic Sea Ice Ties for Second-Lowest Extent on Record

By: Bob Henson , 9:46 PM GMT on September 16, 2016

The National Snow and Ice Data Center (NSIDC) announced on Thursday that the extent of sea ice in the Arctic Ocean has apparently hit its summer minimum. The value of 4.14 million square kilometers (1.60 million square miles) reached on September 10 comes in just behind the record-low extent of 2012 and in a statistical tie with 2007 for second place (see Figure 1). Although refreezing has begun, it’s possible that a late burst of melting could lead to an even lower value in the next few days. This year’s minimum came in below the 4.38 million sq km predicted in August by the median of 39 outlooks submitted by participants in the Sea Ice Prediction Network.


Figure 1. Each year’s minimum in Arctic sea ice extent from 1979, when satellite measurement began, through 2016 (assuming that the September 10 minimum holds). Units are millions of square kilometers. Image credit: Zack Labe, @Zlabe.

Plenty of ice loss without Old Sol’s help
This summer’s polar weather didn’t fit the classic template for major ice loss, which makes the near-record depletion all the more striking and concerning. In 2007, a record-smashing minimum was achieved through weeks of round-the-clock sunshine, together with the Arctic Dipole pattern--an atmospheric setup that creates winds that compact sea ice and shove it to lower latitudes through the Fram Strait between Greenland and Norway. This year got off to a phenomenal head start, as winter temperatures north of the Arctic Circle were far higher than anything on record. Then the weather turned largely cloudy during the crucial period from late June into August, staving off what might otherwise have been a minimum even lower than 2012’s. Still, as NSIDC noted, “the upper ocean was quite warm this summer and ocean-driven melting is important during late summer.” Several weeks of intense storminess in August may have helped to churn up warmer water across the western Arctic, fostering melt from below.

“A large portion of the anomalously low ice can be attributed to the unusual winter/spring,” said Zack Labe, a Ph.D. student analyzing sea ice at the University of California, Irvine. “If we’d had a classic Arctic Dipole pattern this summer, I have no doubt we would have quickly approached or surpassed 2012.”


Figure 2. Comparison of sea ice concentration on September 5 of the years 2012 through 2016 (upper left to lower center), as derived from AMSR2 satellite data (Japan Aerospace Exploration Agency). Lighter areas denote a higher concentration of sea ice. Sea ice extent refers to the amount of ocean with data cells that are at least 15 percent ice-covered. Image credit: JAXA, courtesy A-Team/Arctic Sea Ice Forum.


Figure 3.  Large amounts of open water can be seen in ice extending to the vicinity of the North Pole in this view from NASA’s Terra satellite collected on September 8, 2016. Image credit: NASA Worldview, courtesy slow wing, Arctic Sea Ice Blog.

”Rubble” near the North Pole
Sea ice extent (the amount of ocean with at least 15% ice coverage) is just one way to gauge Arctic sea ice--albeit a useful and important one--and the minimum summer extent is only one data point in that record. Just as you need to know more than the number of named storms to assess the fury of an Atlantic hurricane season, it’s helpful to look at the Arctic sea ice in three dimensions and across the calendar to gauge its true health.

In that broader view, Arctic sea ice is in terrible shape. The amount of multiyear ice--especially sections that survive for five or more years, serving as a bulwark against year-to-year ice loss--has dropped precipitously in the last 10 years. Huge swaths of ice that “survived” the summer of 2016 in terms of extent were actually riddled with gaping cracks and gaps. Figure 2 above shows the pockmarked state of this year’s early-September ice as compared with the previous four years. The eastern Arctic (top part of images) was especially ravaged, as shown more closely in Figure 3 above. Ice described by some observers as “rubble” extended to the vicinity of the North Pole, and a large expanse of open water could be seen behind the Swedish icebreaker Oden as it was moored to an ice floe within two miles of the North Pole on August 28. Areas of open water do occur at times near the North Pole, but the vast expanse of compromised high-latitude ice this year is stunning.


Figure 4. The yacht Northabout cuts through ice-studded waters. This year the Northabout is completing a circumnavigation of the Arctic Ocean as a way to increase attention to the effects of climate change on the region. Image credit: Polar Ocean Challenge.

A disturbingly successful trip around the Arctic
Both the Northeast and Northwest Passages opened up yet again this summer, a dual milestone that was first recorded in 2008. As part of a project called Polar Ocean Challenge, a 50-foot yacht called Northabout has become the first ship known to sail through both passages in a single summer. The yacht is now heading around the south side of Greenland en route to its home base in England. It took the Northabout a mere 14 days to complete the Northwest Passage, where the crew encountered ice only two times. “Whilst we are all delighted to have succeeded, it is extremely worrying to see this lack of ice so starkly,” said expedition leader David Templeman-Adams.

At the Arctic Sea Ice Blog, Neven Acropolis reflected on this odd summer: “I always thought that it would take extreme weather conditions as seen in 2007 for a melting season to end really low: Lots of open skies, warm winds and continuous compaction, just weeks and weeks of the same kind of weather. But given that there's no let-up in the amount of heat flowing into the Arctic--via air and especially ocean--other set-ups can be just as destructive. It will probably be a back-and-forth of high pressure (open skies) and low pressure (dispersal, mixing) that will lead to new records, and eventually an ice-free Arctic…Whatever it is we’re doing to stop this from getting worse in decades to come, we need to do it faster.”

For an update on tropical happenings in the Atlantic and Pacific, see my post from this morning with Jeff Masters. We’ll be back with our next update by Saturday afternoon.

Bob Henson

Arctic Sea Ice

Karl Joins Julia in the Atlantic; Another Major Typhoon to Clip Taiwan

By: Jeff Masters and Bob Henson , 4:00 PM GMT on September 16, 2016

It was a busy early Friday morning for tropical cyclones in the Atlantic, with three storms active at the same time--Ian, Julia and Karl. Late Friday morning, we were down to two named storms, though, thanks to the demise of Tropical Storm Ian in the central Atlantic. Of our two remaining storms, the closer one to land was Tropical Storm Julia, which continued to spin a few hundred miles off the coast of South Carolina. Satellite images on Friday morning showed that Julia continued to struggle with high wind shear, with the low-level center of the storm exposed to view, and the heaviest thunderstorms all to the east of the center. Radar loops from Wilmington, North Carolina showed that Julia’s rains were well offshore. The top winds observed at any offshore buoys on Friday morning were 29 mph, gusting to 36 mph, at buoy 41002, 225 nm south of Cape Hatteras, North Carolina, at 7:50 am EDT. Storm surge levels along the Southeast U.S. coast at 10:30 am EDT Friday were 1.0’ or less.


Figure 1. Latest satellite image of Julia.

Forecast for Julia
Julia is embedded in an atmosphere with very dry air (45 - 50% relative humidity at mid-levels of the atmosphere) and wind shear is expected to stay high, 20 - 35 knots, through Sunday. These conditions should cause the storm to gradually weaken as it meanders off the coast of South Carolina in an atmosphere with weak steering currents. Two of our top models, the GFS and European, predict that high wind shear and dry air should combine to bring about Julia’s demise by Sunday. However, the UKMET model disagrees, predicting that Julia will survive the hostile conditions this weekend, and re-intensify into a strong tropical storm early next week when the wind shear finally relents. The UKMET model keeps Julia wandering off the coast of South Carolina/North Carolina until at least Thursday.


Figure 2. MODIS image of Karl on Friday morning, September 16, 2016. Image credit: NASA.

Tropical Storm Karl continues west with little change
Tropical Storm Karl formed Thursday evening in the waters of the eastern tropical Atlantic, and was headed west at 13 mph towards the Lesser Antilles Islands late Friday morning. Steering currents will likely take Karl far enough to the west-northwest by early next week to keep the storm well clear of the Lesser Antilles Islands, but a potential long-range threat to North America or Bermuda cannot be ruled out at this time. Satellite images on Friday morning showed that Karl was well-organized, with a large circulation and plenty of low-level spiral bands, but the center was nearly completely exposed to view due to high wind shear of 20 - 25 knots, with the storm’s heavy thunderstorms limited to the northeast side of the center. Karl has marginal conditions for development, with relative humidity at mid-levels of the atmosphere near 60% and sea surface temperatures (SSTs) near 27°C (81°F). By Sunday, SSTs will increase to 28°C and wind shear is expected to drop to the low range--less than 10 knots. These conditions favor strengthening. However, the atmosphere surrounding Karl will get drier this weekend, which should slow development.

The track forecast for Karl for the next five days is fairly straightforward, with the steering currents expected to take the storm west, then slightly south of due west, then west-northwest. Karl should clear the northern Lesser Antilles Islands by several hundred miles at the time of its closest approach on Wednesday. As usual, the picture gets pretty murky more than five days into the future. The most probable track painted by the models for Karl a week or more into the future is for the storm to get caught in the steering flow of a strong trough of low pressure passing to its north late next week. This would result in a recurving path for Karl to the north and then northeast, with the storm missing the U.S. East Coast. This is the prediction from about 80% of the 50 members of the 00Z Friday run of the European ensemble forecast, and all 20 members of the 00Z Friday GFS ensemble forecast. In this situation, Bermuda and the Maritime Provinces of Canada might still be at risk from a direct hit, though. The other possibility is that the trough passing to the north of Karl late next week will not be able to capture the storm, and a ridge of high pressure will build in over Karl, forcing it the west or northwest, potentially bringing Karl to a landfall along the U.S. East Coast about ten days from now.

Another African tropical wave may develop early next week
The 00Z Friday runs of our three reliable models for predicting tropical cyclone genesis--the GFS, European, and UKMET models--had one of them, the UKMET model, predict that a new tropical wave expected to come off the coast of Africa on Friday night to develop into a tropical depression early next week. This storm is expected to take a track more to the west-northwest than Karl, and does not appear to be a long-range threat to the Lesser Antilles Islands or North America. In their 8 am EDT Friday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 20% and 50%, respectively. By the middle of next week, this storm will encounter a region more hostile for development, with drier air, higher wind shear and cooler SSTs.

Tropical Storm Ian becomes post-tropical
Tropical Storm Ian made the transition to a powerful extratropical storm with 65 mph winds in the central Atlantic on Friday morning. Ex-Ian will pass close to Iceland on Saturday, potentially bringing sustained winds of 40 - 45 mph to the coast.


Figure 3. Enhanced infrared image from Japan’s Himiwari-8 satellite of Typhoon Malakas at 1430Z (10:30 am EDT) Friday, September 16, 2016.

Category 3 Malakas will brush Taiwan en route to Japan
Just as Super Typhoon Meranti narrowly avoided a direct landfall on the southern tip of Taiwan, it appears that Typhoon Malakas may do the same on the island’s northeast corner, though it will be a very close call. As of 12Z (8:00 am EDT) Friday, the Joint Typhoon Warning Center placed Malakas about 300 miles southeast of Taipei, Taiwan, moving northwest at about 14 mph. Malakas has strengthened to Category 3 strength, with top winds now 125 mph and a pinhole eye evident on satellite imagery. JTWC predicts that Malakas will reach Category 4 strength by Saturday local time.

Malakas is in the midst of a classic tropical cyclone recurvature, so its path will gradually arc rightward, hopefully just in time for its center to miss Taiwan on Saturday. The GFS, European, and UKMET track models all bring Malakas within about 50-75 miles of Taiwan’s northeast corner, perhaps close enough to bring heavy rain and tropical-storm-force winds to the Taipei area. The typhoon’s stronger east side would likely remain offshore, although it could still impact Japan’s Yaeyama Islands. After it recurves, Malakas will be sweeping to the northeast toward Japan’s main island, Honshu. The 00Z Friday runs of the GFS and European models take Malakas directly over Honshu on Tuesday, as depicted in the JTWC outlook below, while the UKMET model solution is slower and a bit further north. Malakas could arrive as a Category 1 typhoon, merging with the remnants of Meranti and bringing torrential rain that could easily top 10” in some locations (see Figure 5 below).


Figure 4. WU depiction of track forecast for Typhoon Malakas from the Joint Typhoon Warning Center as of 8:00 am EDT Friday, September 16.


Figure 5. Accumulated precipitation across eastern Asia from the 06Z Friday run of the GFS model shows the combined effects of Typhoon Malakas and the remnants of Super Typhoon Meranti. Rainfall is shown in millimeters (100 mm = 3.94”). Image credit: tropicaltidbits.com.

East Pacific: So long, Orlene
After surging to hurricane intensity for a second time on Thursday, Tropical Storm Orlene is now on its last legs. Orlene has been enveloped by very dry air (only 20-25% relative humidity at midlevels), which has more than counteracted any potential boost from light wind shear and marginally warm sea-surface temperatures. By Friday morning, Orlene’s tiny core of showers and thunderstorms had almost completely dissipated. As it drifts westward, Orlene will likely be a remnant low by Sunday, perhaps even sooner.

Invest 93E may threaten the Baja Peninsula next week
A new tropical system, Invest 93E, is taking shape a few hundred miles southwest of Acapulco. Thunderstorms remain disorganized within a broad area of low pressure, but 93E will be nourished by a moist atmosphere (RH of 70-80%) and warm SSTs (28-29°C), with only light to moderate wind shear to contend with (10-20 knots). In its tropical outlook issued at 8:00 am EDT Friday, NHC gives 93E a 70% chance of developing into at least a tropical depression by Sunday and a 90% chance by Wednesday. The 00Z and 06Z Friday runs of the GFS model fail to develop 93E, but the European and UKMET models produce a moderately strong tropical storm that parallels the Mexican coast for several days, then potentially arcs toward Baja California and the Southwest U.S. early next week.

We’ll be back with a new post this afternoon on the 2016 Arctic sea ice minimum just reached this week.

Jeff Masters and Bob Henson


Hurricane

From Meranti to Malakas: Another Typhoon Threatens East Asia

By: Bob Henson , 10:18 PM GMT on September 15, 2016

Even as the remains of Super Typhoon Meranti continue to spin down in the rugged mountains of eastern China, there’s another storm on the horizon. Typhoon Malakas, located about 600 miles southeast of Taipei, Taiwan, is gaining strength and could take a swipe at Taiwan on Saturday local time. In its update at 18Z (2:00 pm EDT) Thursday, the Joint Typhoon Warning Center estimated that Malakas had peak winds of 105 mph and was moving northwest at about 16 mph. A tiny eye has been appearing intermittently on satellite imagery in the midst of Malakas’s large circulation.


Figure 1. Enhanced infrared image from Japan’s Himiwari-8 satellite of Typhoon Malakas as of 1900Z (3:00 pm EDT) Thursday, September 15, 2016. Image credit: NOAA/NESDIS.


Figure 2. Track forecast for Malakas as of 1800Z (2:00 pm EDT) Thursday, September 15, 2016.

Malakas is on a classic recurvature track, swinging around a strong upper-level ridge to its northeast. The GFS, European, and UKMET models--our most reliable models for track prediction--all bring Malakas within about 100-150 miles of Taiwan’s northeast corner this weekend. It wouldn’t take much of a westward swing in Malakas’s track to put Taiwan at risk. JTWC is projecting that Malakas will be a high-end Category 3 storm at that point, with top sustained winds of 125 mph, and it could be stronger. JTWC notes that Malakas could go through a period of rapid intensification, as it will be drawing on a moist atmosphere, warm sea-surface temperatures of 29-30°C, and light to moderate wind shear (10-20 knots).If it misses Taiwan, Malakas will be on course to rake some of Japan’s Yaeyama Islands, located 50 to 150 miles east of Taiwan. Early next week, Malakas will likely sweep northeast as a weakening typhoon or tropical storm along or near the spine of Japan’s main island, Honshu.

For such a powerful storm, Meranti’s toll is light
Taiwan and China are breathing sighs of relief after their encounter with formidable Super Typhoon Meranti. Peaking at 190 mph, Meranti’s satellite-estimated sustained winds put the storm in a tie for the tenth strongest winds in global records for any tropical cyclone, including hurricanes and typhoons.

Fortunately, Meranti’s path stayed just off the south coast of Taiwan, and the typhoon had weakened to Category 2 strength with 105 mph sustained winds by the time it made landfall on the China coast in the city of Xiamen. Nevertheless, some 300,000 of Xiamen’s 3.5 million residents were evacuated, and the others went through a wild night. The Xiamen airport recorded wind gusts to 87 mph just ahead of Meranti’s center at 3 am Thursday, along with a one-hour drop in barometric pressure of 19 millibars (from 983 to 964 millibars). Meranti is the strongest typhoon to strike China’s Fujian province in records going back to 1949, according to the nation’s Xinhua news service. The historic Dongguan Bridge, built in 1145, was destroyed by the flood. Rainfall of more than 400 millimeters (15.75”) was reported in Zhejiang province as Meranti slogged inland. According to the insurance broker Aon Benfield, China’s Ministry of Civil Affairs (MCA) reported that Meranti had left at least seven people killed or missing and dozens more injured, with many windows shattered in mid- and high-rise buildings and many trees downed into homes, businesses and vehicles. At least 1.65 million people lost power in Fujian province alone. Several dozen injuries were reported in Taiwan, where more than 600,000 homes lost power. There has been no word yet on the fate of the 3000 residents of Itbayat, the tiny Philippine island just south of Taiwan that was encompassed by Meranti’s eye.

Millions of people were waylaid by cancelled or postponed flight and train schedules as Meranti arrived during East Asia’s Mid-Autumn Festival, a traditional multi-day event commemorating the fall harvest and centered on the full moon that falls between mid-September and mid-October. A two-story-high inflatable moon created as part of the festival went rogue in Meranti’s winds in Fuzhou. The moon caused no major damage but prompted a stir on social media (see embedded video at bottom).


Figure 3. A man walks past a destroyed building in Xiamen in China's eastern Fujian province after Typhoon Meranti made landfall on September 15, 2016.


Figure 4. An aerial view of a flooded road in the city of Fuzhou, about 100 miles north of Typhoon Meranti’s landfall, on September 15, 2016. Image credit: VCG, via Getty Images.

New study: Landfalling typhoons have become more intense since late 1970s
Rapidly intensifying typhoons (of which Meranti is a textbook example) are a growing threat to East and Southeast Asia, according to a study published this month in Nature Geoscience. The study found that the peak winds of typhoons striking the region have increased by 12 - 15% since 1977. This trend appears to be mainly the result of faster intensification rates, as the typhoons are passing over increasingly warmer waters en route to landfall.

Wei Mei and Shang-Ping Xie (Scripps Institution of Oceanography) analyzed the tracks and strengths of nearly 600 typhoons that occurred from 1977 (when the Japan Meteorological Agency began cataloguing typhoon wind speed) through 2013. The authors drew on both JMA and JTWC “best track” data sets, which are the highest-quality typhoon records from each agency. For consistency with JTWC, the JMA wind reports were converted from 10-minute to 1-minute averages. Although a particular storm’s evolution can vary from one dataset to the other, the overall findings of the study were consistent across both datasets. Because of the general increase in peak typhoon strength, the average number of Category 4 and 5 typhoons per year in the Northwest Pacific has jumped from less than 5 to more than 7.

About half of all typhoons in the Northwest Pacific make landfall somewhere, and this is where the trends found in the study are most pronounced. The authors broke the 37-year typhoon record into four clusters, each of which corresponds to a particular region of typhoon development and track. The two clusters that make up most of the landfalling typhoons showed intensification rates that grew by more than 60% over the study period, with an extra 3 knots now being added every six hours during intensification periods. Atmospheric dynamics (including vertical wind shear) don’t show any consistent trend in the study area, but sea surface temperatures have risen, especially just east of Asia. In turn, that has boosted the maximum potential intensity of typhoons approaching the coast of East and Southeast Asia.


Figure 5. Most of the North Pacific Ocean is running warmer than usual right now, as depicted by these departures from average sea surface temperature for September 15, 2016 (in degrees C). Image credit: NOAA Office of Satellite and Product Operations.

Increasing SSTs are a hallmark of our warming planet, and though Mei and Xie did not attempt to directly link the trends they found to human-produced climate change, they do note that the findings on potential intensity are consistent with output from climate models (CMIP5) used in the most recent IPCC report. These models show that SSTs will be warming more quickly this century across the subtropics than over the deep tropics. “The projected ocean surface warming pattern under increasing greenhouse gas forcing suggests that typhoons striking eastern mainland China, Taiwan, Korea and Japan will intensify further,” wrote the authors. “Given disproportionate damages by intense typhoons, this represents a heightened threat to people and properties in the region.”

A previous study found little trend in the strength and frequency of landfalling typhoons from 1950 to 2010. However, the planet’s atmosphere and oceans did not begin their decade-by-decade warming streak until the 1980s. In a global study examining the period 1975-2010, Greg Holland and Cindy Bruyére (National Center for Atmospheric Research, or NCAR) found that more tropical cyclones were reaching Category 4 and 5 intensity over time, with a declining share at the Category 1 and 2 levels. In an email, Holland added: “While not emphasized in our paper, we did find that globally the increasing proportions could be contributed both to increasing intensification rates (in agreement with Mie and Xie) and to longer periods spent over warm oceans, and that both seemed to be equally important.”

Addressing the new study by Mei and Xie, NCAR’s James Done said in an email: “It's a very nice paper, and the results are generally in line with how we think the intensity distribution would change under warming.” The NCAR group has produced a Cyclone Damage Potential index that combines forward speed and storm size with peak winds to gauge a tropical cyclone’s overall ability to wreak havoc. This index has shown a linear increase over the West Pacific for landfalling storms over the last decade, according to a quick analysis by NCAR’s Ming Ge (see Figure 6 below).

Jeff Masters posted earlier today on activity in the Atlantic; he’ll be back on Friday morning with our next update.

Bob Henson


Figure 6. Annual Cyclone Damage Potential (CDP) from 2000 to 2014 for typhoons across the Northwest Pacific. CDP incorporates the size and speed of motion of tropical cyclones as well as their peak sustained winds. Shown are each year's maximum CDP value for all typhoons, the average CDP for all landfalling typhoons (black), and the average CDP for all typhoons, whether or not they make landfall (blue). A longer time period would be needed to see if the apparent increase in CDP for landfalling typhoons is statistically significant. Image credit: Ming Ge and James Done, NCAR.



Video 1. Dan Lindsey (CSU/CIRA) says: “This one is too cool not to share - it's CIRA's Geocolor product from #Himawari showing #Meranti on 13-14 Sep.” Image credit: @DanLindsey77.



Video 2. A model moon rampages through the streets of the Chinese city of Fuzhou after being loosed by Typhoon Meranti. Image credit: Courtesy @shanghaiist.


