Category 6™

Rains From Invest 96L Kill 55 in Mali

By: JeffMasters, 3:40 PM GMT on August 31, 2013

A tropical wave that emerged from the coast of Africa on Friday (Invest 96L) is headed west-northwest towards the Cape Verde Islands. This wave is struggling against high wind shear of 20 - 25 knots. The shear is expected to drop to the moderate level on Sunday, then increase again to the high level on Monday through Thursday. In their 8 am EDT Saturday Tropical Weather Outlook, NHC put the 5-day odds of development at 40%, and the 2-day odds at 40%. Our reliable models for tropical cyclone genesis show little or no development of 96L. This disturbance is unlikely to affect any land areas except the Cape Verde Islands.


Figure 1. MODIS satellite image of Invest 96L off the coast of Africa, taken at 8:30 am EDT on August 31, 2013. Image credit: NASA.

Disturbance 96L moved very slowly across the Sahel region of Africa during the week, dumping torrential rains on Wednesday that triggered flash floods that killed at least 55 people in Mali's capital city, Bamako. Serious flooding also affected neighboring Burkina Faso, Gambia, Guinea, Niger and Senegal. Buildings have collapsed, roads have been rendered impassable, and valuable farmland is submerged beneath flood water, affecting over 200,000 people. More than 100 homes were swept away as the Niger River burst its banks, bringing down bridges and submerging entire streets.


Figure 2. People hold a rope to help rescuers climbing down a roof of a house in a flooded area of Bamako, Mail, on August 28, 2013. At least 55 people have been killed in flash floods caused by torrential rain. Image credit: HABIBOU KOUYATE/AFP/Getty Images)


Figure 3. MODIS satellite image of tropical wave 96L over Mali on August 28, 2013. The storm dumped torrential rains on Mali's capital city, Bamako, which killed at least 55 people. Image credit: NASA.

Disturbance approaching Lesser Antilles no immediate threat to develop
A tropical wave located about 400 miles east of the Lesser Antilles Islands is moving westward at 15 mph, and has changed little over the past three days. The wave has a modest amount of spin, but only a limited amount of heavy thunderstorm activity. In their 8 am EDT Saturday Tropical Weather Outlook, NHC put the 5-day odds of formation of this disturbance at 10%, and the 2-day odds at 0%. Wind shear is a high 20 - 30 knots over the system, and is expected to stay high through Monday. An area of dry air and dust from the Saharan Air Layer (SAL) surrounds the disturbance and is interfering with development. Wind shear may fall to the moderate level in 4 - 5 days when the wave reaches the Central Caribbean, increasing the odds of development then. Of our three reliable models for predicting genesis, the UKMET, GFS, and European models, only the UKMET model develops the disturbance, predicting it will become a tropical depression south of Haiti on Thursday. The wave will spread heavy rains and gusty winds to the Lesser Antilles Islands on Sunday and Monday.