Hurricane Climate Change

Julia More Annoyance Than Threat for U.S.; Keep an Eye on TD 12 in Eastern Atlantic

By: Jeff Masters , 1:56 PM GMT on September 15, 2016

After the surprise emergence of Tropical Storm Julia on Tuesday evening while the center was located over land in northeastern Florida, the storm appears determined to stick around through the weekend and annoy coastal South Carolina and North Carolina with days of intermittent rain showers. Satellite images on Thursday morning showed the classic appearance of a storm struggling with high wind shear, with the low-level center of Julia, now a tropical depression, exposed to view, and the heaviest thunderstorms all to the east of the center. Radar loops from Wilmington, North Carolina showed Julia’s heaviest rains were mostly well offshore, with only one modest band affecting coastal North Carolina. The top winds observed at any offshore buoys on Thursday morning were 27 mph, gusting to 36 mph, at buoy 41004, 47 miles southeast of Charleston, South Carolina, at 7:50 am EDT. Storm surge levels along the Southeast U.S. coast at 9:30 am EDT Thursday were 0.7’ or less.


Figure 1. Latest satellite image of Julia.


Figure 2. Latest radar-estimated rainfall accumulation image for Julia.

Forecast for Julia
Julia is more annoyance than threat, with the storm likely to dump only another 1 - 2” of rain along the immediate coast over the next two days. Wind shear is expected to stay high, 20 - 30 knots, through Sunday, which should prevent any significant intensification as the storm meanders off the coast of South Carolina in an atmosphere with weak steering currents. Our top two models, the GFS and European model, predict that high wind shear and dry air should combine to bring about Julia’s demise by Sunday.


Figure 3. Latest satellite image of TD 12.

Tropical Depression 12 continues west with little change
Tropical Depression Twelve brought heavy rain showers and gusty winds to the Cabo Verde Islands on Wednesday, and is now pulling away from the islands as it heads west at 16 mph. Satellite images on Thursday morning showed that TD 12 was well-organized, with a large circulation and plenty of low-level spiral bands, but the center was completed exposed to view due to high wind shear of 20 - 25 knots, with most of the storm’s heavy thunderstorms far from the center. TD 12 is embedded in a moist atmosphere and has warm SSTs near 27 - 27.5°C (81°F) under it, and may be able to develop into Tropical Storm Karl this weekend, when the shear is expected to drop to the moderate range, 10 - 20 knots. However, the atmosphere surrounding TD 12 will get drier this weekend, which should interfere with development—or possibly dissipate the storm by five days from now, as predicted by the operational GFS model and 17 of its 20 ensemble member forecasts. While most of the 50 members of the European model ensemble show TD 12 eventually recurving out to sea without affecting any land areas, 6 out of 50 of the forecasts show the storm hitting the U.S. East Coast 10+ days from now, so it is too early to assume that TD 12 will be a “fish” storm.

Another African tropical wave may develop next week
The 0Z Thursday runs of our three reliable models for predicting tropical cyclone genesis—the GFS, European, and UKMET models—agreed that a new tropical wave expected to come off the coast of Africa on Friday will develop into a tropical depression early next week. This storm is expected to take a track more to the west-northwest than TD 12, and is less likely to be a long-range threat to the Lesser Antilles Islands or North America. In their 8 am EDT Thursday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 0% and 50%, respectively.

Tropical Storm Ian in the central Atlantic: not long to live
Tropical Storm Ian was accelerating to the northeast in the central Atlantic late Thursday morning, and doesn’t have long to live. On Friday, Ian will become entangled with a cold front and an upper level low pressure system, and will transition to an extratropical storm without ever reaching hurricane status. Ian is not a threat to any land areas.

Invest 93E off the Pacific coast of Mexico may threaten the Baja Peninsula next week
In the Eastern Pacific, an area of low pressure a few hundred miles southwest of Acapulco, Mexico (Invest 93E) has grown more organized, and is a threat to develop into a tropical depression this weekend. Two of our top three models for predicting hurricane genesis, the UKMET and European models, predicted in their 0Z Thursday runs that 93E would develop into a tropical storm which would hit Mexico’s Baja Peninsula on Monday or Tuesday. In their 8 am EDT Thursday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 30% and 80%, respectively.

Storm with subtropical characteristics affecting France and Spain
An extratropical storm (called “Stephanie” by the Free University of Berlin) is in the Bay of Biscay off the west coast of France, and has acquired some subtropical characteristics as it lingers over waters near 23°C (73°F). For more info on this storm, see comment 312 by WU member barbamz in the previous blog post. This storm is expected to move inland near the France/Spain border by Friday morning, spreading heavy rains and gusty winds.

Bob Henson will be back with a new post this afternoon on the typhoon activity in the Western Pacific, along with a a detailed look at a new study finding that landfalling typhoons have increased in intensity in recent decades.

Jeff Masters

Hurricane

Meranti Approaches China Coast After Sweeping Past Taiwan

By: Bob Henson and Jeff Masters , 6:50 PM GMT on September 14, 2016

One of the strongest tropical cyclones in modern weather history, Typhoon Meranti, was approaching the southeast China coast late Wednesday night local time (Wednesday afternoon EDT). As of 16Z (noon EDT), Meranti was located by the Japan Meteorological Agency near 23.8°N, 118.6°E, or about 50 miles southeast of the coastal city of Xiamen in China’s Fujian province. On its steady northwest track at about 12 mph, Meranti should make landfall near or just south of Xiamen at around 4 pm EDT.

Meranti tracked just south of Taiwan’s mountainous south end over the last 24 hours, which disrupted its circulation and put a huge dent in its top sustained winds. You can see the weakening of the typhoon as it passed by Taiwan on this long radar loop from Taiwan compiled by Brian McNoldy. At noon EDT Wednesday, Meranti’s 10-minute average winds were down to 105 mph, according to JMA. The 1-minute average wind used by U.S. agencies is typically around 14% greater than the 10-minute average, so the JMA value would correspond roughly to a 115-mph low-end Category 3 hurricane. Meranti could weaken a bit further before landfall.


Figure 1. This enhanced infrared satellite image shows Typhoon Meranti nearing the coast of China at 1700Z (1:00 pm EDT) Wednesday, September 14, 2016. Image credit: NOAA/NESDIS.

In spite of its decline, Meranti retains the potential to produce a major storm surge along the China coast. It takes some time for the huge amounts of water being pushed by a major hurricane or typhoon to subside, even after the storm itself weakens. Two good examples are Hurricane Katrina and Hurricane/Superstorm Sandy, both of which produced catastrophic storm surge despite significant weakening of their top winds in the hours before landfall. Meranti’s surge would be worst just to the north of its center, and this part of the storm is aimed at the city of Xiamen (population 3.5 million), one of the world’s 20 busiest ports for container shipping. Because China’s coast is so densely populated, storm surge can threaten huge numbers of people. In 1994, Typhoon Fred struck the province of Zhejiang after weakening from a Category 4 to a Category 1 storm. Fred still managed to produce a top surge of just under 9 feet that affected more than 22 million people, as noted by storm surge expert Hal Needham.

Torrential rains are another serious threat from Meranti across east central China. Widespread 3” - 6” rains can be expected near the typhoon’s track as it moves inland, with localized 10” - 15” amounts quite possible. The heavy rain may extend further north later this week, toward Shanghai, as Meranti’s remnants are swept northward. In 1983, Typhoon Wayne followed a very similar track to Meranti, passing just south of Taiwan and weakening from a Category 4 to Category 1 before landfall several dozen miles south of Xiamen. Coastal and inland flooding linked to Wayne in Fujian and Guangdong provinces led to 105 deaths and the collapse of some 30,000 structures.


Figure 2. A local resident removes a rock from a blocked road in southern Pingtung county as Typhoon Meranti slashes southern Taiwan on September 14, 2016. Image credit: Sam YehAFP/Getty Images.


Figure 3. Power lines downed by Meranti partially block the road in southern Taiwan’s Pingtung county on September 14, 2016. Image credit: Sam YehAFP/Getty Images.

Meranti’s impacts on Taiwan and the Philippines
Meranti’s path just south of Taiwan kept the typhoon’s fiercest winds and waves focused on the sparsely populated southeast flank of the island, which greatly reduced the impact on people and property. The South China Morning Post reported that at least one person was missing, 35 were injured, and power and water supply was knocked out to more than 600,000 homes, according to Taiwan’s Central Emergency Operation Centre. The largest city affected was Kaohsiung, on Taiwan’s southwest coast, which received a glancing but still powerful blow as Meranti swept past to the south. Kaohsiung International Airport reported top sustained winds of 71 mph at 12:30 pm local time Wednesday, with a gust to 112 mph shortly thereafter. A number of rainfall amounts topping 500 mm (about 20”) were reported in far southern Taiwan. Xidawushan, in Taiwu Township (in the mountains about 25 miles east of Kaohsiung) reported a total of 799.5 mm (31.48”) from midnight to midnight local time Wednesday, September 14. Of that total, 685 mm (26.97”) fell in just 12 hours. (Thanks to Michael Theusner and Jérôme Reynaud for these rainfall statistics.)

One of the most poignant of many stunning satellites images of Meranti was the view in Figure 4 (below), with Meranti’s eye completely enveloping the tiny Philippines island of Itbayat and its 3,000 residents. As of midday Wednesday, there was no word of conditions on the island. Channel NewsAsia reported that the Philippines government would be sending a civil defense team to Itbayat at a time yet to be determined.


Figure 4. At 17:32 UTC Tuesday (1:32 pm EDT Tuesday or 1:32 am local time Wednesday), the eye of Meranti lay directly over the Philippines’ Itbayat Island, as seen in this visible-light image from Japan’s Himiwari-8 satellite, taken in the bright moonlight that evening. Itbayat recorded sustained winds of 112 mph (10-minute average) and a pressure of 934 mb at 1 am local time, 32 minutes prior to this image. At the time, Meranti was a Category 5 storm with 185 mph winds and a central pressure of 890 mb. Thanks go to WU member Barefootontherocks for posting this image in the blog comments.

History of major typhoon landfalls in China
China has a long history of major typhoon landfalls. Since record keeping in the Northwest Pacific began in 1945, there have been two Category 5 storms to make landfall in China, twelve Category 4 storms, and eighteen Category 3 storms. We constructed the list below using data from the Joint Typhoon Warning Center for the Northwest Pacific, as plotted on NOAA’s Historical Hurricane Tracks website. These storms were all rated Category 3 or stronger in their last 6-hour position before they made landfall in China:

Landfalls South of Hong Kong

Category 5:
160 mph, Super Typhoon Rammasun, July 18, 2014

Category 4:
140 mph, Typhoon Hagupit, September 24, 2008
140 mph, Typhoon Ruby, September 5, 1964
140 mph, Typhoon Kate, September 25, 1955
130 mph, Typhoon Vicente, July 23, 2012
130 mph, Typhoon Betty, October 31, 1953

Category 3:
125 mph, Typhoon Sally, September 9, 1996
125 mph, Typhoon Susan, September 18, 1953
125 mph, Typhoon Rose, August 16, 1971
115 mph, Typhoon Pamela, November 8, 1972
115 mph, Typhoon Ophelia, August 14, 1953
115 mph, Typhoon Winnie, July 1, 1964
115 mph, Typhoon Freda, July 15, 1965

Landfalls North of Hong Kong

Category 5:
160 mph, Super Typhoon Cora, September 5, 1966

Category 4:
150 mph, Super Typhoon Saomai, August 10, 2006
150 mph, Super Typhoon Alice, September 3, 1966
150 mph, Super Typhoon Wanda, August 1, 1956
145 mph, Typhoon Rita, September 1, 1953
140 mph, Typhoon Bilis, August 23, 2000
140 mph, Typhoon Nina, August 16, 1953
130 mph, Typhoon Grace, September 4, 1958

Category 3:
125 mph, Typhoon Joan, August 30, 1959
120 mph, Typhoon Khanun, September 11, 2005
120 mph, Typhoon Wipha, September 19, 2007
120 mph, Typhoon Hope, August 2, 1979
120 mph, Typhoon Amy, July 18, 1991
120 mph, Typhoon Dujuan, September 2, 2003
115 mph, Typhoon Usagi, September 22, 2013
115 mph, Typhoon Viola, July 28, 1969
115 mph, Typhoon Abe, September 14, 1993
115 mph, Typhoon Harriet, July 29, 1952
115 mph, Typhoon Tim, July 10, 1994

Meranti tied for tenth strongest tropical cyclone in history (by wind)
Meranti peaked at 2 pm EDT Tuesday, September 13, with sustained winds of 190 mph. This makes it tied for tenth strongest tropical cyclone in world history (by 1-minute averaged wind speed), according to the “best-track” data from the Joint Typhoon Warning Center and the National Hurricane Center. In addition, Meranti's 890 mb central pressure at the time made it the seventeenth strongest tropical cyclone on record, by pressure.

Officially, here are their top-ten strongest tropical cyclones in world history, by maximum sustained winds:

1) Hurricane Patricia (2015), 215 mph winds (the only Eastern Pacific storm on this list)
2) Super Typhoon Nancy (1961), 215 mph winds, 882 mb. Made landfall as a Cat 2 in Japan, killing 191 people.
3) Super Typhoon Violet (1961), 205 mph winds, 886 mb pressure. Made landfall in Japan as a tropical storm, killing 2 people.
4) Super Typhoon Ida (1958), 200 mph winds, 877 mb pressure. Made landfall as a Cat 1 in Japan, killing 1269 people.
5) Super Typhoon Haiyan (2013), 195 mph winds, 895 mb pressure. Made landfall in the Philippines with 190 mph winds.
5) Super Typhoon Kit (1966), 195 mph winds, 880 mb. Did not make landfall.
5) Super Typhoon Sally (1964), 195 mph winds, 895 mb. Made landfall as a Cat 4 in the Philippines.
5) Super Typhoon Opal (1964), 195 mph winds
5) Super Typhoon Joan (1959), 195 mph winds
10) Super Typhoon Meranti, 190 mph winds, 890 mb pressure.
10) Super Typhoon Tip (1979), 190 mph winds
10) Super Typhoon Vera (1959), 190 mph winds
10) Super Typhoon Louise/Marge (1964), 190 mph winds
10) Hurricane Allen (1980), 190 mph winds (the only Atlantic storm on this list)

However, it is now recognized (Black 1992) that the maximum sustained winds estimated for typhoons during the 1940s to 1960s were too strong. Dr. Hugh Willoughby, former head of NOAA's Hurricane Research Division, had this to say about the winds measured in Super Typhoon Nancy and the other high-end typhoons from this list from the 1960s:

"I would not take the winds seriously because reconnaissance meteorologists estimated them visually. A decade later when I flew with the VW-1 hurricane hunters, we had the same Doppler system used to measure the winds of Typhoon Nancy. It tracked the aircraft motion relative to the (possibly moving) sea surface. It couldn't get a coherent signal in high winds because the beam reflected from both the actual surface (whatever that is) and blowing spray. Visual estimates are dubious because the surface (under the eyewall!) is hard to see unless you are flying below cloud base (200-300 m) and also because appreciably above 115 mph, it's completely white with blowing spray. We used to think that we could estimate stronger winds from the decreasing coverage of slightly greenish patches where the spray was thinner. I now think that we were kidding ourselves. In those days the distinctions among wind gust, sustained one-minute winds, etc., were less well defined than they are now. So we may never know what the 1960s reconnaissance data really means!”

For a complete update on activity in the Atlantic, including unexpected Tropical Storm Julia, see our post from earlier Wednesday. Apologies for the site issues and lack of blog access this morning; we had a disk issue that was causing us trouble.

Bob Henson and Jeff Masters



Video 1. Dan Lindsey (CIRA/CSU) says: “Check out the interesting gravity wave action likely in response to winds interacting w/ terrain.” Image credit: Himiwari-8 satellite loop courtesy @DanLindsey77.



Video 2. Bernadette Woods Placky (Climate Central) says: “Video of #Meranti battering Pintung, Taiwan from my friend Dr. ChiMing Peng.” Image credit: @BernadetteWoods, via @weatherrisk.

Hurricane

Unexpected Tropical Storm Julia Pops Up in Northern Florida; TD 12 Forms

By: Jeff Masters , 4:22 PM GMT on September 14, 2016

In a rare surprise, the Atlantic experienced the formation of a tropical storm with its center located over land on Tuesday evening, when Tropical Storm Julia emerged at 11 pm EDT, centered about five miles west of Jacksonville, Florida. Tropical cyclones (comprising all tropical depressions, tropical storms, and hurricanes) derive their energy from the warm waters of the ocean, so it is very difficult—but not unheard of—for a storm to get its start while centered over land. Julia was helped out by its very large circulation, which pulled a tremendous amount of moisture-laden air from a wide area of ocean. The waters off the coast of Florida that fed Julia featured the exceptional warmth of the Gulf Stream Current, at the time of year when ocean temperatures are at their peak. The Atlantic has had at least one other case of tropical cyclone formation over land: Tropical Storm Beryl in August 1988, which was declared a TD and then a TS while over southeastern Louisiana (thanks go to Boris Konon for this example.)
 

Figure 1. Latest satellite image of Julia.


Figure 2. Latest radar-estimated rainfall accumulation image for Julia.

Radar out of Charleston, South Carolina on Wednesday morning showed that Julia was bringing heavy rains to the coast of Georgia and South Carolina, but these rains were mostly staying offshore. There was no increase in organization or intensity of the echoes apparent, due to the closeness of the storm’s center to land and the presence to a moderately high 15 - 20 knots of wind shear. The main threat of the storm is the heavy rain of 3 - 6” that it is likely to bring to the coast over the next two days. The top winds observed at any coastal sites on Wednesday morning as of noon EDT were sustained winds of 29 mph, gusting to 40 mph, at Fort Pulaski, Georgia at 9:48 am EDT. The strongest offshore buoy winds were 33 mph, gusting to 38 mph, at buoy 41008, 46 miles southeast of Savannah, Georgia, at 10:50 am EDT. Storm surge levels along the Southeast U.S. coast at noon EDT Wednesday were 1.3’ or less.


Figure 3. MODIS image of TD 12 on Wednesday morning, September 14, 2016. Image credit: NASA.

Tropical Depression 12 forms over the Cabo Verde Islands
Tropical Depression Twelve formed in the Cabo Verde Islands off the coast of Africa on Wednesday morning, and was bringing heavy rain showers and gusty winds to the islands as the storm headed west-northwest at 13 mph. Satellite images on Wednesday morning showed that TD 12 was well-organized, with plenty of spin and heavy thunderstorm activity. With wind shear a moderate 10 - 20 knots, a moist atmosphere and warm SSTs near 27 - 27.5°C (81°F), TD 12 is likely to develop into Tropical Storm Karl by Thursday. Ocean temperatures will cool, the atmosphere will get drier and wind shear will increase over TD 12 on Friday, which should weaken the storm—or even dissipate it, as predicted by the GFS model. TD 12 will mostly track to the west-northwest or west over the next five days; it is too early to assume that the storm will recurve to the north and northeast without ever affecting any land areas.

Another African tropical wave may develop next week
The 0Z Wednesday runs of our three reliable models for predicting tropical cyclone genesis—the GFS, European, and UKMET models—agreed that a new tropical wave expected to come off the coast of Africa on Friday will develop into a tropical depression early next week. This storm is expected to take a track more to the northwest than TD 12, and does not appear to be a long-range threat to the Lesser Antilles Islands or North America.

Tropical Storm Ian churning the central Atlantic
Tropical Storm Ian continued its unimposing presence in the central Atlantic late Wednesday morning, about 700 miles east of Bermuda. Ian was headed north at about 19 mph into a region with slightly lower wind shear, which may allow the storm to intensify from its current 50 mph winds to 65 mph winds by Thursday. On Friday, Ian will become entangled with a cold front and and upper level low pressure system, and transition to an extratropical storm. Ian is not a threat to any land areas.

For an update on imposing Typhoon Meranti, which is just hours from landfall in China, see our Wednesday afternoon post, which followed this one. Sorry for the site issues and lack of blog access this morning; we had a disk issue that was causing us trouble.

Jeff Masters

Hurricane

Taiwan, China Brace for Cat 5 Meranti; TS Ian Churns Through Open Atlantic

By: Bob Henson and Jeff Masters , 4:45 PM GMT on September 13, 2016

The newest Jeff Masters/Bob Henson blog post is available at https://www.wunderground.com/blog/. Yesterday's post: Mammoth Super Typhoon Meranti may spare Taiwan a direct hit as it continues barreling toward a potentially destructive landfall in China. Now moving just north of due west at about 15 mph, Meranti will be located near the southern tip of Taiwan by around 8:00 am Wednesday local time (8:00 pm Tuesday EDT). The latest forecast from the Joint Typhoon Warning Center (JWTC) takes the center of Meranti within 50 miles of southern Taiwan, close enough to generate torrential rains, possible landslides, and significant wind damage. Meranti is expected to slam into the southeast coast of China early Thursday local time (8:00 pm EDT Wednesday), perhaps near or just south of the city of Xiamen (population 3.5 million) in China’s Fujian province.


Figure 1. Visible satellite image of Meranti on Tuesday evening local time, September 13, 2016. Image credit: NOAA-NASA and RAMMB/CIRA, courtesy Jon Erdman, weather.com (@wxjerdman).


Figure 2. WU depiction of the JTWC track forecast for Meranti as of 12Z (8:00 am EDT) Tuesday, September 13, 2016.

Meranti among the deepest typhoons in world records
Meranti is a very large and extremely powerful typhoon. Its highest 1-minute sustained winds were 185 mph on Tuesday morning, according to the JWTC. This puts Meranti ahead of Cyclone Winston for the strongest sustained winds for any tropical cyclone of 2016 thus far. (Winston’s top winds were reduced from 185 to 180 mph in post-storm reanalysis.)

Meranti is even more impressive than Winston in another way. Because Meranti is so large, its central pressure is even lower than would be the case for a smaller storm that had the same peak winds. At 1250Z (8:50 am EDT) Tuesday, the Japan Meteorological Agency analyzed Meranti’s central pressure at 890 millibars. This puts Meranti in the elite pantheon of the deepest tropical cyclones ever recorded anywhere on Earth. Several others have had 890 mb central pressure, but only a few have dipped below that mark, including 2 hurricanes in the Atlantic, 1 hurricane in the Northeast Pacific, and 13 typhoons in the Northwest Pacific:

Typhoon Tip (1979) - 870 mb
Hurricane Patricia (2015) - 872 mb
Typhoon June (1975) - 875 mb
Typhoon Nora (1973) - 875 mb
Typhoon Ida (1958) - 877 mb
Typhoon Kit (1966) - 880 mb
Typhoon Rita (1978) - 880 mb
Typhoon Vanessa (1984) - 880 mb
Typhoon Nancy (1961) - 882 mb
Hurricane Wilma (2005) - 882 mb
Typhoon Forrest (1983) - 885 mb
Typhoon Irma (1971) - 885 mb
Typhoon Megi (2010) - 885 mb
Typhoon Nina (1953) - 885 mb
Typhoon Marge (1951) - 886 mb
Hurricane Gilbert (1988) - 888 mb

Most of these typhoon readings are direct measurements, collected aboard reconnaissance flights that were conducted for decades across the Northwest Pacific. With the advent of satellite imagery, regular reconnaissance missions into typhoons were dropped in 1987. It’s possible that several other post-1987 typhoons had central pressures below 890 mb that could not be accurately inferred via satellite. Some of the readings above have been rounded to the nearest 5 mb by the Japan Meteorological Agency, according to WU member 1900hurricane. He adds that Typhoon Judy (1979) had a reconnaissance-measured pressure of 887 mb that’s not part of the JMA database. Thanks to WU member SPShaw for compiling this list.

According to Phil Klotzbach (Colorado State University), only two Northwest Pacific typhoons aside from Meranti are known to have maintained 185-mph sustained winds for at least 18 hours: Tip (1979) and Haiyan (2013).


Figure 3. Visible (left) and infrared (right) VIIRS satellite imagery of the eye and eyewall of Super Typhoon Meranti as of 0508 (1:08 am EDT) Tuesday, September 13, 2016. Image credit: NOAA-NASA and RAMMB/CIRA, courtesy Dan Lindsey (Colorado State University), @DanLindsey77.


Figure 4. Radar image of Super Typhoon Meranti taken at 12:40 EDT September 13, 2016 (12:40 am local time on September 14.) Image credit: Taiwan Central Weather Bureau.

The outlook for Meranti
Even a glancing blow from Meranti is likely to produce significant damage in Taiwan. Meranti is so large that winds above tropical storm force and torrential rains can be expected over much of the island, with much higher winds near the center. The tall mountains of southeast Taiwan will wring out huge amounts of moisture as the storm approaches, with localized rainfall amounts of 25” or more quite possible along east-facing slopes. In addition, enormous waves will batter the southeast coast. Fortunately, this area is sparsely populated, and Meranti is moving at a good clip, which will reduce the chance of even higher rainfall amounts. Taiwan’s second-largest city, Kaohsiung, is located along the nation’s southwest coast, which could get hurricane-force winds as Meranti sweeps by to its south.

Storm surge will not be the biggest threat from Meranti in Taiwan, according to surge expert Hal Needham. “Tropical cyclones have trouble generating substantial storm surge in Taiwan because of the offshore water depth off the east coast is quite deep. Powerful typhoons approach from the east and would need either shallower offshore water or large inlets and bays to generate high storm surge. Those factors are missing from the east coast,” said Needham in an email. “The offshore water depth is more shallow along the west coast, which is also more populated. However, the strongest typhoon winds along a westward-facing coast are blowing offshore because of the counter-clockwise circulation. Typhoons will try to build up storm surge along the ‘back side’ of the storm, but this is not very efficient anywhere in Taiwan.”


Figure 5. Aqua/MODIS image of Marenti at 0510Z (1:10 am EDT) Tuesday, September 13, 2016. Image credit: NASA.

Potential impacts in China
Meranti’s worst impacts may well be in China. Especially if the typhoon avoids a direct hit on Taiwan, it will weaken only partially before reaching the China coast, so there is the potential for major wind damage and storm surge. Should Meranti strike just south of Xiamen, that city and its major port would be at risk of surge impacts. Given the mountainous terrain of China’s Fujian province, we can expect widespread torrential rainfall as a weakening Meranti slows down and grinds its way inland.

Meranti’s projected track is very similar to that of another super typhoon that struck Taiwan back in early July. Super Typhoon Nepartak maintained Category 5 strength with sustained winds of 160 mph and a central pressure of 900 mb until it was just 12 hours from landfall in Taiwan on July 7, 2016. Nepartak made landfall on the southeastern shore of Taiwan as a Category 4 super typhoon with top sustained winds of 150 mph, as estimated by the the Joint Typhoon Warning Center (JTWC), with a central pressure estimated at 930 mb by the Japanese Meteorological Agency (JMA). Nepartak struck the sparsely populated southeast coast of Taiwan and moved offshore north of Kaohsiung, limiting the damage from the storm. Three deaths in Taiwan were blamed on Nepartak, along with $33 million in agricultural damage. National Taiwan University (NTU) buoy NTU2 (located about 170 km southeast of Taitung, Taiwan) recorded a surface pressure of approximately 897 mb as the eye passed over near 8 am EDT July 7. If verified, this would have ranked as the lowest surface pressure ever measured by a buoy in world history. However, we received this update today by email from Ching-Ling Wei from the Institute of Oceanography, National Taiwan University: ”We have recovered the barometer at NTU2 during a buoy service cruise in early August and sent it to the Taiwan's Central Weather Bureau for calibrating the pressure sensor. After a carefully calibration procedure, we corrected the pressure data and obtained the lowest atmospheric pressure of 911.5 hPa instead of 897 hPa when the center of Nepartak was the closest to NTU2.” You can check out the National Taiwan University buoy website http://po.oc.ntu.edu.tw/ for  near-real time data as Super Typhoon Meranti passes by their two buoys. It appears, though, that the buoys lie too far to the north of Meranti’s track to receive hurricane-force winds.


Figure 6. Buoy NTU2 of the Institute of Oceanography, National Taiwan University recorded a lowest atmospheric pressure of 911.5 mb in Super Typhoon Nepartak when the storm made its closest pass on July 7, 2016. A camera on the buoy recorded these boating-unfriendly conditions during the typhoon.

The encounter with the high mountains of Taiwan destroyed the inner core of Nepartak, resulting in the surface circulation separating from the circulation at mid-levels of the atmosphere. A much weakened Tropical Storm Nepartak made landfall in mainland China day later, causing torrential rains that triggered flooding that killed 108 people and caused over $1.4 billion in damage. Meranti should experience less disruption from Taiwan’s mountains than Nepartak, given its more southerly track. According to the National Meteorological Center of China, the region of the coast where Meranti is expected to make landfall received over four inches of rain during the past ten days. It is likely that the soils are near saturation, and widespread destructive flooding can be anticipated from Meranti’s rains.

In the early afternoon Tuesday (U.S. EDT) Meranti was passing very close to two islands owned by the Philippines. At noon EDT Tuesday (midnight local time on Wednesday) , winds at Basco Radar, Philippines were sustained (10-minute average) at 90 mph and the pressure was 936 mb. At 1 am local time, winds at nearby Itbayat were sustained at 112 mph, with a pressure of 934 mb.

Storm chaser James Reynolds is on the southern tip of Taiwan, and will be posting updates to his Twitter feed.

Tropical Storm Ian churning the central Atlantic
Tropical Storm Ian was in the central Atlantic on Tuesday morning, about 900 miles east-southeast of Bermuda, headed north-northwest at 13 mph. Ian was in an environment not conducive for intensification, with wind shear a high 20 knots. Shear is expected to let up only a little this week, making it unlikely Ian will ever attain hurricane strength before getting absorbed by a cold front on Saturday. Ian is not a threat to any land areas.


Figure 7. Regional radar for Florida at 11 am EDT Tuesday, September 13, 2016, showed bands of showers from 93L impacting the coastal waters of Florida.

93L over central Florida showing development as it moves inland
An area of low pressure (Invest 93L) was located just west of Melbourne, Florida on Tuesday morning, and was headed inland, to the north-northwest, at about 10 mph. Satellite images and radar out of Melbourne, Florida showed that 93L was growing more organized Tuesday morning, with an increase in heavy thunderstorm activity, low level spiral bands beginning to form, and some rotation to the cloud pattern beginning to appear. The disturbance was battling high wind shear of 20 - 25 knots. The system will bring heavy rains to much of Florida and portions of southern Georgia on Tuesday. In their 2 pm EDT Tuesday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 40%. This system looked a lot like a tropical depression on radar late Tuesday morning, and was generating sustained winds of 31 mph, gusting to 38 mph, at the Buoy 41009 (23 miles east of Cape Canaveral) at 11:50 am EDT.

95L off the coast of Africa may develop
A large tropical wave (Invest 95L) emerged from the coast of Africa Monday night, and was headed west-northwest at about 10 - 15 mph towards the Cabo Verde Islands. Satellite images late Tuesday morning showed that this wave was well-organized, with plenty of spin and heavy thunderstorm activity. With wind shear a moderate 10 - 20 knots, a moist atmosphere and warm SSTs near 27.5°C (81.5°F), 95L is likely to develop into a tropical depression late this week in the central tropical Atlantic, as predicted by the 0Z Tuesday runs of our three reliable models for predicting tropical cyclone genesis—the GFS, European, and UKMET models. The storm will mostly track to the west-northwest over the next five days, into a region of ocean where few storms eventually end up hitting the Lesser Antilles Islands. However, it is too early to assume that 95L will recurve to the north and northeast without ever affecting any land areas. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 30% and 50%, respectively.


Figure 8. Visible satellite image of Hurricane Orlene as of 1500Z (11:00 am EDT) Tuesday, September 13, 2016. Image credit: NOAA/NESDIS.

Hurricane Orlene on the decline
After topping out as a high-end Category 2 storm with top sustained winds of 110 mph, Hurricane Orlene is now on the downswing. Located about 600 miles west-southwest of Cabo San Lucas, Mexico, as of 11 am EDT Tuesday, Orlene was crawling north-northeast at just 5 mph, with top sustained winds down to 100 mph. A weakness in the upper-level ridge north of Orlene will keep steering currents very weak until Thursday, when the hurricane should resume a westward motion that will keep it away from the Mexican coast. Orlene is over marginally warm waters of 26-27°C, and its slow motion will allow cooler waters to be churned up, so the weakening trend should continue. Orlene is the 10th hurricane of the surprisingly busy East Pacific season of 2016.

Bob Henson and Jeff Masters

Hurricane

Category 5 Meranti Threatens Taiwan; Tropical Storm Ian Forms in the Atlantic

By: Bob Henson and Jeff Masters , 4:36 PM GMT on September 12, 2016

Earth’s strongest tropical cyclone of 2016 thus far is heading for a potentially destructive encounter with Taiwan. A mere 50-mph tropical storm just two days ago, Super Typhoon Meranti was packing top sustained winds of 155 knots (180 mph) at 12Z (8:00 am EDT) Monday, using the 1-minute peak wind standard employed by the Joint Typhoon Warning Center (JTWC) and the National Hurricane Center. (Outside of the U.S., most weather agencies employ a 10-minute wind average; by this standard, Meranti’s peak winds were 115 knots, according to the Japan Meteorological Agency.) Meranti has vaulted to Category 5 strength by taking advantage of nearly ideal conditions, including very warm sea-surface temperatures around 30°C (86°F), very low wind shear (below 10 knots), and a fairly moist mid-level atmosphere (60-70% relative humidity).


Figure 1. Enhanced infrared satellite image from Japan’s Himiwari-8 satellite of Typhoon Meranti at 1500Z (11:00 am EDT) Monday, September 12, 2016. Image credit: NOAA/NESDIS.

Meranti is the planet’s fourth Category 5 storm of the year, following Tropical Cyclone Winston (February, Southwest Pacific Ocean), Tropical Cyclone Fantala (May, Southwest Indian Ocean), and Super Typhoon Nepartak (July, Northwest Pacific Ocean), which struck Taiwan (see below). The globe averages between 4 and 5 Category 5 storms per year. Meranti has now tied Winston for the strongest winds of the year, and its central pressure of 905 mb, as analyzed by JMA at 12Z Monday, puts it just behind Nepartak (900 mb).

Forecast for Meranti
Southern Taiwan faces a serious threat from Meranti. Typhoons this strong will sometimes undergo an eyewall replacement cycle that can trim their peak winds for a day or so, but otherwise it appears Meranti will hang onto most or all of its power until it approaches Taiwan in a couple of days. The latest track forecast for Meranti reflects some major disagreement among models and forecast agencies. The 00Z Monday runs of the GFS and European models take Meranti across the southern tip of Taiwan, while the 00Z UKMET model takes the cyclone on a much more southerly track, which would keep it well offshore. Likewise, the JWTC forecast as of 12Z Monday brings Meranti’s center to the southern tip of Taiwan just after 00Z (8 am local time) Wednesday, while the 12Z Monday outlook from JMA keeps Meranti’s center about 100 miles south of the island. Given the uncertainty and the potential for disaster, southern Taiwan needs to prepare for the possibility of a landfalling super typhoon.


Figure 2. Forecasts for Meranti’s track issued at 12Z (8:00 am EDT) Monday by the Japan Meteorological Agency (left) and the Joint Typhoon Warning Center (right). Image credit: JMA and JTWC.

The GFS and European models project a slight northwest bend to the track that may allow Meranti to grind its way along the southwest coast of the island on Wednesday while maintaining at least Category 3 strength, as reflected in the JTWC outlook. Taiwan’s second-largest city, Kaohsiung, is located along the southwest coast. Taiwan’s southern tip is very sparsely populated, but there are close to 3 million people in the Kaohsiung area.

Taiwan’s second super typhoon of the year
Meranti’s projected track is very similar to that of another super typhoon that struck Taiwan back in early July. Super Typhoon Nepartak maintained Category 5 strength with sustained winds of 160 mph and a central pressure of 900 mb until it was just 12 hours from landfall in Taiwan on July 7, 2016. Nepartak made landfall on the southeastern shore of Taiwan as a Category 4 super typhoon with top sustained winds of 150 mph, as estimated by the the Joint Typhoon Warning Center (JTWC), with a central pressure estimated at 930 mb by the Japanese Meteorological Agency (JMA). Nepartak struck the sparsely populated southeast coast of Taiwan and moved offshore north of Kaohsiung, limiting the damage from the storm. Three deaths in Taiwan were blamed on Nepartak, along with $33 million in agricultural damage. National Taiwan University (NTU) buoy NTU2 (located about 170 km southeast of Taitung, Taiwan) recorded a surface pressure of approximately 897 mb as the eye passed over near 8 am EDT July 7. If verified, this may rank as the lowest surface pressure ever measured by a buoy in world history. A team from National Taiwan University is working to verify that the calibration of the pressure on this buoy was correct.

The encounter with the high mountains of Taiwan destroyed the inner core of Nepartak, resulting in the surface circulation separating from the circulation at mid-levels of the atmosphere. A much weakened Tropical Storm Nepartak made landfall in mainland China day later, causing torrential rains that triggered flooding that killed 108 people and caused over $1.4 billion in damage. The future of Meranti beyond Taiwan hinges in part on whether the storm’s core stays just offshore or whether it gets torn apart by Taiwan’s southern mountains--an outcome too close to call at this point. Track models agree that Meranti will continue on a general west-northwest to northwest track toward the China coast, but with important differences on where Meranti might strike along the coast and how it would behave after that point. Meranti’s size and strength give it the potential to be a catastrophic rainmaker in China.


Figure 3. Radar image of Super Typhoon Nepartak making landfall in southeastern Taiwan taken at 5:30 pm EDT July 7, 2016 (5:30 am local time July 8 in Taiwan.) Image credit: Taiwan CWB.

Another wrinkle now taking shape in the Northwest Pacific
Another factor in Meranti’s track--and a threat in its own right--is Tropical Storm 18, which is gradually strengthening about 300 miles west of Guam. TS 18W could be a Category 3 or 4 typhoon as it approaches Japan’s southern islands on a gradually recurving track. By Wednesday, it’s possible that TS 18W will be close enough to Meranti to trigger the Fuijiwhara effect--the process by which two tropical cyclones begin to rotate cyclonically around a point in between. Should this be the case, it would enhance 18W’s poleward motion--perhaps sending it toward Japan’s populous island of Honshu as a typhoon--while also acting to slow any recurvature of Meranti and potentially increasing its rain- and flood-making potential in China.


Figure 4. Latest satellite image of Orlene.

Yet another East Pacific hurricane: Orlene
The Northeast Pacific notched its 10th hurricane of the 2016 season with the arrival of Hurricane Orlene on Sunday night. This puts the East Pacific well ahead of its long-term average of 8 hurricanes for the entire season (1971-2009). Packing top sustained winds of 90 mph as of the 11 am EDT Monday advisory from NHC, Orlene has a good shot of attaining Category 2 strength later today or tonight as it benefits from extremely low wind shear (around 5 knots) and fairly warm SSTs (27-28°C). Working against Orlene will be cooler waters lurking just below the surface, which are increasingly likely to be churned up as Orlene’s northwestward motion slows to a crawl by Tuesday. A strengthening upper-level ridge should push Orlene westward after its expected midweek stall, nudging it away from land and toward cooler waters.


Figure 5. Latest satellite image of Ian.

Tropical Storm Ian forms in the central Atlantic
Tropical Storm Ian finally decided to spin up into a named storm late Monday morning in the waters of the Central Atlantic, about 1140 miles southeast of Bermuda. Ian is the ninth named storm of the Atlantic hurricane season, bringing our total activity at the approximate halfway point of the season to 9 named storms, 4 hurricanes and 1 intense hurricane. An average season has 11 named storms, 6 hurricanes and 2 intense hurricanes, so we are ahead of schedule for named storms and hurricanes, but near average for intense hurricanes and ACE index, as detailed in our Friday post. Ian is in an environment not conducive for intensification, with wind shear a high 20 knots. Shear is expected to weaken only a little by mid-week, making it unlikely Ian will ever attain hurricane strength as it moves northwards over cooler waters. Ian is not a threat to any land areas.

93L approaching Florida little threat to develop
An area of low pressure (formerly called Invest 93L) was located over the central and northwest Bahamas on Monday morning, and was headed west-northwest at about 10 - 15 mph towards central Florida. Satellite images and long-range radar out of Melbourne, Florida showed that 93L continued to have only a limited amount of heavy thunderstorms, with no sign of a surface circulation center. The disturbance was battling high wind shear of 25 knots and plenty of dry air. The disturbance should move inland over Florida on Tuesday without developing, but will bring some heavy thunderstorms to the central and northwest Bahamas on Monday, and central and northern Florida on Tuesday. In their 8 am EDT Monday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 10%.

Bob Henson and Jeff Masters



Figure 6. CIRA/CSU's Dan Lindsey posted this mesmerizing loop of Typhoon Meranti on Monday. It features images collected by the Himiwari-8 satellite at 500-meter resolution every 2.5 minutes. Image credit: @DanLindsey77.

Hurricane

Rapid Intensification Watch: Orlene and Meranti Spin Up in Pacific

By: Bob Henson , 5:17 PM GMT on September 11, 2016

While the North Atlantic is on the tepid side this weekend in terms of tropical cyclones, we have two potentially fearsome storms in the North Pacific. One is unlikely to hit land; the other is taking a bead on Taiwan. The latter is Typhoon Meranti, located about 900 miles southeast of Taipei as of 12Z (8:00 am EDT) Sunday. Meranti’s top winds, as reported by the Joint Typhoon Warning Center using the 1-minute U.S. standard, jumped from 40 to 85 mph in the 24 hours leading up to 12Z Sunday. This meets the National Hurricane Center (NHC) definition of rapid intensification: an increase in the maximum sustained winds of a tropical cyclone of at least 30 knots (35 mph) in 24 hours.

Meranti’s central core of showers and thunderstorms (convection) is expanding and consolidating quickly, with excellent upper-level outflow evident on satellite. Sea surface temperatures will be holding near 30°C (86°F) along Meranti’s path. Mid-level relative humidity will climb from around 70% toward 80%, and wind shear will be dropping to or below 10 knots. All of these factors point toward Meranti becoming a formidable typhoon.


Figure 1. Enhanced infrared image from the Himiwari-8 satellite of Typhoon Meranti at 1500Z (11:00 am EDT) Sunday, September 11, 2016. Image credit: NOAA/NESDIS.

The outlook for Meranti
JTWC predicts that rapid intensification may continue over the next 24 - 48 hours, with an expected Category 4 strength of at least 125 knots (145 mph) by the time of Meranti’s projected landfall in Taiwan on Wednesday local time. On average, Taiwan gets a landfalling typhoon this strong about once per year, with 14 such landfalls occurring between 2000 and 2015. The 00Z Sunday runs of the GFS, European, and UKMET models all predict that Meranti will be a major typhoon passing across or near southern Taiwan, with the UKMET keeping Meranti just offshore. Although Taiwan’s largest city, Taipei, is located near the north tip of the island, the second-largest city, Kaohsiung, is in the far southwest. Even in this well-prepared nation, a major typhoon can cause significant damage and loss of life. Meranti should recurve along or near the coast of eastern China later in the week, perhaps dumping 10” or more of rain as far north as Shanghai and across parts of Japan.

East Pacific: Orlene gathers strength
The Northeast Pacific got its 16th tropical depression and 15th tropical storm of the year this weekend with the christening of Tropical Storm Orlene. This is the earliest occurrence of a year’s 16th tropical cyclone (including depressions) in the East Pacific since the frenetically busy 1992 season, which used the same list of names as 2016. In that year, Hurricane Orlene became a tropical storm on September 3. Nine additional named storms followed, culminating in Tropical Storm Zeke on October 26.


Figure 2. Latest infrared satellite image of Tropical Storm Orlene.

This year’s Orlene is off to a healthy start, with a solid convective core and top winds up to 50 mph as of the 11 am EDT Sunday advisory from NHC. Located well out to sea--nearly 700 miles southwest of Cabo San Lucas, Mexico--Orlene is in an environment primed for rapid growth. The 12Z Sunday SHIPS model shows very low wind shear (5 - 10 knots), warm SSTs of 28-29°C (82-86°F), and a reasonably moist middle atmosphere (relative humidity of 55-60%). Working against Orlene will be cooler waters lurking just below the surface, which are increasingly likely to be churned up as Orlene’s northwestward motion slows to a crawl by Tuesday. The SHIPS model gives Orlene a 44% chance of gaining at least 25 knots of strength by Monday morning, and a 39% chance of gaining 45 knots of intensity by Monday evening. The latter would push Orlene into the Category 2 bracket. A strengthening upper-level ridge should push Orlene westward after its midweek stall, nudging it away from land and toward cooler waters.


Figure 3. Visible satellite image of Invests 92L, 93L, and 94L as of 1615Z (12:15 pm EDT) Sunday, September 22, 2016. Image credit: NASA/MSFC Earth Science Office.