Jeff Masters

Hurricane

August Ends With a Whimper for the Atlantic Hurricane Season

By: JeffMasters, 3:19 PM GMT on August 30, 2013

The end of August is upon us, and with just one day remaining in the month, the remarkably quiet Atlantic hurricane season of 2013 is highly likely to be just the 6th season since the Hurricane Hunters began flying in 1944 without a hurricane forming by the end of August (the other years: 2002, 2001, 1988, 1984, and 1967.) Although there have been two tropical storms in August (Erin and Fernand), these storms were weak and short-lived, and August 2013 had one of the lowest Accumulated Cyclone Energy (ACE) totals on record for an August in the Atlantic. ACE is calculated as the square of the wind speed every 6 hours for every named storm with at least 40 mph sustained winds (scaled by a factor of 10,000 for usability.) Since the damage potential of a hurricane is proportional to the square or cube of the maximum wind speed, ACE is not only a measure of tropical cyclone activity, but also a measure of the damage potential. During the 20-year period 1981 - 2010, the Atlantic averaged 104 ACE units, and the 20-year average ACE by the end of August was 30. So far in 2013, we've managed just 9 ACE units, with only 1.9 of those occurring in August. Since the current active hurricane period we are in began in 1995, only 2002 had a lower ACE by this point in the year (4.9), and only 1997 had a lower August ACE (zero.) Both were El Niño years, when we expect hurricane activity to be low due to high wind shear. If we go back to the beginning of reliable Atlantic ACE statistics in 1966, when good satellite data first became available, only five other years had August ACE values comparable to 2013's. Three of those years were El Niño years, and the other two had ocean temperatures in the hurricane Main Development Region (MDR, from 10 - 20°N, 20 - 70°W) that were more than 0.5°C (0.9°F) cooler than in 2013. This year's combination of no El Niño, warm MDR SSTs, and an exceptionally low August ACE is an event unparalleled in the historical record, going back to 1966.


Figure 1. August Accumulated Cyclone Energy (ACE) totals in August 2013 were among the lowest on record for an August in the Atlantic. Other Augusts with low ACE in the Atlantic all occurred during El Niño years, or when sea surface temperatures (SSTs) were cool.

Why the quiet season?
The main reason for the quiet August has been the large amount of dry, stable air over the Atlantic. This dry air has two sources: the Sahara desert of Africa, and sinking air from aloft, which warms and dries as it sinks. Even so, I find it highly perplexing that activity has been so low when all of the other factors--lack of an El Niño, low wind shear, an active African Monsoon spitting out plenty of tropical waves, and above average ocean temperatures--have favored development. Instability has increased over the tropical Atlantic over the past few days, thanks to the influence of the MJO and a Convectively Coupled Kelvin Wave (CCKW) bringing rising air to the Atlantic. However, there do not appear to be any tropical waves positioned to take advantage of this increased instability and intensify to hurricane strength over the coming week (though 96L could become a tropical storm.) The next tropical wave with a shot at becoming a hurricane will not exit the coast of Africa until September 7. If we make it all the way to September 11 without a hurricane in the Atlantic, it will beat the record set by Hurricane Gustav of 2002 for latest date of formation of the season's first hurricane, going back to when the Hurricane Hunters first began flying in 1944. Even if we do get a first-half of September hurricane, the steering pattern features a strong trough of low pressure over the U.S. East Coast, which would have high odds of recurving any hurricane that manages to form out to sea, without affecting any land areas.


Figure 2. Vertical instability over the tropical Atlantic in 2013 (blue line) compared to average (black line.) The instability is plotted in °C, as a difference in temperature from near the surface to the upper atmosphere. Thunderstorms grow much more readily when vertical instability is high. Instability has been much lower than average during most of August, due to outbreaks of dry air from Saharan Air Layer (SAL) , and an unusual amount of dry, sinking air in the tropical Atlantic. Low instability reduces the potential for tropical storm formation. During the last few days of August, instability has increased, which should raise the odds of tropical storm formation. Image credit: NOAA/NESDIS/CIRA.

Quiet in the Atlantic
A tropical wave located about 900 miles east of the Lesser Antilles Islands is moving westward at 15 mph, and has changed little over the past two days. The wave has a modest amount of spin, but only a limited amount of heavy thunderstorm activity. In their 8 am EDT Friday Tropical Weather Outlook, NHC left the 5-day odds of formation of this disturbance at 20%, and the 2-day odds at 10%. Wind shear is a moderate 10 - 20 knots over the system, but there is an area of dry air and dust from the Saharan Air Layer (SAL) to the north and west that is interfering with development. Our three reliable models for predicting genesis, the UKMET, GFS, and European models, do not develop the disturbance during the next five days. The wave will likely spread heavy rains and gusty winds to the Lesser Antilles Islands by Monday, when the wave will encounter an area of high wind shear positioned over the Northern Lesser Antilles Islands. This high wind shear will likely decrease the odds of development beginning on Monday.