Atlantic: Three invests all in a row
Three tropical waves continue to vie for attention in the North Atlantic, although only the easternmost one shows any immediate sign of development. Invest 94L features a large circulation, with access to very warm SSTs (around 28°C) and a moist atmosphere. However, 94L’s convection has waxed and waned, and much of it is strung along a north-south axis rather than consolidating around a center. NHC gives 94L a 70% chance of developing into at least a tropical depression by Tuesday and a 80% chance by Friday. Its window of opportunity may close long before then, as the 12Z Sunday SHIPS model shows wind shear increasing to 20-30 knots by Monday, with significant mid-level drying. Our leading models for hurricane genesis, the UKMET, GFS, and ECMWF, insist that 94L has a very good shot at becoming a tropical depression if not a tropical storm by Monday, though only about half of the ECMWF ensemble members from 00Z Sunday bring 94L to tropical storm strength. I’m highly skeptical that 94L will develop into a named storm; if it does, I suspect it will be a highly sheared, asymmetric system rather than a classic, well-formed tropical cyclone. Upper-level troughing will steer 94L toward the north-northwest, keeping it well out to sea.

The Atlantic’s other two systems of interest are barely clinging to life. In fact, both were downgraded on Sunday morning from their “invest” (investigative area) status, the rating that calls for dedicated track models and other specialized attention from NHC. Across the eastern Bahamas, former Invest 93L remains highly disorganized, with no surface circulation associated with it. Ex-93L should limp west-northwestward across the Bahamas and into Florida over the next several days with some heavy showers and thunderstorms, but no major models are making it even a minimal tropical storm. In the southeast Gulf of Mexico, just north of western Cuba, former Invest 92L is even more poorly organized, with very little convection evident on satellite. Computer models agree that ex-92L is likely to dissipate in the open Gulf over the next day or so. NHC gives both systems a near-zero chance of development through the next five days.

We’ll be back with our next update by midday Monday. Happy birthday, Dr. Masters!

Bob Henson


Hurricane

Litte Change to--or Threat From--Atlantic Invests 92L, 93L, and 94L

By: Jeff Masters , 3:56 PM GMT on September 10, 2016

An area of low pressure (Invest 92L) with a well-defined surface circulation was located a few hundred miles west of the Florida Keys on Saturday morning. Satellite images and long range radar out of Key West showed that 92L had a well-defined surface circulation center, but the center was completely exposed to view, with almost no heavy thunderstorms—the telltale sign of a storm struggling with dry air and high wind shear. The storm’s heaviest thunderstorms were being kept to the southeast side of the center of circulation by high wind shear of 20 - 30 knots, due to strong upper-level winds out of the northwest. The disturbance was also battling plenty of dry air, as seen on water vapor satellite loops, and the 8 am EDT Saturday run of the SHIPS model showed the humidity at mid-levels of the atmosphere was about 60%, which is marginal for tropical storm formation. Sea surface temperatures (SSTs) were a very warm 30°C (86°F), though.


Figure 1. Latest satellite image of 92L.

Forecast for 92L
There is little model support for the development of 92L, with none of the 00Z Saturday operational versions of our three reliable models for predicting tropical cyclone genesis--the GFS, UKMET and European models--predicting development. During the next five days, 92L should experience dry mid-level air with a relative humidity of 55 - 65%, and see moderately high wind shear of 15 - 25 knots, as the storm heads west at about 5 mph through the southern Gulf of Mexico, according to the 8 am EDT Saturday run of the SHIPS model. These conditions are quite marginal for development. In their 8 am EDT Saturday Tropical Weather Outlook, NHC dropped their 2-day and 5-day development odds to 10%.

Tropical wave 93L just north of Puerto Rico little threat to develop
An area of low pressure (Invest 93L) located about 200 miles north of Puerto Rico on Saturday morning was headed west-northwest at about 15 mph. Satellite images show that 93L continues to have only a limited amount of heavy thunderstorms, with no sign of a surface circulation center. The disturbance was battling plenty of dry air, as seen on water vapor satellite loops. The 8 am EDT Saturday run of the SHIPS model showed the humidity at mid-levels of the atmosphere was about 45 - 50%, which is almost always is too dry to support tropical storm formation. Conditions were otherwise favorable for development, with wind shear a low 5 - 10 knots, and sea surface temperatures (SSTs) a warm 29.5°C (85°F.)


Figure 2. Latest satellite image of 93L.

Forecast for 93L
There is very little model support for the development of 93L, with none of the 00Z Saturday operational versions of our three reliable models for predicting tropical cyclone genesis--the GFS, UKMET and European models--predicting development. The 8 am EDT Saturday run of the SHIPS model predicted that 93L would find a moister surrounding atmosphere by Sunday, but that wind shear would increase to the moderate range. These conditions are quite marginal for development. In their 8 am EDT Saturday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 10%. Invest 93L will continue moving west-northwest to northwest at about 15 mph through Sunday, then slow down to a forward speed of about 10 mph on Monday and Tuesday. This should put the storm near the coast of northern Florida by Tuesday.


Figure 3. Latest satellite image of 94L.

Tropical wave 94L in the central tropical Atlantic likely to develop
A large tropical wave located about 1000 miles east of the Lesser Antilles Islands on Saturday morning (Invest 94L) was headed west-northwest at 10 mph. Satellite images showed that 94L remained well-organized, with a large circulation and an increasing amount of heavy thunderstorms. With wind shear expected to be in the low to moderate range through Monday, along with plenty of warm water and a moist atmosphere with 70% humidity at middle levels of the atmosphere, 94L is likely to develop into a tropical depression by Monday, as predicted by our top three models for hurricane genesis, the UKMET, GFS and European models. In their 8 am EDT Saturday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 70% and 90%, respectively. The long-range models are showing 94L will likely be a “fish storm”, and take a west-northwesterly then northwesterly track into the Central Atlantic to a location where few storms ever become a threat to the Lesser Antilles Islands or North America. The next name on the Atlantic list of storms is Ian.

Tropical Storm Meranti headed for Taiwan
In the Northwest Pacific, Tropical Storm Meranti is intensifying and is expected to become a major typhoon that will threaten Taiwan by Wednesday. We’ll have more on this storm in future posts.

Jeff Masters

Hurricane

Three Non-Threatening Atlantic Invests to Watch: 92L, 93L, and 94L

By: Jeff Masters and Bob Henson , 6:32 PM GMT on September 09, 2016

An area of low pressure with a well-defined surface circulation developed on Friday afternoon in the Florida Straits, just east of the Florida Keys. This disturbance had been designated as Invest 92L earlier in the week, but then dropped, once the system lost nearly all of its heavy thunderstorms. Now, NHC is once more calling this system 92L. Satellite images and long range radar out of Key West show that 92L has well-defined surface circulation center, but almost no heavy thunderstorms. The storm’s heaviest thunderstorms were being kept to the southeast side of the center of circulation by high wind shear of 20 - 25 knots from strong upper-level winds out of the northwest. The disturbance was also battling plenty of dry air, as seen on water vapor satellite loops, and the 8 am EDT Friday run of the SHIPS model showed the humidity at mid-levels of the atmosphere was about 55 - 60%, which is marginal for tropical storm formation. Sea surface temperatures (SSTs) were a very warm 30°C (86°F), though. Surface winds measured by the ASCAT satellite instrument were as high as 35 mph at 10:28 am EDT Friday.


Figure 1. MODIS visible satellite image of 92L taken early Friday afternoon, September 9, 2016. Image credit: NASA.

Forecast for 92L
There is little model support for the development of 92L, with none of the 00Z Friday operational versions of our three reliable models for predicting tropical cyclone genesis--the GFS, UKMET and European models--predicting development. Only 4% of the 50 ensemble members of the European model predicted development, and none of the 20 members of the GFS model ensemble did so. During the next five days, 92L should experience dry mid-level air with a relative humidity of 60 - 65%, and see moderately high wind shear of 15 - 25 knots, as the storm heads west at about 5 mph through the southern Gulf of Mexico, according to the 8 am EDT Friday run of the SHIPS model. These conditions are quite marginal for development. In their 2 pm EDT Friday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 40%. 92L will likely bring winds gusts near 40 mph and rain squalls to the Florida Keys Friday evening through Saturday afternoon.

Tropical wave 93L just northeast of Puerto Rico little threat to develop
An area of low pressure located 100 - 200 miles northeast of Puerto Rico on Friday afternoon was headed west-northwest at about 15 mph. This system was designated Invest 93L by NHC on Thursday morning. Satellite images and long range radar out of Puerto Rico show that 93L has only a limited amount of heavy thunderstorms, with no sign of a surface circulation center. The disturbance was battling plenty of dry air, as seen on water vapor satellite loops, and the 8 am EDT Friday run of the SHIPS model showed the humidity at mid-levels of the atmosphere was about 45 - 50%, which is normally too dry to support tropical storm formation unless wind shear is very low. Wind shear was a moderate 10 - 20 knots over 93L, and sea surface temperatures (SSTs) were a warm 29°C (84°F.)


Figure 2. Latest satellite image of 93L.

Forecast for 93L
There is very little model support for the development of 93L, with none of the 00Z Friday operational versions of our three reliable models for predicting tropical cyclone genesis--the GFS, UKMET and European models--predicting development. Only 8% of the 50 ensemble members of the European model predicted development, and none of the 20 members of the GFS model ensemble did so. The 8 am EDT Friday run of the SHIPS model predicted that 93L would find a moister surrounding atmosphere by Sunday, but that wind shear would grow high, greater than 20 knots, by then. These conditions are quite marginal for development. In their 2 pm EDT Friday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 10% and 10%, respectively. Invest 93L will continue moving west-northwest to northwest at about 15 mph over the next two days, then slow down to a forward speed of about 10 mph on Monday and Tuesday. This should put the storm a few hundred miles southeast of the coast of South Carolina by Tuesday.


Figure 2. Latest satellite image of 94L.

Tropical wave 94L in the central tropical Atlantic likely to develop
A large tropical wave midway between the coast of Africa and the Lesser Antilles Islands (Invest 94L) was headed west-northwest at 20 mph Friday afternoon. Satellite images showed that 94L was becoming well-organized, with a large circulation and an increasing amount of heavy thunderstorms. With wind shear expected to be in the low to moderate range through Tuesday, along with plenty of warm water and a moist atmosphere with 70% humidity at middle levels of the atmosphere, 94L is likely to develop into a tropical depression by early next week, as predicted our top three models for hurricane genesis, the UKMET, GFS and European models. In their 2 pm EDT Friday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 50% and 80%, respectively. The long-range models are showing 94L will likely be a “fish storm”, and take a west-northwesterly then northwesterly track into the Central Atlantic to a location where few storms ever become a threat to the Lesser Antilles Islands or North America. The next name on the Atlantic list of storms is Ian.


Figure 4. NASA-NOAA's Suomi NPP satellite captured this image of Hurricane Gaston (Category 2, 105 mph winds) in the central North Atlantic on August 30, 2016, at 1625Z (12:25 p.m. EDT). Gaston was the only major hurricane of the first half of the 2016 Atlantic hurricane season, peaking as a Category 3 storm with 120 mph winds on August 29. Image credit: NOAA / NASA Goddard MODIS Rapid Response Team.

Midpoint review: A fairly active, not-too-destructive Atlantic season
Now that we’re at the climatological midpoint of the Atlantic hurricane season (September 10 or 11, depending upon which statistics one uses), it’s a good time to review how this season is shaping up. We can compare this season to what one might expect from climatology based on data compiled by Phil Klotzbach (Colorado State University). As a climatological benchmark, Klotzbach uses the 30-year period from 1981 to 2010, which is almost evenly split between the pre-1995 inactive period and the post-1995 active period in the Atlantic. Here’s where we stand as of September 9 relative to the amount of activity that an average season would have produced by now:

8 named storms (typical by 9/9 is 6.5)
4 hurricanes (typical by 9/9 is 2.7)
1 major hurricane (typical by 9/9 is 1.2)
40.7 ACE, or accumulated cyclone energy (typical by 9/9 is 46.2)

This season is running a bit below average on ACE, as well as the number of hurricane days and major-hurricane days (not shown here). We are considerably above average on the number of named storms and hurricanes. If we continued the rest of the season at exactly this pace, we'd end up with 16 named storms, 8 hurricanes, and 2 majors. That would line up pretty well with the consensus of the various forecast groups as summarized by Phil Klotzbach. Here again, we're running ahead of the consensus on the number of named storms and hurricanes, but a bit under expectation on the amount of ACE. The four tropical storms this year were all quite weak (Bonnie, Colin, Danielle, Fiona). About 60% of the entire season’s ACE thus far--more than every other tropical cyclone combined--was generated by long-lived Hurricane Gaston, which stayed far out to sea. The most destructive Atlantic system of the year thus far, Hurricane Hermine, produced total economic damages that could approach $1 billion.  We’ve had one quite deadly storm this year: Hurricane Earl, which hit Belize as a Category 1 storm on August 4, then made a second landfall on Mexico’s Bay of Campeche coast on August 6. Earl killed 67 people and left 12 others missing; 57 of the fatalities were in Mexico, due to flooding and mudslides.

NOAA: La Niña a no-show
On Thursday, NOAA cancelled its La Niña Watch, as the atmosphere has failed to respond to sea-surface temperatures in the borderline La Niña range. If La Niña indeed fails to materialize, then the rest of the Atlantic season may end up less active than predicted by some of the higher-end outlooks that anticipated La Niña’s emergence.

We'll have an update on Saturday afternoon.

Jeff Masters and Bob Henson

Hurricane

U.S. Endures its Sultriest Summer Nights on Record

By: Bob Henson , 11:53 PM GMT on September 08, 2016

The broiling summer of 2016 placed fifth hottest among the 122 summers since records began in 1895 for the contiguous U.S., according to NOAA analyses released on Thursday. Even more impressive, this past summer (June through August) saw the highest average minimum temperature on record--certainly no surprise to people across the country who endured one muggy night after another. The average daily minimum for June through August 2016 was a balmy 60.81°F, beating the record of 60.70°F set in 2010. The average daily summer low in the contiguous 48 states has climbed about 1.4°F in the last century. That’s double the increase of 0.7°F in the average daily summer high.


Figure 1. Average daily minimum temperatures for the contiguous U.S. for each summer from 1895 to 2016. Image credit: NOAA/NCEI

Wetter, hotter American summer
In a climate warmed by increasing greenhouse gases, computer models and basic theory agree that nights should warm more quickly than days. In large part this is because of increasing atmospheric moisture that keeps nighttime temperatures up, even when that moisture doesn’t actually produce rain. Climate Central has a handy site that allows you to calculate the ongoing increase in summer dew point temperatures for more than 100 cities around the nation. In larger cities, there’s another human-produced climate effect in the mix: the urban heat island, whereby the pavement and buildings of metro areas heat up by day and release the heat by night. The urban heat island is only a small part of the global temperature picture, since rural areas and oceans are heating up as well.

California, Connecticut, and Rhode Island each saw their warmest summer on record in 2016, while many other states had a top-ten warmest summer. Some U.S. cities set records for their longest stretches above a certain temperature, and a few also saw their warmest summer overall, as noted by Chris Burt in a WU blog earlier this week. Among those setting all-time records for average summer temperature, including both daily highs and daily lows:

Anchorage, Alaska: 60.7° (previous 60.2° in 2015)
Charleston, South Carolina: 84.1° (previous 83.4° in 2011)
Las Vegas, Nevada: 93.1° (previous 92.5° in 2007)

See Chris’s post for other examples.


Figure 2. Statewide rankings for average temperature during June-August 2016, as compared to each June-August since 1895. Darker shades of orange indicate higher rankings for warmth, with 1 denoting the coldest month on record and 122 the warmest. Image credit: NOAA/NCEI.


Figure 3. Statewide rankings for average precipitation during June-August 2016, as compared to each June-August since 1895. Darker shades of green indicate higher rankings for moisture, with 1 denoting the driest month on record and 122 the wettest. Image credit: NOAA/NCEI.


Figure 4. Corn stalks were stunted by lack of rain in this Barnstead, New Hampshire, field on September 2, 2016. The drought in southern New England and dry spells this summer further north mean that fall foliage could come earlier this year and not last as long in some areas. Image credit: AP Photo/Jim Cole.

Boston sees its driest summer on record, while the central U.S. was soaked
Large parts of the western and eastern U.S. had to deal with a scarcity of rain along with the heat this summer. It was the 24th wettest out of the past 122 summers nationwide, but that is largely due to the moisture surplus over the central U.S., centered on the Mississippi Valley. Eight states in this corridor had a top-ten wettest summer. Meanwile, the rest of the nation saw precious little rain overall. Wyoming, South Carolina, and Massachusetts all had top-ten driest summers. It’s been especially parched in New England, where parts of eastern Massachusetts and southern New Hampshire are now in extreme drought, according to Thursday’s U.S. Drought Monitor. Boston had its driest summer in records going back to 1872. Only 3.92” fell from June through August, breaking the record of 3.97” set in 1957.

August was both very warm (17th hottest) and very wet (2nd wettest) for the contiguous U.S. as a whole. Eight states saw their hottest August on record: Pennsylvania, Maryland, Delaware, New Jersey, New York, Massachusetts, Rhode Island, and Connecticut. One state had its wettest August. Unsurprisingly, that was Louisiana, where a slow-moving low pressure system dumped massive amounts of rain and triggered unprecedented flooding in and near Baton Rouge. According to the August 2016 Catastrophe Report from insurance broker Aon Benfield, damage from the flood was estimated at $10 - $15 billion, which will likely make it the second most expensive non-hurricane related flood in U.S. history, behind the $35 billion in damage from the summer 1993 flooding in the Midwest.


Figure 5. Mud-covered belongings line the floor of a home after floodwater receded on August 17, 2016, in Denham Springs, Louisiana. Image credit: Brendan Smialowski/AFP/Getty Images.

Climate change made the Louisiana floods more likely
The mammoth rainfall in August that dumped 10” - 30” on a large part of southern Louisiana was probably twice as likely to occur because of the influence of human-produced greenhouse gases, according to a rapid-response study released on Tuesday. Researchers at Climate Central, NOAA, Princeton University, and the Royal Netherlands Meteorological Institute analyzed output from several major climate model runs, covering both past and future climates, to see how this type of heavy-rain event has changed across the Central Gulf Coast from east Texas to western Florida.

Such an event was calculated to be at least 40% more likely—and probably twice as likely—due to the influence of climate change. Even in today’s climate, a rainfall of this size and scope remains a rarity, one that would now be expected once every 550 years at any particular point in the region, the study found. However, the expected frequency of getting such an event somewhere within the entire Gulf Coast study region is now about every 25-30 years—that is, frequent enough to play into infrastructure, building, and insurance decisions. Put another way, the intensity of a typical 25-30 year rainfall event in this region has gone up by at least 10%. When it comes to flooding, a small increase in water level can lead to a huge increase in impact.

This week’s report is part of the World Weather Attribution project, coordinated by Climate Central. The project brings in researchers around the globe to carry out prompt analyses of high-profile weather events and their potential links to climate change. Such studies have typically taken a year or two to complete, so the project has zeroed in on ways to produce these studies more quickly, while an event is still high on the public’s mind. For example, scientists can extract relevant detail from recent comprehensive model runs, rather than carrying out entirely new model runs from scratch.

Bernadette Woods Placky (Climate Central) mentioned another benefit in an email: “People usually assess and rebuild immediately after an event. This information can help inform that process, from the public to insurance to emergency managers to other decision makers. When we wait 1-2 years for this information, the plans are already finished or in the works.”

We’ll be back with our next update by Friday afternoon.

Bob Henson

Climate Summaries Climate Change Flood

Newton Sweeps into U.S. with Strong Wind, Heavy Rain, Deep Moisture

By: Bob Henson and Jeff Masters , 4:12 PM GMT on September 08, 2016

Former Hurricane Newton swept from Mexico’s Gulf of California into southeast Arizona between 1 and 2 pm MDT Wednesday afternoon. Analyzed as a 45-mph tropical storm at noon MDT Wednesday, the fast-weakening Newton was classified as a remnant low by NHC a couple of hours after its international border crossing. In its final discussion on the system, NHC said: “Based on the data available to us at this time, we do not think that Newton moved into southern Arizona as a tropical cyclone.” Only a handful of systems have been officially designated as tropical storms while in Arizona, including Hurricane Katrina (1967), Hurricane Lester (1992), and Hurricane Raymond (1989). In 1997, Hurricane Nora tracked up the Colorado River valley along the Arizona/California border as a tropical storm.


Figure 1. Natural-color satellite image of Newton over southeast Arizona at 2035Z (4:35 pm EDT) Wednesday, September 7, 2015, about an hour after it crossed into the state. Image credit: CSU/RAMMB/CIRA.

Newton still brought a dose of strong wind and torrential rain to the state. About 40 miles south of Tucson, a remote automated weather station located at 7120 feet near the top of Mt. Hopkins recorded several hours of high wind from the east while Newton was still a tropical storm in Mexico. The winds peaked at 9:19 am MDT with a report of 52 mph gusting to 66 mph. Other high-altitude stations also reported strong gusts, including 58 mph at Miracle Valley (elevation 7677 feet). Rainfall totals of 3” - 5” were scattered along the track of Newton at higher elevations across southeast Arizona and southwest New Mexico, with 1” - 2” common at lower elevations. A PWS on the summit of Mt. Graham reported more than 4”.

Newton’s presence lives on in the form of deep tropical moisture funneled well ahead of the storm into the Central Plains and Midwest across a preexisting frontal system. At Lincoln, IL, the atmospheric sounding taken at 12Z (8:00 am EDT) Thursday showed 2.54” of total precipitable water (TPW)--the amount of water vapor in a column of air from the surface upward. This is the second-highest TPW recorded on any date since regular balloon-borne soundings began in the Lincoln/Peoria area in 1949, topped only by 2.66” on July 8, 1949. Showers and thunderstorms dropped 3” - 5” rains on Wednesday afternoon and evening across parts of southeast Kansas, eastern Iowa, and western Michigan (see Figure 2). The NWS Weather Prediction Center is calling for a slight risk of rainfall exceeding flash flood thresholds on Thursday and Friday across parts of Kansas, Missouri, Illinois, and Indiana.


Figure 2. Multisensor analysis of total rainfall (in inches) from 12Z (8:00 am EDT) Wednesday, September 7, to 12Z Thursday, September 8. Image credit: NOAA/NWS Advanced Hydrologic Prediction Service.

Tropical wave 93L near the Lesser Antilles little threat to develop
An area of low pressure located about 300 miles east of the northern Lesser Antilles Islands late Thursday morning was  headed west-northwest at about 15 mph. This system was designated Invest 93L by NHC on Thursday morning. Satellite images show that 93L has developed plenty of spin but only a small amount of heavy thunderstorm activity. Spinning clouds in mid to upper levels of the storm briefly created the false appearance that 93L had an eye Thursday morning, but the system has had no signs of a surface circulation center thus far. The disturbance was battling plenty of dry air, as seen on water vapor satellite loops, and the 8 am EDT Thursday run of the SHIPS model showed the humidity at mid-levels of the atmosphere was about 45 - 50%, which is normally too dry to support tropical storm formation unless wind shear is very low. Wind shear was a moderate 10 - 20 knots over 93L, and sea surface temperatures (SSTs) were a  warm 29°C (84°F.)


Figure 3. Latest satellite image of 93L.

Forecast for 93L
There is very little model support for the development of 93L, with none of the 00Z Thursday operational versions of our three reliable models for predicting tropical cyclone genesis--the GFS, UKMET and European models--predicting development. Only 4% of the 50 ensemble members of the European model predicted development, and none of the 20 members of the GFS model ensemble did so. The 8 am EDT Thursday run of the SHIPS model predicted that wind shear would remain moderate, 10 - 20 knots, over the next five days, but the atmosphere would remain very dry, which should make any development slow to occur. In their 8 am EDT Thursday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 10% and 20%, respectively. Invest 93L will continue moving west-northwest to northwest at about 15 mph over the next five days, which should put it a few hundred miles southeast of the coast of South Carolina by Tuesday.

African tropical wave may develop early next week
A tropical wave that emerged from the coast of Africa on Tuesday was bringing a modest area of disorganized heavy thunderstorms to the waters of the tropical Atlantic about 500 miles southwest of the Cabo Verde Islands on Thursday morning, as seen on Meteosat satellite images. This tropical wave could develop into a tropical depression by early next week, predicted our top three models for hurricane genesis, the UKMET, GFS and European models, in their 0Z Thursday runs. In their 8 am EDT Thursday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 10% and 60%, respectively. The long-range models are showing a west-northwesterly track for this storm into the Central Atlantic to a location where few storms ever become a threat to the Lesser Antilles Islands or North America.

Bob Henson and Jeff Masters


Hurricane

Tropical Storm Newton Makes a 2nd Mexican Landfall, Spreads Heavy Rains to SW U.S.

By: Jeff Masters , 3:22 PM GMT on September 07, 2016

Heavy rains have moved into Southeast Arizona as Tropical Storm Newton plows northwards at 18 mph. As of the 11 am EDT Wednesday advisory, Newton had top sustained winds of 50 mph and was located just 135 miles south of Tucson, Arizona. Newton made two landfalls in Mexico during the previous 36 hours. The first came early Tuesday morning, when Newton hit the southern tip of the Baja Peninsula with sustained winds of 90 mph, with the eye of the storm passing directly over the resort town of Cabo San Lucas. After crossing the spine of the Baja Peninsula on Tuesday, Newton weakened to a tropical storm with 70 mph winds and made landfall in southwest Mexico near Bahia Kino early Wednesday morning. Here are some peak wind gusts measured during and after this final landfall, according to NHC:

• 66 mph: Guaymas, Mexico
• 64 mph: East of Bahia Kino
• 50 mph: Hermosillo International Airport

The storm is being blamed for two deaths, with three others missing, from a capsized fishing boat near the town of La Paz on the Baja Peninsula. Newton is expected to cross into southern Arizona as a tropical depression or weak tropical storm on Wednesday afternoon, and dissipate over the dry, rugged terrain of Arizona by Wednesday night.


Figure 1. A woman wades through a street flooded by the heavy rains brought on by Hurricane Newton in Cabo San Lucas, Mexico, Tuesday, Sept. 6, 2016. (AP Photo/Eduardo Verdugo)


Figure 2. MODIS visible satellite image of Hurricane Newton on Tuesday morning, September 6, 2016. At the time, Newton was a Category 1 storm with 80 mph winds, located over the center of Mexico’s Baja Peninsula. Image credit: NASA.

High moisture available to Newton
By late Wednesday morning, Newton was bringing heavy rains to southeastern Arizona, southern New Mexico and southwest Texas. Widespread heavy rains of 1 - 3” and the risk of flash floods are expected in the region, and a Flash Flood Watch is posted. Ocean temperatures in the waters off the southwest Mexico coast, including the Gulf of California, are about 1°C (1.8°F) above average, which is allowing high amounts of water vapor to evaporate into the atmosphere and feed Newton’s heavy rains. This rich moisture was streaming into the Southwest U.S., where on Wednesday morning, four upper-air observing stations—Amarillo, TX, El Paso, TX, Phoenix, AZ, and Tucson AZ—recorded top-ten amounts of moisture on record for the month of September (1st, 2nd, 3rd, and 7th highest, respectively.) The upper-air soundings measured total precipitable water (TPW)—the amount of water that would result if one condensed all the water vapor in a column above and precipitated it out. Upper-air balloon soundings began in the U.S. in 1948.

New African tropical wave may develop late this week
A tropical wave that emerged from the coast of Africa on Tuesday was bringing a modest area of disorganized heavy thunderstorms near the Cabo Verde Islands on Wednesday morning, as seen on Meteosat satellite images. This tropical wave could develop into a tropical depression by the weekend, a few hundred miles west-southwest of the Cabo Verde Islands, predicted the 00Z Wednesday run of the UKMET model. Our other two reliable models for predicting tropical cyclone genesis, the GFS and European models, were less impressed with this tropical wave than in their previous runs, and showed only weak development of the system late this week, due to dry air coming off the coast of Africa. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 20% and 70%, respectively. The long-range models are showing a west-northwesterly track for this storm into the Central Atlantic to a location where few storms ever become a threat to the Lesser Antilles Islands or North America.

Satellite images on Wednesday morning showed that the tropical wave (Invest 92L) that was over the central Caribbean near Hispaniola on Tuesday had grown very weak and disorganized. NHC no longer considers 92L an area of interest, and stopped issuing model forecasts for it.


Figure 3. MODIS visible satellite image of Post Tropical Cyclone Hermine on Tuesday morning, September 6, 2016. At the time, NHC was about to issue its final advisory on the storm. Image credit: NASA.

Hermine still spinning off the coast of Long Island
Post-Tropical Cyclone Hermine is still gradually spinning down as it meanders over the cool waters just over 100 miles southeast of Long Island, New York. Hermine was bringing extensive cloud cover to the Northeast, but little in the way of strong winds or precipitation. On Tuesday afternoon at 2 pm EDT, the National Hurricane Center (NHC) judged that Hermine no longer represented a danger, and issued its last advisory on the storm. This ended a marathon 19-day period where they tracked the storm first as Invest 99L, then as TD 9, Tropical Storm Hermine, Hurricane Hermine, and finally, Post-Tropical Cyclone Hermine. What’s left of Hermine is expected to get absorbed by a low pressure system passing to its north on Thursday, and Hermine should no longer be identifiable as a separate entity by Friday.

Record September heat in Europe and the Mideast
All-time September heat records for Spain and Portugal fell this week, thanks to a massive dome of high pressure that settled over the region that sent temperatures soaring as high as 46.4°C (115.5°F) in Sanlucar La Mayor, Spain—a new September European heat record, if verified. Perhaps more extraordinary was the record September heat in the Middle East this week; Mitribah, Kuwait recorded 51.2°C (124.2°F) on September 4—the hottest reliably measured temperature on record so late in the year, world-wide. Wunderground’s weather historian, Christopher C. Burt, has the details in his Wednesday post, Hottest Temperature Ever Measured in September for Europe.

Jeff Masters

Hurricane

Sudden Storm, Lingering Storm: Hurricane Newton Hits Baja, Hermine Still Spinning

By: Jeff Masters , 4:30 PM GMT on September 06, 2016

Hurricane Newton is battering Mexico’s Baja Peninsula after whipping into existence on Sunday and intensifying with frightening speed. Newton became a tropical depression on Sunday afternoon in the waters a few hundred miles south-southeast of the tip of Mexico’s Baja Peninsula, and rapidly grew into a hurricane in just 24 hours. Just 36 hours after the first advisory on the new tropical depression was issued, Newton was battering the southern tip of Baja with sustained winds of 90 mph, with the eye of the storm passing directly over the resort town of Cabo San Lucas. On Tuesday morning, an automated station at an altitude of 735 feet in Cabo San Lucas recorded sustained winds of 79 mph, gusting to 116 mph, between 2:10 - 2:20 EDT. An automated station along the central spine of the Baja Peninsula at Sierra La Laguna at an elevation of 6,394 feet recorded sustained winds of 73 mph, gusting to 136 mph, at 3:20 am EDT Tuesday. Newton’s sudden explosion into a landfalling hurricane didn’t give much time for evacuations and preparations, but the Mexican civil defense system for hurricanes is one of the best in the world, and I am hopeful that they got everyone to safety in time. Since reliable record keeping of intensification rates of Atlantic hurricanes began in 1970 (when regular satellite coverage became available), the fastest an Atlantic storm has gone from its first appearance as a tropical depression to hurricane strength is 18 hours, in the case of Hurricane Humberto of 2007. There were seven storms that accomplished the feat in 24 hours. I haven’t studied the statistics on this matter for the Eastern Pacific.


Figure 1. MODIS visible satellite image of Hurricane Newton on Monday, September 5, 2016, at 2:45 pm EDT. At the time, Newton was about to become a Category 1 storm with 75 mph winds. Image credit: NASA.

Forecast for Newton
Newton will finish traversing the southern portion of the Baja Peninsula by Tuesday afternoon, then emerge over the warm waters of the Gulf of California for 6 - 12 hours before making a final landfall on the coast of northwestern Mexico early Wednesday morning. Due to the narrowness of the Gulf of California and the high mountains on either side, plus an increase in wind shear, Newton will continue to weaken until its final landfall on Wednesday morning. Heavy rains will be the main threat from Newton over the next two days, with up to 18” expected in Baja. A significant pulse of moisture will surge into southeastern Arizona and southern New Mexico on Wednesday afternoon through Thursday, bringing widespread heavy rains of 1 - 3” and the risk of flash floods. A Flash Flood Watch is posted for portions of southeast Arizona, southern New Mexico, and southwest Texas.


Figure 2. Winds of 65 mph churn the ocean in this view taken inside of Hermine by an Air Force hurricane hunter aircraft on September 5, 2016. Image credit: LM TSgt Banks.

Hermine gradually winding down
Post-Tropical Cyclone Hermine is gradually spinning down as it meanders over the cool waters just over 100 miles southeast of Long Island, per the 11 am NHC advisory.  New York, where a Tropical Storm Warning continues. Data from an Air Force Hurricane Hunter flight early Tuesday morning and recent buoy observations suggested Hermine’s top sustained surface winds had decreased to 60 mph by late morning Thursday. Hermine is close enough to the Mid-Atlantic and Northeast shorelines to produce rough surf, rip currents and a storm surge of 1 - 2 feet, but has dropped very little rainfall over the past day. The maximum storm surge observed on the U.S. coast late Tuesday morning was 2.0 feet in Atlantic City, New Jersey. Wind gusts from Hermine on Long Island on Tuesday morning included 37 mph at Farmingdale, 37 mph at Shirley, 37 mph at JFK Airport, and 32 mph at La Guardia Airport. At the Montauk Point Lighthouse, on the eastern tip of Long Island, sustained winds of 58 mph and gusts to 64 mph were reported at 12:20 am EDT Tuesday.


Figure 3.Visible VIIRS image of Hermine as of 1755Z (1:55 pm EDT) Monday, September 5, 2016. Image credit: NASA.

Forecast for Hermine
Hermine is now over waters cooler than 26°C—too cool to support tropical redevelopment. Furthermore, the storm is no longer receiving energy from atmospheric dynamics (from contrasts between warm moist air and cool dense air.) Without either source of energy to sustain it, Hermine will gradually spin down until it dissipates. Dissipation is predicted to occur by Thursday afternoon by both the European model and the GFS model. There is a chance of tropical-storm-force winds (sustained winds of 39 mph or more) and bursts of heavy rain on eastern Long Island and over far southeast Massachusetts before dissipation occurs. Storm surge will be less than two feet, but some minor coastal flooding and beach erosion will occur. For the latest on local impacts, check the local statements compiled on the NHC website.

92L little threat to develop
Satellite images on Tuesday morning showed that a tropical wave (Invest 92L) was bringing heavy rains to Hispaniola. The activity was much weaker and less organized than on Monday. A strong and persistent ridge of high pressure should keep 92L on a west to west-northwest path, and the system will bring some heavy rains to Jamaica and eastern Cuba on Wednesday and western Cuba on Thursday. Development is being arrested by dry air, and the 2 am EDT Tuesday SHIPS model forecast for 92L showed increasing dry air and wind shear for the latter part of the week, which should keep any development from occurring. The latest 0Z Tuesday operational runs of our three top models for forecasting tropical cyclone genesis--the GFS, European and UKMET models--did not show development of the system over the next five days. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC dropped their 2-day and 5-day development odds to 0% and 10%, respectively.

New African tropical wave may develop late this week
A tropical wave expected to leave the coast of Africa on Tuesday could develop into a tropical depression by next weekend, a few hundred miles west-southwest of the Cabo Verde Islands, predicted the 00Z Tuesday runs of the UKMET, GFS and European models. The Sahara Desert dust and dry air machine will be moderately active during the week, and development of this new tropical wave will likely be hindered by dry air. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC gave this future system 2-day and 5-day development odds of 10% and 60%, respectively. The long-range models are showing a west-northwesterly track for this storm into the Central Atlantic to a location where few storms ever become a threat to the Lesser Antilles Islands or North America.

Jeff Masters



Video 1. Heavy rain and high winds sweep across Nantucket Island on Monday, September 5, 2016. Image credit: Blair Perkins, @xplorenantucket.

Hurricane

Pesky Hermine Keeps Laboring; Strengthening Newton Heads for Baja California

By: Jeff Masters and Bob Henson , 4:04 PM GMT on September 05, 2016

The two-week marathon that gave us a string of names and a landfalling hurricane--Invest 99L, Tropical Depression 9, Tropical Storm Hermine, Hurricane Hermine, and Post-Tropical Cyclone Hermine--rolls onward, with no immediate end on tap. Hermine remains a strong post-tropical cyclone, with data from an Air Force Hurricane Hunter flight on Monday morning supporting peak surface winds of 70 mph as of the 11 am EDT advisory from the National Hurricane Center.

Located about 230 miles southeast of Montauk, NY, on the eastern tip of Long Island, Hermine is now moving northwest at about 6 mph. Hermine has gotten close enough to the Mid-Atlantic and Northeast shorelines to produce rough surf and riptides, but stayed enough out to keep weather impacts minimal thus far. The maximum storm surge observed on the U.S. coast late Monday morning was 1.6 feet in Chatham, Massachusetts. Showers and thunderstorms far north of Hermine’s center have been gradually shifting toward the west since Sunday, and it will be a windy, showery Labor Day on parts of Cape Cod, Long Island, and nearby islands. Tropical storm warnings remain in effect from eastern Long Island and eastern Connecticut to the south shore of Cape Cod in Massachusetts, with coastal flood advisories along the Mid-Atlantic coast of New Jersey, Delaware, and Virginia, as well as parts of Chesapeake Bay.


Figure 1. Infrared satellite image of Hermine as of 1337Z (9:37 am EDT) Monday, September 5, 2016. The brightest colors correspond to the coldest (highest) cloud tops, although few of these clouds were producing rainfall over New England except for southeast Massachusetts. Image credit: NASA/MSFC Earth Science Office.

Hermine’s brush with Long Island and Cape Cod
For days, computer models had suggested that Hermine would be re-intensifying on Monday and perhaps regaining some tropical characteristics. It now appears that process has been torpedoed by a timing problem. On Sunday, Hermine was located near the north edge of the Gulf Stream, where it had access to 26-28°C waters (at least 2°C above average). The idea was that Hermine would take advantage of that oceanic warmth while an upper-level low to its west would give Hermine a dynamical boost and pull it closer to the U.S. coast. However, the upper low’s influence was delayed, in part because Hermine moved so far east over the weekend. Now Hermine is approaching waters cooler than 26°C, too cool to support much in the way of tropical redevelopment.

Computer models now agree on weakening Hermine gradually over the next couple of days, but residents of coastal New York and southern New England would be smart to keep an eye on it. The 00Z Monday runs of the GFS, European, and UKMET models--our three best track models--keep Hermine moving to the northwest or even west-northwest, bringing it by Tuesday within 200 miles of New York City and only about 100-150 miles south or southeast of Montauk. The models then start to move Hermine east-northeast on Wednesday at varying speeds, taking it just south of Cape Cod. If nothing else, Hermine’s approach will keep surf high and rip current dangerous, especially along beaches east and north of New York City and along the New Jersey coast. There is a chance of tropical-storm-force winds (sustained winds of 39 mph or more) and bursts of heavy rain on eastern Long Island and over far southeast Massachusetts. Storm surge will be less than previously feared, but some minor coastal flooding remains possible, and beach erosion may mount over the next several days. For the latest on local impacts, check the local statements compiled on the NHC website.


Figure 2. Visible satellite image of Invest 92L at 1500Z (11:00 am EDT) Monday, September 5, 2016. Image credit: CIMSS/SSEC/University of Wisconsin-Madison.

Little change to 92L
Satellite images on Monday morning showed that the moderate level of heavy thunderstorm activity associated with a large tropical wave passing through the Lesser Antilles Islands (Invest 92L) had changed little since Sunday. 92L brought winds near tropical storm-force to the Lesser Antilles on Sunday night, according to data from the ASCAT satellite. No rotation of the storm’s echoes was apparent on Martinique radar, though. The wave is fighting a large amount of dry air, which is keeping heavy thunderstorm activity relatively sparse.

A strong and persistent ridge of high pressure should keep 92L on a west to west-northwest path, and the system will be near Jamaica by Wednesday and approach the western tip of Cuba and Mexico’s Yucatan Peninsula by Thursday night. If 92L does develop, it probably won’t last long. The 8 am EDT Monday SHIPS model forecast for 92L showed light to moderate wind shear of 5 - 15 knots through through Tuesday, rising to the high range, 20 - 25 knots, for the latter part of the week. The latest 0Z Monday operational runs of our three top models for forecasting tropical cyclone genesis--the GFS, European and UKMET models--did not show development of the system over the next five days. Almost all of the ensemble members of the 0Z Monday runs of the GFS and European model that did develop 92L showed the storm being destroyed by high wind shear and dry air before reaching Jamaica on Wednesday. In their 8 am EDT Monday Tropical Weather Outlook, NHC dropped their 2-day and 5-day development odds to 10% and 20%, respectively.

New African tropical wave may develop late this week
A tropical wave expected to leave the coast of Africa on Tuesday could develop into a tropical depression by next weekend, a few hundred miles west-southwest of the Cabo Verde Islands, predicted the 00Z Monday runs of the UKMET and European models. The Sahara Desert dust and dry air machine will be moderately active during the week, and development of this new tropical wave will likely be hindered by dry air. In their 8 am EDT Monday Tropical Weather Outlook, NHC gave this future system 2-day and 5-day development odds of 0% and 20%, respectively. The long-range models are showing a west-northwesterly track for this storm into the Central Atlantic to a location where few storms ever become a threat to the Lesser Antilles Islands.


Figure 3. Latest satellite image for Tropical Storm Newton.

Hurricane warnings for Tropical Storm Newton along Mexico’s Baja California Peninsula
A hurricane warning is up for the southern tip of Mexico’s Baja California Peninsula, including Cabo San Lucas, as a strengthening Tropical Storm Newton heads north-northwest at 12 mph. Newton formed in Mexico’s Pacific waters about 200 miles southwest of Puerto Vallarta late Sunday night, and is already bringing very heavy rains and gusty winds to southwestern Mexico, as seen on satellite loops. The Hurricane Hunters are scheduled to investigate Newton on Monday afternoon.

Newton’s top winds had increased to 60 mph as of the 11 am EDT advisory from NHC, and an eye was beginning to appear on satellite imagery. There is potential for Newton to strengthen quite a bit more before its expected landfall on southern Baja California late Tuesday. The 12Z Monday output from the SHIPS statistical model shows a high potential for rapid intensification, with a 40% chance of Newton’s top winds increasing by at least 40 knots (46 mph) in 24 hours. That would make Newton a Category 2 storm by landfall. None of the 06Z Monday computer model runs show that amount of intensification, and it’s possible that dynamical factors not included in the SHIPS model will keep Newton from its full potential.

Newton’s north-northwest motion should take it off the Baja California peninsula and over the very warm waters of the Gulf of California by Wednesday, which could allow to be at or near tropical storm intensity as it approaches Arizona late Wednesday. Ahead of Newton’s large circulation, moisture will surge through the Gulf of California into the U.S. Southwest on Tuesday and Wednesday, fueling showers and thunderstorms. Heavier rains may develop late Wednesday into Thursday, especially over southeast Arizona, as the core of Newton moves inland. Newton’s moisture could also enhance thunderstorms over the southern and central Great Plains late this week.

We’ll be back with our next update on Tuesday. To our readers in the U.S. and Canada, Happy Labor Day!

Jeff Masters and Bob Henson


Figure 4 WU depiction of official forecast for Tropical Storm Newton as of 11 am EDT Monday, September 6.

Hurricane

Hermine to Linger Offshore after Soaking Southeast; Dangerous Surge Still Possible

By: Jeff Masters and Bob Henson , 4:49 PM GMT on September 04, 2016

The storm now known as Post-Tropical Cyclone Hermine continues to spin east of the mid-Atlantic coast with top sustained winds of 70 mph, as of the 11 am EDT Sunday advisory from the National Hurricane Center. Hermine was located about 295 miles south-southeast of the east end of Long Island, NY, and about 310 miles east-southeast of Ocean City, MD. Tropical Storm Warnings remained in effect at 11 am EDT Sunday from Cape Charles Light, Virginia, to west of Watch Hill, Rhode Island, and for locations along Delaware Bay. A Tropical Storm Watch is in effect from Watch Hill, RI, to Sagamore Beach, MA, as well as Block Island, Martha’s Vineyard, and Nantucket.

Hermine moved further east than expected on Saturday, leaving the weather surprisingly mellow on the Mid-Atlantic and New York coastline--but it is still possible that dangerous storm surge could arrive over the next couple of days (see below).


Figure 1. Visible satellite image of Hermine as of 1515Z (11:15 am EDT) Sunday, September 4, 2016. The solid white areas at top coreespond to Hermine’s intense showers and thunderstorms, which are located well north of the low-level center of circulation. Image credit: NOAA/NESDIS.