Figure 3. MODIS satellite image of the tropical disturbance 650 miles east of the Lesser Antilles, taken at 2:30 pm EDT on August 30, 2013. A small closed circulation had developed well to the west of an area of increasing heavy thunderstorms. Image credit: NASA.

A well-organized tropical wave emerging from the coast of Africa (Invest 96L) is headed west-northwest towards the Cape Verde Islands. This wave is expected to take a west-northwesterly track into an area with drier air and higher wind shear, and would likely not be able to make the long trek across the Atlantic to threaten North America or the Caribbean Islands. In their 8 am EDT Friday Tropical Weather Outlook, NHC put the 5-day odds of development at 60%, and the 2-day odds at 40%. The GFS model predicts that 96L will develop, but the UKMET and European models show little or no development.

Wunderblogger Lee Grenci has an excellent new post, "Atlantic Hurricane Season: The Saharan Air Layer and Vertical Wind Shear".

I plan on having a relaxing Labor Day weekend, but will do at least one update. Have a great weekend, everyone!

Jeff Masters

Hurricane

Genesis of New Atlantic Tropical Cyclones: Which Model Should You Trust?

By: JeffMasters, 3:29 PM GMT on August 29, 2013

It's been a remarkably quiet late August in the Atlantic for hurricanes, with nary an "Invest", and precious few yellow 10% probability circles on NHC's Tropical Weather Outlook. When will this remarkable hurricane drought come to an end? The computer models we use to track hurricanes have conflicting ideas about this. Which model should we believe, and how far into the future can these models successfully predict genesis events of new tropical depressions in the Atlantic? Some answers come in a 2013 paper by a group of scientists led by Florida State's Daniel Halperin, just accepted for publication in the journal Weather and Forecasting, "An evaluation of tropical cyclone genesis forecasts from global numerical models." It turns out that two of the three most reliable models for predicting the genesis of tropical cyclones up to four days in advance are also the our top-performing models for predicting hurricane tracks: the American GFS model and the European ECMWF model. The UKMET model is also quite good at predicting tropical cyclone genesis events. The Canadian GEM model and the Navy NOGAPS model (now succeeded by the Navy NAVGEM model) do less well. When two or more models make the same genesis forecast, the odds of the event actually occurring are increased considerably. The models have improved greatly in making genesis forecasts in recent years; back in 2007, when our top three models made a 4-day genesis forecast, these verified only 17 - 28% of the time. By 2011, the hit rate had increased to 45 - 50%. However, the models still miss most genesis events. In 2011, the probability of detection of a tropical cyclone genesis event was only 8% - 23%, meaning that 77 - 92% of time, a tropical depression or tropical storm formed without the model predicting that it would form. The best model to use for looking at Atlantic tropical cyclone genesis in 2011 was the UKMET model, which combined a relatively high probability of detection rate with a low false alarm rate.


Figure 1. The probability of making a correct tropical cyclone genesis forecast for all forecast hours (06 to 96) by model, for the period 2004 - 2011. In 2011, three models--the GFS, UKMET, and European--made genesis forecasts that were correct 45 - 50% of the time. The Canadian GEM model and the Navy NOGAPS model did less well. Data taken from Halperin et al., 2013, "An evaluation of tropical cyclone genesis forecasts from global numerical models." Weather and Forecasting, doi: http://dx.doi.org/10.1175/WAF-D-13-00008.1

The paper looked at 135 Atlantic genesis events over the period 2004 - 2011, as predicted by five global computer weather forecast models: the GFS, ECMWF, UKMET, NOGAPS, and Canadian GEM. The regional GFDL and HWRF hurricane models were not considered, as they use output from the GFS model to drive them, and are not good at making genesis forecasts. The study only looked at forecasts made up to four days in advance. While there have been cases where I've seen genesis predictions made 7 - 10 days in advance come true, these are rare, and the "hit" rate of successful genesis forecasts even four days in advance is low. In the paper, a "hit" was defined as a forecast that successfully predicted genesis within 24 hours and 345 miles of the observed time and location. A "false alarm" occurred when the model predicted a storm that never developed. The other categories considered were "late genesis" events where a storm formed more than 24 hours before it was predicted to, and "early genesis", where a storm formed more than 24 hours after it was predicted to. Here are some details on each model's ability to make Atlantic tropical cyclone genesis forecasts:

European ECMWF model: The model is reluctant to predict genesis, and misses many genesis events (it had only an 8% probability of detection in 2011.) However, when it does predict genesis, it usually happens, with only a 16% false alarm percentage in 2011. The European model exhibits preferred regions of genesis, with over 60% of its genesis forecasts occurring in the MDR: 10-20°N, 60-20°W. Although the ECMWF misses many tropical cyclone genesis events in the Gulf of Mexico (as do the other models), when it does forecast genesis there, it almost always occurs. The model was highly prone to making late genesis forecasts.

American GFS model: The GFS model improved substantially in its genesis forecasts beginning in 2010, most likely due to a major model upgrade in 2010. The GFS is more aggressive at predicting genesis than the European model, and is less likely to miss a genesis event (22% probability of detection in 2011.) However, the incidence of false alarms was 32% in 2011, double what the European model had. Like the European model, the GFS exhibits preferred regions of genesis, with nearly 60% of its genesis forecasts occurring in the MDR: 10-20°N, 60-20°W.

UKMET model: The UKMET is more aggressive at predicting genesis than the European model, and is less likely to miss a genesis event (20% probability of detection in 2011.) The incidence of false alarms was 18% in 2011, similar to what the European model had. Like the European and GFS models, the UKMET exhibits preferred regions of genesis, with more than 67% of its genesis forecasts occurring in the MDR: 10-20°N, 60-20°W. The model was prone to making late genesis forecasts.

Canadian GEM model: The Canadian model was the least likely to miss a formation event, with a 23% probability of detection in 2011. False alarms have been a major issue, though, and the Canadian model generated the second highest number of bogus genesis events of any of the five models evaluated (42% of all its genesis forecasts in 2011 were false alarms.) The Canadian model does not seem to have a preferred region of genesis—all types of genesis events occur across the entire basin. The model performs best over the main development region (MDR; 10-20°N, 60-20°W), Caribbean Sea, and Gulf of Mexico (GOM).

Navy NOGAPS model: This model was retired in 2012, but we may be able to assume that its successor, the NAVGEM model, will have some of the same characteristics. False Alarms have been a major issue, and the NOGAPS model generated the highest number of bogus genesis events of any of the five models evaluated (60% of all its genesis forecasts in 2011 were false alarms.) The model also had a low probability of detection, just 9%. NOGAPS model does not seem to have a preferred region of genesis—all types of genesis events occur across the entire basin. Despite the high levels of hurricane activity in 2004 and 2005, the NOGAPS model failed to successfully forecast any genesis events those years.

Sources of Model Data
You can view 7-day ECMWF and 16-day GFS forecasts on wunderground's wundermap with the model layer turned on.
Longer ten-day ECMWF forecasts are available from the ECMWF web site.
FSU's experimental hurricane forecast page (CMC, ECMWF, GFDL, GFS, HWRF, and NAVGEM models)
NOAA's HFIP model comparison page (GFS, ECMWF, FIM, FIM9, UKMET, and CMC models.)
Experimental HFIP models

Very Quiet in the Atlantic
A tropical wave that came off the coast of Africa on Sunday is midway between Africa and the Lesser Antilles Islands. This disturbance is moving westward at 15 mph, has a modest amount of spin, but only a limited amount of heavy thunderstorm activity. In their 8 am EDT Thursday Tropical Weather Outlook, NHC left the 5-day odds of formation of this disturbance at 20%, but increased the 2-day odds of formation to 10%. Wind shear is a moderate 10 - 20 knots over the system, but there is an area of dry air and dust from the Saharan Air Layer (SAL) to the north and west that is interfering with development. With the exception of the NAVGEM model, there is little support from the models for developing the disturbance during the next five days. The wave could spread heavy rains and gusty winds to the Lesser Antilles Islands on Sunday and Monday.