On Saturday, Hermine’s strong winds drove a large storm surge to the coasts of Virginia and northern North Carolina, causing extensive street flooding and beach erosion. The highest water level was observed at Cape Hatteras, North Carolina--a storm tide of 4.68 feet (height of the water above mean sea level), which was the highest water level observed at that station since it was established in 2010 (previous record: 4.02’ during Hurricane Sandy on October 29, 2012.) The high surge in the North Carolina Outer Banks flooded the only road into these vulnerable barrier islands, NC 12. While there was some sand deposited on the road, no major damage was reported by the North Carolina DOT, and the road was open on Sunday. Both bridges connecting NC 12 to the mainland were closed by high winds for a period on Saturday; the high winds caused a fatal crash of a semi-truck on the US-64 bridge, bringing Hermine’s death toll to two people.

High storm surges were also observed on Saturday along the southern Virginia coast at the entrance to Chesapeake Bay, where three tide gages recorded a storm surge between 3.5 and 4.0 feet. Duck, NC received a 3’ storm surge. On Sunday morning, Hermine’s storm surge had abated along the coast, and was less than two feet everywhere, as seen using our wundermap with the Storm Surge layer turned on, or the NOAA Tides and Currents page for Hermine.


Figure 2. Cars drive on the flooded NC Highway 12 in Hatteras, N.C., Saturday, Sept. 3, 2016 after Tropical Storm Hermine passed the Outer Banks. (AP Photo/Tom Copeland) 

Hermine brings heavy rains
Hermine’s rains had mostly ended along the U.S. coast on Sunday, but the storm had left very heavy rains in its wake. Inland flooding was relatively minimal, thanks in large part to the dry conditions that have prevailed across much of the Southeast over the last few weeks in this drought-stricken summer. According to the 11 am Sunday NOAA Storm Summary, the top rain amounts by state were:

Florida, 18.89” at Baskin
North Carolina, 13.34” at Cedar Island
South Carolina, 9.93” at Myrtle Beach AFB
Georgia, 6.37” at Alma
Virginia, 6.50” at Holiday Island


Figure 3. Observed rainfall for the 7-day period ending at 8 am EDT Sunday, September 4, 2016. Hermine brought 7-day rainfall amounts of 10+ inches (pink colors) to coastal portions of Florida, South Carolina and North Carolina. The highest storm-total rainfall amounts over a 7-day period were near the Tampa Bay, Florida region; 18.89” fell at Baskin and 15.27” at Largo, just north of St. Petersburg. Image credit: NWS/AHPS.

The forecast for Hermine
Thankfully, it appears that Hermine’s impacts along the Mid-Atlantic and Northeast coast will be considerably less than feared just a day ago. Hermine is still expected to track very slowly north from Sunday through Tuesday, as it becomes entangled with a weak upper-level trough to its west. Our three best track models--the Euro, GFS, and UKMET--now agree that Hermine should remain at least 150-200 miles off the Mid-Atlantic and Northeast coasts. This is a bigger margin of safety than Saturday’s model runs had implied. Because Hermine scooted farther east than expected on Saturday, it will now be harder for any potential westward wobble to make much of a difference for conditions along the coast. The official NHC track as of 11 am Sunday reflects the model consensus for a slow northwestward arc in Hermine’s path that would bring it as far west as about 70°W on Monday into Tuesday. Models vary on how quickly Hermine begins shifting east of Cape Cod and out to sea for good, with the ECMWF now calling for this to happen on Tuesday, the GFS on Wednesday, and the UKMET on Thursday. The slower solutions keep Hermine a bit closer to the coast before its departure.

Winds: Even though Hermine is classified as a post-tropical cyclone, it may regain some tropical characteristics over the next day or two as it hovers near the boundary of the Gulf Stream, where sea-surface temperatures of 26-28°C (79-82°F) are at least 2°C above average. Hermine’s peak winds are predicted by NHC to increase to minimal hurricane force (65 knots, or 74 mph) from Monday into Tuesday. Any hurricane-strength winds would be focused in a small area near Hermine’s center. However, tropical storm force winds already extend up to 205 miles from the center, and it is possible that some coastal areas will experience sustained tropical storm force winds (39 mph or greater) as Hermine wobbles its way slowly northward. Such winds are most likely on eastern Long Island, Cape Cod, and nearby islands, including Nantucket and Martha’s Vineyard. Note that Hermine could still be classified as a post-tropical cyclone even if its top winds regain hurricane force.


Figure 4. WU depiction of the official NHC track for Hermine as of 1500Z (11 am EDT). NHC may continue to keep Hermine classified as a post-tropical cyclone even if its peak winds regain hurricane-force strength, as depicted here.

Rainfall: Hermine’s low-level center was devoid of showers and thunderstorms on Sunday morning. We can expect some redevelopment around the north and west sides of the center by Sunday night and Monday, as Hermine shifts toward a more subtropical-cyclone structure. Extensive heavy rains now appear unlikely along the Mid-Atlantic and Northeast coast, given Hermine’s expected distance and the large amount of dry air being entrained on Hermine’s northwest side. Scattered showers and thunderstorms around Hermine’s periphery could drop 1” or 2” at some coastal locations. If Hermine’s precipitation shield grows bigger than expected on Monday, those rains could edge a bit farther inland.

Storm surge: The threat of a severe storm surge has gone down since Saturday due to Hermine’s more eastward location, but storm surge remains a significant threat that could produce major flooding in some locations. There is still high confidence on the long duration of this event, which raises the odds of back-bay flooding that could intensify over several days as water is continually pushed inland.

Shown below are NWS predictions as of midday Sunday for the potential peak storm surge levels at selected locations. The total water height above sea level, called the storm tide, will vary up or down from these numbers by several feet depending on whether the peak falls at high vs. low tide. Access to barrier islands of New Jersey and New York may be compromised by surge-related flooding. For the latest on potential impacts, be sure to check local statements compiled on the NHC website.

Maryland beaches: 2’ or more
Delaware beaches:  3’ or more
New Jersey beaches:  3’ or more
NY Harbor, Long Island (south shore and east bays): 2’ - 4’
Eastern Long Island Sound, Eastern South Shore Bays of Long Island: 2’ - 4’


Figure 5. Latest satellite image of 92L.

92L growing more organized
Satellite images on Sunday morning showed that the moderate level of heavy thunderstorm activity associated with a large tropical wave passing through the Lesser Antilles Islands on Sunday (Invest 92L) had increased in intensity and organization since Saturday. The storm was bringing winds near tropical storm-force to the Lesser Antilles on Sunday morning. La Desirade (Guadeloupe) reported sustained winds of 38 mph, gusting to 57mph at 9 am Sunday in a strong rain band that dumped 0.14” (3.6 mm) of rain; at 11 am, winds had decreased to a sustained 27 mph at the site. Some rotation of the storm’s echoes was apparent on Martinique radar, so we have to watch this tropical wave today to monitor it for further signs of development.

A strong and persistent ridge of high pressure should keep 92L on a fairly straightforward westward path, and the storm will move through the Lesser Antilles Islands on Sunday and be near Jamaica by Wednesday. If 92L does develop, it probably won’t last long. The 8 am EDT Sunday SHIPS model forecast for 92L showed moderate wind shear of 15 - 20 knots through through Tuesday, rising to the high range, 20 - 35 knots, for the latter part of the week. The latest 0Z Sunday operational runs of our three top models for forecasting tropical cyclone genesis--the GFS, European and UKMET models--did not show development of the system over the next five days. More than 40% of the 70 ensemble members of the 0Z Sunday runs of the GFS and European model did show 92L developing into a tropical depression or weak tropical storm by Monday, but all of them showed the storm being destroyed by high wind shear and dry air before reaching Jamaica on Wednesday. In their 8 am EDT Sunday Tropical Weather Outlook, NHC kept their 2-day and 5-day development odds at 20% and 30%, respectively.

A tropical wave expected to leave the coast of Africa on Tuesday could develop into a tropical depression by next weekend, a few hundred miles west-southwest of the Cabo Verde Islands, predicted the 00Z Sunday runs of the UKMET and European models. The NASA/GMAO model predicted that Sahara Desert dust and dry air machine would be moderately active during the week, and development of this new tropical wave will likely be hindered by dry air.

90E in the Eastern Pacific a threat to Mexico’s Pacific coast
A large and well-organized tropical disturbance (Invest 90E) is already bringing very heavy rains and gusty winds to southwestern Mexico, as seen on satellite loops. These conditions are likely to continue for the next few days, and NHC is predicting total rainfall amounts of 5 - 10”, with isolated totals up to 15”, across the southern Mexico states of Oaxaca, Guerrero, Michoacan, Colima, Jalisco, and Nayarit. The European and UKMET models predict that 90E will develop into a tropical storm that will hit Mexico’s Baja Peninsula late this week. In their 8 am EDT Tropical Weather Outlook NHC gave 90E 2-day and 5-day odds of development of 80% and 90%, respectively. The Hurricane Hunters are on call to investigate this system on Monday.

We'll be back with our next update around midday Monday.

Jeff Masters and Bob Henson

Hurricane Flood

High Winds From Hermine Lash Mid-Atlantic, Drive Dangerous Storm Surge

By: Bob Henson and Jeff Masters , 4:48 PM GMT on September 03, 2016

Tropical Storm Warnings were flying from northern North Carolina to New York and Connecticut late Saturday morning as Tropical Storm Hermine emerged over the waters off the coast of North Carolina. Hermine rolled off the Outer Banks of North Carolina near Nags Head around 8:00 am EDT Saturday, moving east-northeast at 15 mph, and was located about 80 miles southeast of Norfolk, VA as of the 11:00 am EDT advisory from NHC. Hermine completed the transition from a tropical storm to an extratropical storm Saturday morning, and is now officially called Post-Tropical Cyclone Hermine. A post-tropical cyclone is a storm that derives its energy from atmospheric dynamics and contrasts rather than from the heat of the ocean. In a post-tropical cyclone, the circulation is often tilted northward from the surface to upper levels, resulting in an asymmetric comma shape typical of a mid-latitude winter storm. Radar and satellite imagery shows this configuration in place for Hermine: the strongest thunderstorms on Saturday morning were more than 200 miles northeast of Hermine’s low-level center. The new version of Hermine is just as powerful as the old one, though, with top winds now back up to 65 mph. Tropical-storm-force winds extend out more than 200 miles northeast of Hermine’s center. Duck Pier, NC, recently reported a sustained wind of 58 mph gusting to 73 mph.


Figure 1. MODIS visible satellite image of Hermine taken at 12:30 pm EDT September 3, 2016. At the time, Hermine was a post-tropical cyclone with top sustained winds of 70 mph. Note the asymmetric shape of the storm, with a comma-like configuration, instead of circular. Image credit: NASA.

Some of Hermine’s top winds thus far, courtesy of weather.com:

    •    79 mph at C-Tower, south of St. George Island, south of Apalachicola at an elevation of 115 feet, with sustained winds of 61 mph late Thursday
    •    75 mph early Thursday evening near Indian Shores Beach in western Pinellas County, Florida
    •    67 mph in Keaton Beach, Florida
    •    64 mph gusts at Florida State University's football stadium in Tallahassee 
    •    62 mph at Elizabeth City, North Carolina, early Saturday morning.
    •    62 mph at St. Petersburg's Albert Whitted Airport and near Clearwater Beach on Thursday evening.
    •    59 mph near Folly Island, South Carolina on Friday evening.
    •    58 mph near Dewees Island, South Carolina with sustained winds of 40 mph on Friday afternoon.
    •    55 mph at Norfolk International Airport, Virginia, Saturday morning.
    •    54 mph at Shaw Air Force Base.
    •    53 mph in Apalachicola, Florida on Thursday evening and in Brunswick, Georgia on Friday afternoon.
    •    52 mph at Clearwater Beach, Florida later Thursday evening, and a 51 mph gust at Cedar Key, Florida.
    •    51 mph at Virginia Beach, Virginia, early Saturday morning.
    •    49 mph in Charleston, South Carolina on Friday evening.


Figure 2. Residents look at Alligator Point Road, which collapsed during the storm surge from Hurricane Hermine at Alligator Point, Florida on September 2, 2016. (Mark Wallheiser/Getty Images) 

Hermine brings heavy rains
Hermine is being fed by unusually high amounts of atmospheric water vapor, and continues to dump very heavy rains along its path. According to the 11 am Saturday NOAA Storm Summary, the top rain amount in North Carolina was 13.34” at Cedar Island; South Carolina’s highest was 9.93” at Myrtle Beach AFB, and Georgia’s highest amount was 6.37” at Alma. Hermine had already dumped 3.95” on Virginia Beach, Virginia, with more rain to come.


Figure 3. Observed rainfall for the 1-day period ending at 8 am EDT Saturday, September 3, 2016. Hermine brought 24-hour rainfall amounts of 6+ inches (pink colors) to coastal portions of South Carolina and North Carolina. The highest storm-total rainfall amounts over a 4-day period were near the Tampa Bay, Florida region; 18.89” fell at Baskin and 15.27” at Largo, just north of St. Petersburg. Hermine’s rains mostly missed Florida’s Lake Okeechobee region; on Thursday, the Army Corps of Engineers decided that the lake’s vulnerable dike is safe at the current water level, and elected not to increase the amount of polluted water released from the lake into its outflow canals that lead to the ocean. In May and June, large releases of Lake Okeechobee water to relieve pressure on the dike caused massive algae blooms and serious water quality issues along both coasts of Florida. Image credit: NWS/AHPS.

Hermine’s strange journey ahead: a unusual danger for the Mid-Atlantic and Northeast coast
Despite its new classification as a post-tropical system, Hermine will continue to be tracked through advisories by the National Hurricane Center until it no longer poses a threat to land. They will have a difficult job: over the next few days, Hermine will take one of the oddest and most unsettling trajectories in memory for a named storm along the U.S. East Coast. The upper-level trough that pulled Hermine northeastward is now leaving it behind, and steering currents will become very weak. As a result, Hermine will spin for several days in the region east of the Mid-Atlantic and south of New England, gradually working its way northward. Because Hermine will be slowing to a crawl close to the north edge of the Gulf Stream on Sunday and Monday, it will be near or atop sea-surface temperatures of 26-28°C (79-82°F), which is at least 2°C above average and more than warm enough to support tropical development. Instability in the atmosphere will be enhanced by some residual cold air aloft, a fragment of the departing trough. As a result of all this, Hermine is likely to re-organize from Sunday into Monday into a more symmetric, warm-core system, perhaps embedded within the weak upper low fragment.

It’s unclear whether Hermine will again be technically classified as a tropical storm or hurricane rather than a post-tropical storm, but that point is moot in terms of impact. Computer models are near-unanimous in bringing Hermine’s top winds up to or above hurricane strength for at least a few hours on Sunday night or Monday. NHC’s official outlook at 11 am NHC outlook puts Hermine at minimal hurricane strength from Sunday night to Tuesday morning. Hermine’s peak winds are expected to remain stronger than their current 65 mph until at least Wednesday.

Our best track models--the GFS, Euro, and UKMET--agree that Hermine will remain within about 150 to 250 miles of the coast till at least the middle of next week before a more definitive move out to sea. Because steering currents are so weak, we cannot yet be certain how long Hermine will linger nearby. We can expect some erratic small-scale motion, perhaps including one or more clockwise or counterclockwise loops, especially from Sunday through Tuesday. Any of these small jogs might bring Hermine closer to, or farther from, the mid-Atlantic and Northeast coastline. Fortunately, there is no sign of any large-scale upper-level feature that would pull Hermine well inland, and eventually the polar jet stream will dip far enough south to haul Hermine out to sea.


Figure 4. Forecast from the 06Z Saturday run of the GFS model (one of our three most reliable track models) for the surface center of Hermine. From top left to lower right, the valid time periods are 00Z (8:00 pm EDT) Saturday, September 3, through 00Z Thursday, September 8. Wind speeds are shown in knots; add 15% for miles per hour. Green colors denote winds of at least tropical storm strength (34 knots or 39 mph); purple colors denote minimal hurricane strength (65 knots or 74 mph). Image credit: tropicaltidbits.com.

Major storm surge flooding expected along mid-Atlantic, Northeast coast
The next phase of Hermine could easily be more destructive than its Florida landfall. An unusually prolonged period of high surf, beach erosion, and storm-surge flooding will unfold along the coastline from Delaware to New York. In some places, the peak surge could be comparable to that experienced during Hurricane Irene in 2011, with at least some level of high water persisting for days on end. There is very high confidence on the long duration of this event, which raises the odds of back-bay flooding that could intensify over several days as water is continually pushed inland. Multiple days of strong wind and heavy surf are likely to produce enormous amounts of damaging beach erosion.

One complicating factor with Hermine is that the amount of coastal flooding may end up far out of proportion to what local weather conditions would lead you to expect. The water will be driven into the coast mainly by the processes unfolding offshore. There will likely be a sharp cutoff to the heaviest rain and highest wind associated with Hermine (see Figure 5 below). Outside of this zone, it could be merely breezy and partly cloudy; inside it, you could experience torrential rain and tropical storm-force wind. The transition zone may wobble inland or offshore at times, making it hard to gauge exactly what to expect at any given spot. Tourists and residents should keep this nontraditional storm behavior in mind. It would be prudent to avoid flood-prone areas and roadways near the coast even if the weather doesn’t seem particularly threatening. Rip currents will add to the danger of the rough surf, making this a very good Labor Day weekend to stay off the beach. Unlike most tropical and winter storms, the storm surge and associated flooding may persist for several days in some areas.

Shown below are NWS predictions as of midday Saturday for the potential peak storm surge levels over the next several days. The total water height above sea level, called the storm tide, will vary up or down from these numbers by several feet depending on whether the peak falls at high vs. low tide. For the latest on potential impacts, be sure to check local statements compiled on the NHC website.

Southside Hampton Roads, VA: 5.5’ to 7.0’, with ocean waves greater than 15’ and bay waves of 6-9’
Eastern Shore of the Chesapeake Bay, VA: 5 - 5.5’, with 3-5’ waves on James River
Atlantic City, NJ:  2’ - 4’
Sandy Hook, NJ:  2’ - 4’
Coastal New York and Long Island Sound, including parts of Connecticut and New Jersey: Significant surge with moderate to major coastal flooding (details to come)
Coastal Rhode Island and Massachusetts: 1’ - 2’ feet, except 2’ - 3’ possible from Westerly to Point Judith, RI


Figure 5. Projected 5-day precipitation totals from 12Z (8:00 am EDT) Saturday, September 3, to Thursday, September 8. A huge contrast is evident between the torrential amounts expected near the center of Hermine--where more than 20” of rain could fall--and the much lesser amounts over most inland areas. A small nudge in Hermine’s notion could bring significantly heavier amounts to the coastline than shown here. Image credit: NOAA/NWS Weather Prediction Center.

What’s next in the Atlantic?
We’re two weeks into the peak part of the Atlantic hurricane season, with the half-way point coming up on September 11. Sea surface temperatures (SSTs) are at their maximum now, basin-wide wind shear is at its minimum and the African monsoon is at its peak, so we should expect near-average activity with one or two more named storms in the coming two weeks. The Madden Julian Oscillation (MJO), a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days, is currently weak, and will not act to discourage Atlantic tropical storm formation, like it was doing in mid-August, when it was centered in the Western Pacific. The only major impediment for tropical storm formation would seem to be a higher than average amount of dry air coming off the coast of Africa for this time of year.


Figure 6. MODIS visible satellite image of 92L taken on Saturday morning September 3, 2016. Image credit: NASA.

Satellite images show that shower activity has increased over the past day in association with a large tropical wave located about 500 miles east of the Lesser Antilles Islands (Invest 92L). However, the tropical wave was embedded in a major area of dust and dry air from the Sahara Desert, which was slowing development and making 92L’s thunderstorms disorganized. The 8 am EDT Saturday SHIPS model forecast for 92L shows low wind shear and warm SSTs for the next four days--favorable for development--as it moves west at 15 - 20 mph, but shows the air surrounding 92L will grow even drier as it enters the eastern Caribbean, which should keep any development slow to occur. The latest 0Z Saturday runs of our three top models for forecasting tropical cyclone genesis--the GFS, European and UKMET models--did not show development of the system over the next five days. A strong and persistent ridge of high pressure should keep 92L on a fairly straightforward west to west-northwest path, and the storm will move through the Lesser Antilles Islands on Sunday and be near Jamaica and Haiti on Tuesday. By Wednesday, when 92L will enter the Western Caribbean, it will find a moister environment and have more potential for development. In their 2 pm EDT Saturday Tropical Weather Outlook, NHC bumped up their 2-day and 5-day development odds to 20% and 30%, respectively. The Hurricane Hunters are scheduled to investigate 92L on Monday afternoon, if necessary.

A tropical wave expected to leave the coast of Africa on Tuesday could develop into a tropical depression by Wednesday a few hundred miles west-southwest of the Cabo Verde Islands, predicted the 00Z Saturday run of the UKMET model. The 0Z Saturday run of the European model and 06Z Saturday run of the GFS model did not develop this tropical wave over the next five days, but did show development after that time. The NASA/GMAO model predicted that Sahara Desert dust and dry air machine would be moderately active during the week, and this new tropical wave will likely have its development hindered by dry air.

Lester cruising well north of Hawaii
Hawaii’s prolonged two-part threat of the past week from Hurricane Madeline and Hurricane Lester is finally winding down. As of 11 am EDT (5 am HST) Saturday, Lester was located 260 miles east of Honolulu, with top sustained winds down to 100 mph. As expected, Lester is tracking parallel to the islands on a west-northwest track, and it appears that the model consensus was on target in keeping that track far enough north of the islands (about 100-150 miles) to avoid major trouble. The Hurricane Watch for the islands has now been cancelled, but dangerous surf is still expected.

Bob Henson and Jeff Masters

Hurricane Flood

Hermine Hits Florida, Heads Towards Mid-Atlantic

By: Jeff Masters and Bob Henson , 4:35 PM GMT on September 02, 2016

Now a tropical storm, Hurricane Hermine hit the coast of Florida near St. Marks at approximately 1:30 am EDT Friday, September 2, 2016, as a Category 1 storm with sustained winds of 80 mph. Hermine was Florida’s first hurricane strike in nearly eleven years, since Hurricane Wilma of October 2005, and was the first hurricane to strike the U.S. since Hurricane Arthur hit North Carolina on July 3, 2014 as a Category 2 storm with 100 mph winds. One death is being blamed on Hermine so far, due to a falling tree in Marion County, about 150 miles southeast of where the center of the storm made landfall. As of 11 am EDT Friday, Hermine was located inland about 50 miles west-southwest of Savannah, GA, moving northeast at 18 mph. Hermine’s top sustained winds were down to 50 mph.


Figure 1. Radar image of Hurricane Hermine from the Tallahassee, Florida radar near the time of landfall, at 1:16 am EDT September 2, 2016.