A tropical wave expected to emerge from the coast of Africa on Friday and track over the Cape Verde Islands is also showing little support for development from the models. This wave is expected to take a northwesterly track, and would likely not be able to make the long trek across the Atlantic to threaten North America or the Caribbean Islands. In their 8 am EDT Thursday Tropical Weather Outlook, NHC put the 5-day odds of development at 20%.

Wunderblogger Lee Grenci has an excellent new post, "Atlantic Hurricane Season: The Saharan Air Layer and Vertical Wind Shear".

California's Rim Fire Now 6th Largest in State History, and is 30% Controlled
California's massive Rim Fire in Yosemite National Park continues to grow, but an army of over 4,000 firefighters are making headway against the blaze, and had attained 30% containment of the fire as of 9:20 am EDT on Thursday. According to Inciweb, fire has burned over 192,700 acres. This moves the fire into 6th place for largest fire in state history, according to statistics tabulated by wunderground's weather historian, Christopher C. Burt in his latest post, "The Worst Wild Fires in U.S. History". The Rim Fire will have difficultly surpassing California's largest fire on record--the Cedar Fire in San Diego County of October 2003. That fire burned 273,246 acres (430 square miles). California has had its driest year-to-date period, so it is no surprise that the state is experiencing an unusually large fire this summer.


Video 1. Time-lapse photography shows various perspectives of the 2013 Rim Fire, as viewed from Yosemite National Park. The first part of this video is from the Crane Flat Helibase. The fire is currently burning in wilderness and is not immediately threatening visitors or employees. The second half of the video is from Glacier Point, showing Yosemite Valley, and how little the smoke from the fire has impacted the Valley.

Jeff Masters

Hurricane

Rim Fire California's 7th Largest on Record, and is 23% Contained

By: JeffMasters, 2:22 PM GMT on August 28, 2013

California's massive Rim Fire in Yosemite National Park continues to grow, but firefighters took advantage of less windy conditions on Tuesday and had attained 23% containment of the fire as of 9:20 am EDT on Wednesday. According to Inciweb, the Rim Fire has burned 187,000 acres. This ranks as the 7th largest fire in state history, and largest fire on record in the California Sierra Mountains, says wunderground's weather historian, Christopher C. Burt in his latest post, "The Worst Wild Fires in U.S. History". The Rim Fire will likely climb to be the 5th biggest fire in California's history by this weekend, but will have difficultly surpassing California's largest fire on record--the Cedar Fire in San Diego County of October 2003. That fire burned 273,246 acres (430 square miles), destroyed 2,820 buildings, and killed 15 people. California has had its driest year-to-date period, so it is no surprise that the state is experiencing an unusually large fire this summer. It would also not be a surprise if the state sees another huge fire this year, as peak California fire activity usually comes in September and October, during the end of the six month-long dry season, and when the hot, dry Santa Ana winds tend to blow.


Figure 1. California's Rim Fire as captured by a member of the International Space Station on August 26, 2013. Lake Tahoe is visible at the top, and smoke from the fire obscures the northern portion of Yosemite National Park, and streams into Nevada. Image credit: NASA.


Figure 2. Outbuilding and pickup truck burned by the Rim Fire. Image credit: USFS-Mike McMillan

Weather conditions over the next five days where the Rim Fire is burning are expected to near average, with high temperatures in the upper 80s to low 90s and moderate winds. Air quality alerts for smoke have been posted over portions of California and Nevada, and Reno, Nevada is under a dense smoke advisory. The dense smoke will result in unhealthy air quality in the city, and prolonged outdoor activities are being discouraged. Travel may be difficult due to reduced visibility less than five miles on some area highways.