Hermine brings a storm surge of 7.5’ to Cedar Key
Hermine brought a storm surge in excess of three feet to over 200 miles of the Florida Gulf Coast, from Apalachicola to Tampa Bay. The highest storm surge observed at any tide gage was 7.5’ at Cedar Key, Florida. The water level (storm tide, which includes storm surge and tide) rose to 7.64’ above mean sea level at 1:36 am EDT September 2; according to a Friday blog post by storm surge expert Dr. Hal Needham, this was the 5th highest water level since records began in 1880 at Cedar Key, and the highest in 23 years—since the Superstorm of March 1993. Hermine brought a maximum storm surge of approximately 4.2’ to Apalalchicola, 3.7’ at Tampa and 3.5’ at St. Petersburg. Late Friday morning, the storm surge from Hermine was still more than two feet high from Cedar Key to Tampa, and had built to one foot at Charleston, South Carolina.


Figure 2. Still frame from a September 1, 2016 Weather Channel Facebook video of the Cedar Key storm surge.


Figure 3. A street is blocked from debris washed up from the tidal surge of Hurricane Hermine Friday, Sept. 2, 2016, in Cedar Key, Florida. AP Photo/John Raoux.

High winds from Hermine cause widespread power outages
Hermine struck a relatively unpopulated stretch of the Florida coast with few observation sites. The strongest winds on land appear to have been at a Personal Weather Station (PWS) at Alligator Point, Florida in the northern eyewall of Hermine: sustained winds of 59 mph, gusting to 78 mph, at 10:25 pm EDT September 1, 2016. This station experienced the calm of the eye for over 50 minutes; it is interesting to see the upward spike in temperature of 3°F that occurred in the eye just after midnight. The highest winds at an offshore site were measured at Tyndall AFB Tower C, located about 20 miles south of Apalachicola, Florida: sustained winds of 61 mph, gusting to 79 mph, at 8:50 pm EDT. The anemometer on the tower is at an elevation of 35 meters, which is higher than the standard 10 meters used to reference surface winds, so these winds need to be scaled down to what they would be at a height of 10 meters for a valid comparison to other surface wind measurements. There were no measurements of hurricane-force surface winds during Hermine’s life, except by the Hurricane Hunters. Hermine’s strong winds brought down large numbers of trees, and about 253,000 customers were without power at the height of the storm on Friday morning in Florida. An additional 70,000 customers lost power in Georgia.


Figure 4. Observed rainfall for the 1-day period ending at 8 am EDT Friday, September 2, 2016. Hermine brought 24-hour rainfall amounts of 6+ inches to portions of northern Florida and Southern Georgia. The highest storm-total rainfall amounts over a 4-day period were near the Tampa Bay, Florida region; 18.89” fell at Baskin and 14.60” at Largo, just north of St. Petersburg. Hermine’s rains mostly missed Florida’s Lake Okeechobee region; on Thursday, the Army Corps of Engineers decided that the lake’s vulnerable dike is safe at the current water level, and elected not to increase the amount of polluted water released from the lake into its outflow canals that lead to the ocean. In May and June, large releases of Lake Okeechobee water to relieve pressure on the dike caused massive algae blooms and serious water quality issues along both coasts of Florida. Image credit: NWS/AHPS.

Extremely high moisture available to Hermine
Near record-warm ocean temperatures in the Gulf of Mexico are evaporating near-record amounts of water vapor into the atmosphere for Hermine to feed off of, and this moisture-laden air is surging northeastwards with Hermine. At 8 am EDT Friday, the upper-air balloon sounding at Charleston, South Carolina measured 2.60” of total precipitable water (TPW)—the amount of water that would result if one condensed all the water vapor in a column above and precipitated it out. This value ranked as the 9th highest TPW measurement of the 47,000+ balloon soundings taken at the site since 1948. On Thursday, Tampa, Florida measured its 2nd highest TPW measurement on record: 2.80”, during a special 2 pm EDT balloon sounding.

Correction: We reported earlier that an astonishing 3.25" of TPW was recorded this morning at the upper air sounding site at Nassua in the Bahamas--more than 5 standard deviations above normal. This reading was based on a daily summary prepared by the National Weather Service. However, Brian Brettschneider pointed out that this must be an error, since the site's previous highest TPW was only 2.8", and a cross check of sounding data from the University of Wyoming reveals that the TPW at Nassau this morning was actually 1.60".

The forecast for Hermine: Major storm impacts still to come
As of Friday morning, Tropical Storm Warnings extended up the Atlantic coast to Virginia, with Tropical Storm Watches now extending all the way to the Connecticut coast and all of Long Island, New York, including the New York metropolitan area. Hermine will continue rolling along or near the Southeast coast on Friday, gradually weakening before it emerges off North Carolina’s Outer Banks on Saturday. By that point, Hermine will have undergone at least a partial conversion into an extratropical (post-tropical storm), deriving energy from atmospheric dynamics rather than from the heat energy of the ocean. By later in the weekend, however, Hermine may again be resembling a tropical cyclone, as upper-level winds along its path weaken and upper-level ridging to its west, north, and east blocks its path.

Computer models are in agreement that Hermine will slow down Sunday into Monday in the Atlantic east of Delaware and south of Long Island, NY, roughly 150 to 200 miles offshore. The models suggest that Hermine may even carry out one or two tight cyclonic loops in this area. From Sunday morning through Wednesday morning, the official NHC forecast moves Hermine only about 200 miles to the northeast—an average motion of only 3 mph.


Figure 3. The official track forecast for Hurricane Hermine as of 11 pm EDT Thursday. Hermine’s dramatic slowdown is evident in the period from Sunday to Tuesday.

On Sunday, Hermine will be positioned near the north edge of the Gulf Stream, with SSTs of 27-28°C (81-82°F). These are more than 2°C above average for this time of year and more than adequate to support tropical development, regardless of whether Hermine is classified as a tropical or post-tropical storm by that point. As shown above, the 11 am EDT Friday outlook from NHC increases Hermine’s winds to the Category 1 threshold of 65 knots (75 mph) on Monday morning, with the center about 150 miles east of the Maryland shore. Hermine will likely weaken only gradually after that point, as it spins and crawls over the next several days. (Note that even if Hermine does become post-tropical, NHC will continue issuing advisories on Hermine as long as it remains a significant threat to land.)


Figure X. Sea surface temperatures for the last week of August 2016 (left) and departures from normal for the time of year (right), both in degrees C. Image credit: NOAA/NHC.


Torrential rains were pushing northward from Georgia to North Carolina on Friday morning, extending from the coast up to 200 miles inland. Flash flood watches are in effect from Georgia to Virginia along a belt northwest of tropical storm warnings. Rains of 4” - 8” totals will be widespread through this area, especially close to the coast. The dry weather in recent weeks will help tamp down the risk of large-scale flooding, although flash floods are still a risk whenever very heavy rain falls in a short amount of time. Rains of 1” - 5” and gusty winds will work their way up the Delaware and New Jersey coastlines from Saturday into Sunday, with the heaviest rains perhaps staying just offshore of the New York coast. There will likely be a sharp cutoff on the western side of the heaviest rain, so travelers and residents should be aware that a journey of just 50 miles or so toward or away from the coast could produce big changes in the weather you experience. Along with high surf, rip current risk will be high across many beaches for the next several days.

Major impacts possible on Mid-Atlantic and Northeast shoreline
Hermine is shaping up to be a prolonged, high-impact event for coastlines from Delaware to Massachusetts. As the center moves very slowly, Hermine will push vast amounts of water toward beaches, bays, and inlets. The winds and seas rotating around Hermine will have the greatest impact along the coast of central and northern New Jersey, Long Island, and Long Island Sound—the same areas hit hardest by Hurricane Sandy in 2012. Because Hermine is much weaker than Sandy and will not be plowing onshore as Sandy did, the highest water levels along the immediate coast are expected to stay well below the Sandy peaks, and coastal winds will be considerably less. However, because Hermine will be positioned offshore for several days, there will be a cumulative impact from relentless, battering waves and swells, and beach erosion could be severe. In addition, water will pile up in back bays over multiple tidal cycles, producing a prolonged rise in water levels and the potential for significant flooding in those areas.


Figure X. Although Tropical Storm Hermine is expected to remain off the mid-Atlantic coast, it will still generate large storm surge to its north and west. Guidance from the NOAA ESTOFS model indicates that the storm surge from Hermine could reach or exceed 4 - 5 feet on Sunday night along most of the New Jersey coast, the western Long Island coast, and western Long Island sound. Tides could either add or subtract as much as 2 - 3 feet to this total. This map is not an official forecast of storm surge or local impacts, since the timing and intensity of any storm surge is likely to evolve over the next several days. Image credit: NOAA Ocean Prediction Center.


Based on the GFS model forecast, NOAA’s ESTOFS storm surge model projects that a storm surge greater than 5 feet is possible along parts of the New Jersey coast at the western end of Long Island Sound late Sunday (see graphic). If this occurred at high tide, it could produce total water heights of more than 7 feet above sea level, which would be comparable to the expected surge impact of a Category 1 hurricane in the western sound, which adjoins low-lying LaGuardia International Airport.

The record surge from Sandy was in a class by itself, and Hermine will not cause the massive destruction of homes and businesses that Sandy did. However, in some locations, Hermine’s surge could be on par with levels observed in other noteworthy hurricanes and extratropical storms, including Hurricane Irene of 2011. This could lead to extensive disruption of everyday life in coastal areas of New Jersey and New York. The exact surge impact and the hardest-hit locations will depend on Hermine’s exact track and intensity Sunday and Monday, which are still too far away to pin down. It’s important to note that this surge-related flooding could easily occur even if the heaviest rains from Hermine remain just offshore, so residents may be at risk even where it hasn’t rained much. On Friday morning, the New York NWS cautioned that “Moderate to possibly major coastal flooding is probable” during high tide as early as midday Sunday, with storm surge of 3-5 feet atop high tide in western Long Island Sound and New York Harbor. The storm surge from Hurricane Irene in New York Harbor (omitting the tide) was 3.8 feet. Those living or traveling in flood-prone areas should keep abreast of NWS local statements on Hermine and heed any and all evacuation advice from local officials.

Weakening Gaston heads through the western Azores
A Tropical Storm Warning is up for the islands of the western and Central Azores as Tropical Storm Gaston speeds east-northeast at 18 mph through the western Azores. Down to just 70 mph winds as of 11 am EDT Friday, Gaston is expected to produce total rain accumulations of 1 to 3 inches over the western and central Azores through Saturday, when it is expected to become post-tropical. On Wednesday morning, Gaston peaked as an impressive Category 3 hurricane with 120 mph winds, becoming the season’s only major hurricane in the Atlantic thus far. The Azores only averages one hurricane strike per decade, and has already seen one this year: 2016’s first Atlantic storm, Hurricane Alex, which struck the island of Terceira in the central Azores on January 15 as a bizarrely out-of-season tropical storm in January. Alex did minimal damage and caused no direct deaths.


Figure X. Infrared satellite image of Hurricane Lester as of 1445Z (10:45 am EDT and 4:45 am HST) Friday, September 2, 2016.

Hurricane Watch continues for Hawaii ahead of Category 2 Lester
It’s quite rare to have two separate parts of the United States threatened by two hurricanes at the same time. So it was on Thursday night, with Hurricane Lester continuing to bear down on Hawaii. A hurricane watch is now in effect for the islands of Maui, Molokai, Lanai, Kahoolawe, and Oahu, including the Honolulu area. The Big Island has been dropped from this watch, since Lester is now too far north for its west-northwest track to affect the island.

As of 11 am EDT Friday (5:00 am HST), Lester was about 435 miles east of Hilo. Lester is now a Category 2 storm, with top sustained winds down to 110 mph as Lester travels over waters churned up by Hurricane Madeline just a couple of days ago. Computer models continue to agree on taking Lester along a west-northwest path that will parallel the Hawaiian island chain on Saturday, most likely as a Category 1 hurricane. The model guidance is now in fairly close and consistent agreement on a track just north of the islands, but the nearness of that track—perhaps within 100 miles—is enough to require continued vigilance. Huge surf can be expected regardless of the exact track. If Lester stays north as expected, the islands will be on its weaker left-hand side, reducing the chance of any major impact.

Invest 92L trying to organize in tropical Atlantic
Satellite images show that shower activity has increased in association with a large tropical wave located about 1000 miles east of the Lesser Antilles islands. Originally designated Invest 92L on Monday, this wave was no longer deemed worthy by NHC as an “Invest”, and they stopped issuing their suite of model forecasts for the system Tuesday through Thursday. The tropical wave was embedded in a major area of dust and dry air from the Sahara Desert, which was preventing development. However, on Friday morning, NHC resumed their interest in this system, and the latest SHIPS model forecast for 92L shows low wind shear and warm SSTs for the next five days, favorable for development, as it moves west at 10 - 15 mph. The latest 0Z Friday runs of our three top models for forecasting tropical cyclone genesis--the GFS, European and UKMET models—did not show development of the system over the next five days, and the large region of dry air that 92L is embedded in will keep any development slow. A strong and persistent ridge of high pressure should keep 92L on a fairly straightforward west to west-northwest path, and the storm will likely move through the Lesser Antilles Islands on Sunday, and be near Hispaniola on Monday or Tuesday. In their 8 am EDT Friday Tropical Weather Outlook, NHC kept their 2-day and 5-day development odds at 10% and 20%, respectively.

We’ll be back with our next update by midday Saturday.

Jeff Masters and Bob Henson

Hurricane

Major Storm Surge, Massive Rains as Hurricane Hermine Sweeps Ashore

By: Bob Henson and Jeff Masters , 4:14 AM GMT on September 02, 2016

Widespread storm surge was barreling into Florida’s northeast Gulf Coast late Thursday night with the approach of Hurricane Hermine. The warm waters of the eastern Gulf fueled an well-advertised strengthening of Hermine on Thursday afternoon and evening. Hermine was an 80-mph Category 1 hurricane as of the midnight update from the National Hurricane Center. NHC placed the center of Hermine about 40 miles southeast of Tallahassee, FL, just an hour or two from making landfall. Thunderstorms were wrapped around a semi-distinct eye, and heavy bands of rain were clearly evident on radar. An especially intense belt of rain was moving across the northernmost FL peninsula late Thursday.

A Hurricane Warning remained in effect from Suwanee River to Mexico Beach, FL. A variety of other hurricane and tropical storm watches and warnings plastered the Gulf and Atlantic coasts from Florida all the way to northern New Jersey (see below for more on Hermine’s expected track). With Hurricane Gaston also active in the Central Atlantic, we now have multiple hurricanes in the Atlantic for the first time since the first week of September 2012, when Hurricane Leslie and Hurricane Michael were both active. Hermine will be the first hurricane to strike Florida since Wilma hit South Florida as a Category 3 storm in October 2005. Hermine will also be the first hurricane to strike the U.S. since Hurricane Arthur hit North Carolina on July 3, 2014 as a Category 2 storm with 100 mph winds.


Figure X. Hermine approaching landfall in Florida at 6:15 pm EDT September 1, 2016.

Observations this evening
A Personal Weather Station (PWS) at Alligator Point in the northern eyewall of Hermine measured sustained winds of 59 mph, gusting to 78 mph, at 10:25 pm EDT, and had picked up 2.15” of rain since 11 am.

Tyndall AFB Tower C, located about 20 miles south of Apalachicola, Florida, recorded sustained winds of 61 mph, gusting to 79 mph, at 8:50 pm EDT. The anemometer on the tower is at an elevation of 35 meters, which is higher than the standard 10 meters used to reference surface winds, so these winds need to be scaled down to what they would be at a height of 10 meters for a valid comparison to other surface wind measurements.

A PWS on Cedar Key, Florida recorded sustained winds of 44 mph, gusting to 52 mph, at 11:15 pm EDT.

A coastal C-MAN station at Keaton Beach, Florida, in the stronger eastern eyewall of Hermine, measured sustained winds of 45 mph, gusting to 58 mph, at 11 pm EDT.


Figure 2. Regional radar of Hurricane Hermine at 11 pm EDT September 1, 2016, with three time traces of the storm surge at water gages in Apalachicola, Cedar Key and Tampa, Florida. The storm surge (green line) had peaked in Tampa at this time, but was still rising at Apalachicola and Cedar Key. Surge images were taken from NOAA Tides and Currents page for Hermine.


Storm surge a major concern overnight
Even after Hermine’s expected landfall between 1 and 2 am Friday near the head of Apalachee Bay, the strong winds east and south of its center will continue to shove water into the bay and points east along Florida’s Big Bend. This is the bay’s first direct hit from a hurricane in half a century—since Alma, in 1966. NHC warned that surge levels could reach as high as 9 feet in parts of Apalachee Bay and the Big Bend. Fortunately, this is one of the most sparsely settled parts of the Florida coast, but widespread inundation in such places as the flood-prone community of Cedar Key will have a major impact. By 11 pm EDT Thursday, the surge had climbed to 7.2 feet at Cedar Key and 4.0 feet at Apalachicola. HIgh tide will be around 3 am EDT in Cedar Key, which means that the total water level (surge plus tide) could still be rising even after the surge itself peaks.

According to the U-Surge project, headed by storm surge expert Hal Needham, Hermine is likely to produce the highest water level for many locations between St. Marks and Clearwater since the March 1993 “Storm of the Century”. That extremely powerful winter-type storm produced a record surge of 6 to 12 feet across the region.

The forecast for Hermine
Hermine will be rolling along or near the Southeast coast on Friday, emerging off North Carolina’s Outer Banks by Saturday. At that point, Hermine will probably have undergone at least a partial conversion into an extratropical (post-tropical storm), deriving energy from atmospheric dynamics rather than from the heat energy of the ocean.

By later in the weekend, however, Hermine may regain some of its tropical characteristics. Computer models indicate it will slow down Sunday into Monday and perhaps even carry out a tightly looping path east of the Delmarva and south of Long Island, NY. Ocean temperatures are more than 2°C above average in this region. This may be warm enough to allow Hermine’s winds to restrengthen close to hurricane force near its center by Monday or Tuesday, regardless of whether it is classified as a tropical or post-tropical storm by that point. Even if it does become post-tropical, NHC will continue issuing advisories on Hermine as long as it remains a significant threat to land.


Figure 3. The official track forecast for Hurricane Hermine as of 11 pm EDT Thursday. Hermine’s dramatic slowdown is evident in the period from Sunday to Tuesday.

The biggest impact from Hermine after landfall will be a swath of torrential rain stretching from far north Florida to eastern North Carolina, extending from the coast up to 200 miles inland. Flash flood watches are in effect from Georgia to Virginia along a belt northwest of tropical storm warnings. Rains of 5” - 10”, with local totals on the order of 15”, will affect far northern Florida and southern Georgia, with 4” - 8” totals widespread up to the Carolinas. Rains of 1” - 5” and gusty winds will work their way up the Delaware, New Jersey, and New York coastlines from Saturday into Sunday and perhaps Monday--a very unwelcome prospect for Labor Day beach goers.

A zone of very heavy rain (perhaps 5” to 10”) and stronger wind could develop in parts of the eastern Delmarva and/or southern New Jersey on Sunday and Monday, depending on how close ex-Hermine gets to the Mid-Atlantic coast when it slows down.

Hurricane Watch continues for Hawaii ahead of Category 3 Lester
It’s quite rare to have two separate parts of the United States threatened by two hurricanes at the same time. So it was on Thursday night, with Hurricane Lester continuing to bear down on Hawaii. As of 11 pm EDT Thursday (5:00 pm HST), Lester was about 600 miles east of Hilo. Lester is again a major hurricane, with top sustained winds of 125 mph. Computer models agree on taking Lester along a west-northwest path that will parallel the Hawaiian island chain on Saturday. Uncertainty remains over how strong Lester will be at that point (most likely a Category 1 hurricane) and how close its path will be to the islands. The model guidance is now in fairly close agreement on a track just north of the islands, but perhaps within 100 miles—close enough to require continued vigilance.

We’ll be back on Friday with more than one update on the evolving Hermine situation.

Bob Henson and Jeff Masters

Hurricane

Hurricane Hermine Poised to Make First Hurricane Strike on Florida in 11 Years

By: Jeff Masters , 10:32 PM GMT on September 01, 2016

Widespread storm surge was barreling into Florida’s northeast Gulf Coast late Thursday night with the approach of Hurricane Hermine. The warm waters of the eastern Gulf fueled an well-advertised strengthening of Hermine on Thursday afternoon and evening. Hermine was an 80-mph Category 1 hurricane as of the midnight update from the National Hurricane Center. NHC placed the center of Hermine about 40 miles southeast of Tallahassee, FL, just an hour or two from making landfall. Thunderstorms were wrapped around a semi-distinct eye, and heavy bands of rain were clearly evident on radar. An especially intense belt of rain was moving across the northernmost FL peninsula late Thursday.

A Hurricane Warning remained in effect from Suwanee River to Mexico Beach, FL. A variety of other hurricane and tropical storm watches and warnings plastered the Gulf and Atlantic coasts from Florida all the way to northern New Jersey (see below for more on Hermine’s expected track). With Hurricane Gaston also active in the Central Atlantic, we now have multiple hurricanes in the Atlantic for the first time since the first week of September 2012, when Hurricane Leslie and Hurricane Michael were both active. Hermine will be the first hurricane to strike Florida since Wilma hit South Florida as a Category 3 storm in October 2005. Hermine will also be the first hurricane to strike the U.S. since Hurricane Arthur hit North Carolina on July 3, 2014 as a Category 2 storm with 100 mph winds.


Figure X. Hermine approaching landfall in Florida at 6:15 pm EDT September 1, 2016.

Observations this evening
A Personal Weather Station (PWS) at Alligator Point in the northern eyewall of Hermine measured sustained winds of 59 mph, gusting to 78 mph, at 10:25 pm EDT, and had picked up 2.15” of rain since 11 am.

Tyndall AFB Tower C, located about 20 miles south of Apalachicola, Florida, recorded sustained winds of 61 mph, gusting to 79 mph, at 8:50 pm EDT. The anemometer on the tower is at an elevation of 35 meters, which is higher than the standard 10 meters used to reference surface winds, so these winds need to be scaled down to what they would be at a height of 10 meters for a valid comparison to other surface wind measurements.

A PWS on Cedar Key, Florida recorded sustained winds of 44 mph, gusting to 52 mph, at 11:15 pm EDT.

A coastal C-MAN station at Keaton Beach, Florida, in the stronger eastern eyewall of Hermine, measured sustained winds of 45 mph, gusting to 58 mph, at 11 pm EDT.


Figure 2. Regional radar of Hurricane Hermine at 11 pm EDT September 1, 2016, with three time traces of the storm surge at water gages in Apalachicola, Cedar Key and Tampa, Florida. The storm surge (green line) had peaked in Tampa at this time, but was still rising at Apalachicola and Cedar Key. Surge images were taken from NOAA Tides and Currents page for Hermine.


Storm surge a major concern overnight
Even after Hermine’s expected landfall between 1 and 2 am Friday near the head of Apalachee Bay, the strong winds east and south of its center will continue to shove water into the bay and points east along Florida’s Big Bend. This is the bay’s first direct hit from a hurricane in half a century—since Alma, in 1966. NHC warned that surge levels could reach as high as 9 feet in parts of Apalachee Bay and the Big Bend. Fortunately, this is one of the most sparsely settled parts of the Florida coast, but widespread inundation in such places as the flood-prone community of Cedar Key will have a major impact. By 11 pm EDT Thursday, the surge had climbed to 7.2 feet at Cedar Key and 4.0 feet at Apalachicola. HIgh tide will be around 3 am EDT in Cedar Key, which means that the total water level (surge plus tide) could still be rising even after the surge itself peaks.

According to the U-Surge project, headed by storm surge expert Hal Needham, Hermine is likely to produce the highest water level for many locations between St. Marks and Clearwater since the March 1993 “Storm of the Century”. That extremely powerful winter-type storm produced a record surge of 6 to 12 feet across the region.

The forecast for Hermine
Hermine will be rolling along or near the Southeast coast on Friday, emerging off North Carolina’s Outer Banks by Saturday. At that point, Hermine will probably have undergone at least a partial conversion into an extratropical (post-tropical storm), deriving energy from atmospheric dynamics rather than from the heat energy of the ocean.

By later in the weekend, however, Hermine may regain some of its tropical characteristics. Computer models indicate it will slow down Sunday into Monday and perhaps even carry out a tightly looping path east of the Delmarva and south of Long Island, NY. Ocean temperatures are more than 2°C above average in this region. This may be warm enough to allow Hermine’s winds to restrengthen close to hurricane force near its center by Monday or Tuesday, regardless of whether it is classified as a tropical or post-tropical storm by that point. Even if it does become post-tropical, NHC will continue issuing advisories on Hermine as long as it remains a significant threat to land.


Figure 3. The official track forecast for Hurricane Hermine as of 11 pm EDT Thursday. Hermine’s dramatic slowdown is evident in the period from Sunday to Tuesday.

The biggest impact from Hermine after landfall will be a swath of torrential rain stretching from far north Florida to eastern North Carolina, extending from the coast up to 200 miles inland. Flash flood watches are in effect from Georgia to Virginia along a belt northwest of tropical storm warnings. Rains of 5” - 10”, with local totals on the order of 15”, will affect far northern Florida and southern Georgia, with 4” - 8” totals widespread up to the Carolinas. Rains of 1” - 5” and gusty winds will work their way up the Delaware, New Jersey, and New York coastlines from Saturday into Sunday and perhaps Monday--a very unwelcome prospect for Labor Day beach goers.

A zone of very heavy rain (perhaps 5” to 10”) and stronger wind could develop in parts of the eastern Delmarva and/or southern New Jersey on Sunday and Monday, depending on how close ex-Hermine gets to the Mid-Atlantic coast when it slows down.

Hurricane Watch continues for Hawaii ahead of Category 3 Lester
It’s quite rare to have two separate parts of the United States threatened by two hurricanes at the same time. So it was on Thursday night, with Hurricane Lester continuing to bear down on Hawaii. As of 11 pm EDT Thursday (5:00 pm HST), Lester was about 600 miles east of Hilo. Lester is again a major hurricane, with top sustained winds of 125 mph. Computer models agree on taking Lester along a west-northwest path that will parallel the Hawaiian island chain on Saturday. Uncertainty remains over how strong Lester will be at that point (most likely a Category 1 hurricane) and how close its path will be to the islands. The model guidance is now in fairly close agreement on a track just north of the islands, but perhaps within 100 miles—close enough to require continued vigilance.

We’ll be back on Friday with more than one update on the evolving Hermine situation.

Bob Henson and Jeff Masters

Hurricane

Intensifying Tropical Storm Hermine Beginning to Close Off an Eye

By: Jeff Masters and Bob Henson , 4:48 PM GMT on September 01, 2016

Tropical Storm Hermine is gathering strength as it steams north-northeastwards at 14 mph towards the Florida Panhandle, and appears poised to give Florida its first hurricane strike in nearly eleven years when it crosses the coast late tonight or early Friday morning. An Air Force hurricane hunter aircraft was in Hermine early Thursday afternoon, and found top surface winds of 65 mph to the east of the center, and a central pressure that had fallen to 991 mb. The aircraft recorded a distinct double wind maximum on either side of the center, evidence that Hermine was not far from from closing off an eye. This process was also apparent on satellite images, which showed a band of intense thunderstorms rotating around the north side of Hermine’s center, forming the northern portion of an eyewall. At this rate, Hermine should have a fully closed eye by late afternoon, and could be a Category 1 hurricane by Thursday evening. It’s a good thing Hermine didn’t get its act together a day earlier, or we might be looking a a major hurricane in the Gulf of Mexico today. Wind shear continued to be a moderate 10 - 20 knots on Thursday morning, but the direction of the shear was from the west-southwest, where the upstream air is not quite as dry, as seen on water vapor satellite imagery. Sea surface temperatures (SSTs) near the center remained favorable for development, near 30°C (86°F). 


Figure 1. Radar composite of Tropical Storm Hermine taken on Thursday morning, September 1, 2016 by NOAA hurricane hunter aircraft N43RF (flight path shown in red, with the flight path of the simultaneous flight by the NASA Global Hawk aircraft shown in yellow.) Image credit: NOAA/AOML/HRD Twitter feed.


Figure 2. Latest satellite image of Hermine.

Hermine expected to become a Category 1 hurricane
The latest Thursday morning runs of our top models are in solid agreement that Hermine will make landfall along the Florida Big Bend coast on Thursday evening near midnight. In their 11 am EDT Thursday Wind Probability Forecast, NHC’s highest odds for getting tropical storm force winds of 34+ mph from Hermine along the Gulf Coast of Florida were 84%, 83%, and 71%, respectively, for St. Marks, Cedar Key and Apalachicola, Florida. The SHIPS model on Thursday morning predicted moderately favorable conditions for intensification, with wind shear staying a moderate 10 - 20 knots through landfall on Thursday night. SSTs will be a very warm 30°C (86°F), and mid-level relative humidity was predicted to be a reasonably moist 65%. Our three best intensity models--the HWRF, DSHIPS and LGEM models--were in agreement with their latest runs available late Thursday morning on a landfall intensity of 70 - 75 mph--a borderline strong tropical storm or minimal Category 1 hurricane.

The Gulf Coast of Florida can receive large storm surges, due to the extensive stretch of shallow continental shelf waters offshore that extend up to 90 miles from the coast, and NHC has increased their maximum storm surge forecast to 5 - 8’ above ground along a stretch of the Florida coast to the right of where the center is expected to make landfall. Hermine is a large storm, with tropical storm-force winds that extend out up to 140 miles east and southeast of the center, and will likely deliver a storm surge of at least 3 feet to a 150-mile stretch of the Florida Gulf Coast and potentially 1-3 feet along a 150-mile stretch of the Georgia and southern South Carolina coast. Early Thursday afternoon, Hermine continued to create storm surge heights over 1’ along the entire Gulf Coast from New Orleans, Louisiana to Naples, Florida. According to the NOAA Tides and Currents page for Hermine, the maximum storm surge early Thursday afternoon was approximately 2.0’ at Cedar Key and Apalachicola, Florida.

Extremely rich moisture available to Hermine
Near record-warm ocean temperatures in the Gulf of Mexico are evaporating near-record amounts of water vapor into the atmosphere for Hermine to feed off of. At 8 am EDT Thursday, the upper-air balloon sounding at Jacksonville, Florida measured 2.41” of total precipitable water (TPW)--the amount of water that would result if one condensed all the water vapor in a column above and precipitated it out. This value ranked in the upper 1% of all TPW measurements taken at the site since 1948. According to the National Weather Service, Jacksonville’s all-time greatest precipitable water sounding was 2.82” on July 20, 1993. Near-record TPW values around 2.5” were analyzed by satellite over the Gulf of Mexico on Thursday morning, and the NAM model predicted a large surge of this rich moisture will accompany Hermine along its track up the Southeast U.S. coast into North Carolina.


Figure 3. Precipitable water (blue = high values, in millimeters) is running as high as 65-75 mm (2.6” - 3.0”) across the eastern Gulf of Mexico in association with Tropical Storm Hermine. This moisture will swing northeast as Hermine makes landfall and moves across the Southeast U.S. Image credit: tropicaltidbits.com.

Heavy rains expected across Southeast, Mid-Atlantic
After landfall, Hermine will become embedded in a cold front as it sweeps through Georgia, and the storm will begin transitioning to a powerful extratropical storm, deriving energy from atmospheric dynamics rather than from the heat energy of the ocean. This extra energy source should allow Hermine to maintain tropical storm intensity as it speeds to the northeast along the Southeast U.S. coast. In their 11 am EDT Thursday Wind Probability Forecast, NHC gave odds of at least 30% for tropical storm-force winds to affect the entire U.S. coast from northern Florida to Virginia. With Hermine now expected to track near or just inland from the Southeast coast, we can expect copious rainfall of 4” - 8”, with locally higher amounts, in a swath extending about 200 miles inland from the coast (see Figure 4) from late Thursday into Friday. Parts of the Southeast have been dealing with a parched, hot summer, so some moisture will be welcome, although these amounts could be excessive in places.

Hermine is expected to become fully extratropical by Saturday night, when it will be offshore from North Carolina. On Sunday and Monday, ex-Hermine is expected to stall out off the Mid-Atlantic coast, as the storm becomes entangled with an upper-level trough of low pressure. Rains of 1” - 4” should sweep along the Delaware, New Jersey, and New York coastlines from Saturday into Sunday and perhaps Monday--a very unwelcome prospect for Labor Day beach goers. A zone of even heavier rain may emerge in parts of the eastern Delmarva and/or southern New Jersey, with the location hinging on how close ex-Hermine gets to the Mid-Atlantic coast as it slows down.

Flash flood watches are in effect from Georgia to North Carolina along a swath northwest of tropical storm warnings. Recent weeks have been relatively dry for much of the Southeast and mid-Atlantic, which will help reduce the risk of major flooding, but an event like Hermine can easily outweigh that factor. As it appears now,  Hermine is not likely to be a billion-dollar hurricane. However, the huge amounts of rain it will unleash on the U.S. coast could set the stage for a follow-up hurricane to be an extremely serious flood disaster, should another hurricane visit the region in September.


Figure 4. NWS precipitation outlook for the 5-day period from 12Z (8:00 am EDT) Thursday, September 1, 2016, through 12Z Tuesday, September 6. Image credit: NOAA/NWS Weather Prediction Center.

Links
Links
Storm surge from NOAA Tides and Currents page for Hermine
Storm surge expert Dr. Hal Needham's new post, New Coastal Flooding Products Help Us Track Hermine's Storm Surge
NWS Local Statements on Hermine
NWS Excessive Rainfall Forecasts
Radar loops from Brian McNoldy
Loews Don CeSar webcam from St. Pete’s Beach (thanks to WU member LuckySD for this link)
Blue Parrot Cafe webcam in Apalalachicola (thanks go to WU member captainhunter for this link)

Madeline angles south of Hawaii; hurricane watch for Lester
Tropical Storm Madeline, a mighty Category 4 hurricane just two days ago, limped across the Pacific just south of Hawaii’s Big Island on Thursday morning, producing much less impact than feared. Madeline was located 180 miles south of South Point, HI, as of the 11:00 am EST (5:00 am HST) advisory from the Central Pacific Hurricane Center (CPHC). Madeline’s top winds were holding steady at 50 mph, but the storm is expected be a depression by Friday as it continues moving west over open water, eventually arcing to the west-northwest.

Tropical storm warnings for Madeline were cancelled across Hawaii at 2:00 am HST (8:00 am EDT) Thursday. No major damage or injury had yet been reported, though we’ll have to wait till after daybreak Hawaii time to know for sure. Hilo racked up more than 4” of rain during Hermine’s approach on Tuesday and Wednesday, and rainfall was no doubt heavier along the east slopes of the Big Island. Tropical storm force winds were observed as far away from Madeline as Hawaii’s northwest coast, where a mesonet station at the Kohala Ranch Resort reported sustained winds of 44 mph at 2:38 pm HST Wednesday and a gust to 60 mph at 11:03 am HST.


Figure 5. Infrared image of Tropical Storm Madeline (left) and Hurricane Lester (right) at 1200Z (8:00 am EDT and 2:00 am HST) Thursday, September 1, 2016. Image credit: NASA/MSFC Earth Science Office.

Next in Hawaii’s unusual hurricane lineup is Hurricane Lester, located about 750 miles east of Hilo as of the 11 am EDT advisory from CPHC. A Hurricane Watch has been issued for Hawaii’s Big Island, Maui, Molokai, Lanai, and Kahoolawe. Although Lester retains a distinct eye in infrared satellite imagery, its top sustained winds were down to 105 mph, making it a Category 2 storm. Wind shear along Lester’s path remains quite low at 5-10 knots, but dry air at mid-levels of the atmosphere (relative humidities only around 35-40%) is feeding into Lester’s shield of thunderstorms, which has become less intense in satellite imagery. Also, Lester is passing over a corridor of waters churned up by Madeline, which may have pushed SSTs below the 26°C benchmark for tropical development, as suggested in daily SST mapping. The SHIPS model predicts SSTs in the 26-27°C range for the next several days, with little change in wind shear or relative humidity, so Lester may weaken only gradually.

Lester’s potential path is worrisome enough to require vigilance. Computer models agree that Lester will angle west-northwest on a path that parallels the Hawaiian island chain, but they disagree on how close that path will be to the islands. The 00Z and 06Z runs of the HWRF model take Lester about 100 miles north of the islands as a Category 1 hurricane, with the GFDL runs considerably closer and equally strong. The 00Z and 06Z GFS runs are similar to the HWRF but with Lester as a strong tropical storm, and the European model also keeps Lester about as far north. Some members of the 00Z Euro and GFS ensembles are considerably further south: the GFS ensemble average (GEFS) moves Lester across Hawaii northwest of the Big Island, and the Euro average is similar, though a shade further north. These conflicting messages from the models underscore the need for continued caution. The official 11 am EDT track from CPHC moves Lester just north of the islands from Saturday into early Sunday as a minimal Category 1 hurricane, bringing Lester to about 100 miles north of Oahu.


Figure 6. Official track for Hurricane Lester as of 11:00 am EDT (5:00 am HST) Thursday.

Weakening Gaston heads toward the Azores
A Tropical Storm Warning is up for the islands of the western and Central Azores as Hurricane Gaston continues to speed east-northeast at 23 mph towards the islands, where the storm is expected to arrive on Friday morning. On Wednesday morning, Gaston peaked as an impressive Category 3 hurricane with 120 mph winds. Since then, higher wind shear and a traverse over cooler waters near 26.5°C (80°F) has caused weakening, and Gaston was a Category 1 hurricane with 80 mph winds at 11 am EDT Thursday. This weakening trend should continue, resulting in Gaston arriving in the western Azores on Friday morning as a tropical storm with 65 mph winds. The Azores only averages one hurricane strike per decade, and has already seen one this year: 2016’s first Atlantic storm, Hurricane Alex, which struck the island of Terceira in the central Azores on January 15 as a bizarrely out-of-season tropical storm in January. Alex did minimal damage and caused no direct deaths.


Figure 7. MODIS visible satellite image of Hurricane Gaston on Wednesday afternoon, August 31, 2016. At the time, Gaston was a Category 3 storm with 115 mph winds. Image credit: NASA.

92L in the central tropical Atlantic embedded in dry air
A large tropical wave with plenty of spin but no heavy thunderstorm activity is located midway between Africa and the Lesser Antilles islands. Originally designated Invest 92L, this wave was no longer deemed worthy by NHC as an “Invest”, and they are no longer issuing their suite of model forecasts for the system. The tropical wave was embedded in a major area of dust and dry air from the Sahara Desert, and this dry air will greatly interfere with development over the coming days as 92L heads west at 15 - 20 mph across the tropical Atlantic. The latest 0Z Thursday runs of our three top models for forecasting tropical cyclone genesis--the GFS, European and UKMET models--had one of the three, the UKMET, showing development of the system over the next five days. A strong and persistent ridge of high pressure should keep 92L on a fairly straightforward west to west-northwest path, and the storm will likely move through the Lesser Antilles Islands on Sunday night, and be near Hispaniola on Tuesday. In their 8 am EDT Thursday Tropical Weather Outlook, NHC reduced their 2-day and 5-day development odds to 0% and 20%, respectively.

We’ll be back with a new post later today.

Jeff Masters and Bob Henson





Hurricane

Florida’s First Hurricane Warning in 4 Years as Hermine Intensifies to 65 mph

By: Jeff Masters , 12:31 PM GMT on September 01, 2016

Tropical Storm Hermine is gathering strength as it steams north-northeastwards at 10 mph towards the Florida Panhandle, and appears poised to give Florida its first hurricane strike in nearly eleven years when it crosses the coast late tonight or early Friday morning. A NOAA hurricane hunter aircraft was in Hermine early Thursday morning, and found top surface winds of 65 mph to the east of the center, and a central pressure that had fallen to 992 mb. Buoy 42003, located about 65 miles to the east-southeast of Hermine’s center, had sustained winds of 50 mph gusting to 60 mph at 5:50 am EDT Thursday. Wave heights at the buoy had built from 8 feet to 19 feet over the previous 24 hours. Strong winds from Hermine continued to create storm surge heights over 1’ along the entire Gulf Coast from New Orleans, Louisiana to Naples, Florida on Thursday morning. The maximum surge so far from the storm was about 2.5’ at Cedar Key, Florida on Wednesday evening. Satellite images on Thursday morning showed a steadily organizing storm, with heavy thunderstorms building near the storm’s center and some significant low-level spiral bands forming. It’s a good thing Hermine didn’t get its act together a day earlier, or we’d be looking a a major hurricane in the Gulf of Mexico today. Wind shear continued to be a moderate 10 - 15 knots on Thursday morning, but the direction of the shear had switched to the west-southwest. This switch allowed Hermine to begin intensifying more rapidly, since the upstream air was not quite as dry to the west-southwest, as seen on water vapor satellite imagery. Sea surface temperatures (SSTs) near TD 9’s center remained favorable for development, near 30.5°C (87°F).


Figure 1. Blended visible/IR satellite image of Tropical Storm Hermine at dawn, 7:45 am EDT September 1, 2016. Image credit: Navy Research Lab NexSat page.


Figure 2. The Hurricane Warning for Hermine is Florida’s first one since August 25, 2012, when Hurricane Isaac threatened South Florida and the Panhandle. During the period 2006 - 2015, only three storms provoked a Hurricane Warning in Florida. Image credit: Iowa State University.

Intensity forecast: Hermine expected to become a Category 1 hurricane
The SHIPS model on Thursday morning predicted moderately favorable conditions for intensification, with wind shear staying a moderate 10 - 15 knots through landfall on Thursday evening. SSTs will be a very warm 30 - 30.5°C (86 - 87°F), and mid-level relative humidity was predicted to be a reasonably moist 65%. Our three best intensity models--the HWRF, DSHIPS and LGEM models--were in agreement with their latest runs available early Thursday morning on a landfall intensity of 75 mph—minimum Category 1 hurricane strength. The Gulf Coast of Florida is highly vulnerable to large storm surges, due to the extensive stretch of shallow continental shelf waters offshore that extend up to 90 miles from the coast, and NHC has increased their maximum storm surge forecast to 5 - 7’ above ground along a stretch of the Florida coast to the right of where the center is expected to make landfall. Hermine is a large storm, with tropical storm-force winds that extend out up to 140 miles east and southeast of the center, and will likely deliver a storm surge of at least 3 feet to a 150-mile stretch of the Florida Gulf Coast and a 150-mile stretch of the Georgia and southern South Carolina coast.


Figure 3. Radar estimated rainfall between August 29 - and 7:44 am EDT September 1, 2016 from the Tampa radar. Swaths of 2 - 4” of rain (yellow colors) were common over Florida, with some areas of 4 - 6” near Tampa and Melbourne. Heavy rain will be the main threat from Hermine, with rainfall amounts of 5 - 10" expected along its track through northern Florida and southern Georgia.


Figure 4. Projected 7-day rainfall from 12Z (8:00 am EDT) Thursday, September 1, through 12Z September 8, 2016. Rainfall amounts of 5 - 10” are expected along Hermine’s path across Florida and along the Southeast U.S. coast. Image credit: NOAA/NWS Weather Prediction Center.

Track forecast for Hermine: a Florida Gulf Coast landfall, followed by a run up the Southeast coast
The latest Thursday morning runs of our top models are in solid agreement that Hermine will make landfall along the Florida Big Bend coast on Thursday evening near midnight. In their 5 am EDT Thursday Wind Probability Forecast, NHC’s highest odds for getting tropical storm force winds of 34+ mph from Hermine along the Gulf Coast of Florida were 74%, 70%, and 61%, respectively, for Apalachicola, St. Marks and Cedar Key, Florida. After landfall, Hermine will become embedded in a cold front as it sweeps through Georgia, and the storm will begin transitioning to a powerful extratropical storm, deriving energy from atmospheric dynamics rather than from the heat energy of the ocean. This extra energy source should allow Hermine to maintain tropical storm intensity as it speeds to the northeast along the Southeast U.S. coast. In their 5 am EDT Thursday Wind Probability Forecast, NHC gave odds of at least 30% for tropical storm-force winds to affect the entire U.S. coast from northern Florida to Cape Hatteras, North Carolina. Hermine is expected to become fully extratropical by Saturday night, when it will be offshore from North Carolina. On Sunday and Monday, ex-Hermine is expected to stall out off the Mid-Atlantic coast, as the storm becomes entangled with an upper-level trough of low pressure. The storm won’t be named Hermine anymore, but perhaps we should call it “Her-Mean”, since this very large and wet storm will bring high winds and heavy rains to much of the Mid-Atlantic and southern New England coasts during the Sunday and Monday portion of the Labor Day holiday weekend.

Bob and I will have a full update on the tropics, including the latest on Hermine, around noon EDT today.

Jeff Masters

Hurricane


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Category 6™

About

Cat 6 lead authors: WU cofounder Dr. Jeff Masters (right), who flew w/NOAA Hurricane Hunters 1986-1990, & WU meteorologist Bob Henson, @bhensonweather