Links to follow
Wunderground's weather historian, Christopher C. Burt's latest post, "The Worst Wild Fires in U.S. History".
Our wundermap with the fire layer turned on is a good way to track the fire perimeter and smoke plume.
Crane Flat webcam
Time lapse of the Crane Flat webcam for 8 hours on Tuesday.
Yosemite Fire Example of How Droughts Amplify Wildfires, August 26 post by Andrew Freedman of climatecentral.org

Tropical wave midway between Africa and the Lesser Antilles downgraded
A tropical wave that came off the coast of Africa on Sunday is midway between Africa and the Lesser Antilles Islands. This disturbance is moving westward at 10 - 15 mph, has a modest amount of spin, but has lost nearly all of the limited heavy thunderstorm activity it had. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC downgraded the 5-day odds of formation of this disturbance from 30% to 20%. Wind shear is a moderate 10 knots over the system, but there is an area of dry air and dust from the Saharan Air Layer (SAL) to the north that is interfering with development. For the next few days, though, the wave will experience upward-moving air from a Convectively-Coupled Kelvin Wave (CCKW) that is moving eastwards across the Atlantic at 25 - 35 mph. This interaction may contribute to development. With the exception of the NAVGEM model, there is little support from the models for developing the disturbance during the next five days. The wave could spread heavy rains and gusty winds to the Lesser Antilles Islands as early as Sunday.

A tropical wave expected to emerge from the coast of Africa on Friday and track over the Cape Verde Islands is developed by the GFS and European models. This wave is expected to take a northwesterly track, and would likely not be able to make the long trek across the Atlantic to threaten North America or the Caribbean Islands.

Jeff Masters

Fire

Fernand's Rains Kill 13 in Mexico

By: JeffMasters, 1:51 PM GMT on August 27, 2013

Tropical Storm Fernand's life as a tropical storm was brief, lasting just 15 hours at tropical storm strength on Sunday evening and Monday morning. But heavy rains from Fernand have caused the greatest death toll from a 2013 Atlantic named storm yet: thirteen people. The victims all died in in flash floods in Mexico's mountainous terrain west of Veracruz, due to landslides that came down upon houses. Fernand hit Veracruz at 12:45 am EDT Monday morning as a tropical storm with 50 mph winds, and dissipated on Monday evening. The other deaths from this year's named storms include three people killed from floods due to Tropical Storm Barry (two in Mexico and one in El Salvador), and one person killed (in the Dominican Republic) due to flooding rains from Tropical Storm Chantal.


Figure 1. Twenty four-hour rainfall amounts over Mexico ending at 8 am Monday August 26, 2013. Tropical Fernand dumped more than 150 mm (5.90") over a few isolated regions. Image credit: Conagua.


Figure 2. A man is seen outside his home, flooded by the heavy rains of tropical storm Fernand in the city of Veracruz, Veracruz State, on August 26, 2013. Mudslides crashed through several homes in eastern Mexico on Monday, killing at least 13 people. Photo credit: KORAL CARBALLO/AFP/Getty Images.

A tropical wave off the coast of Africa with potential to develop
A tropical wave that came off the coast of Africa on Sunday is located about 300 miles southwest of the Cape Verde Islands. This disturbance is moving westward at about 15 mph, has a modest amount of spin, but is relatively thin on heavy thunderstorm activity. It has not yet earned status as an area of interest ("Invest") by NHC, but they are giving the wave a 30% chance of developing by Sunday. WInd shear is a moderate 10 - 20 knots over the disturbance, but there is an area of dry air and dust from the Saharan Air Layer (SAL) to the north that will likely interfere with development. On Wednesday, the wave will begin encountering upward-moving air from a Convectively-Coupled Kelvin Wave (CCKW) that is moving eastwards across the Atlantic at 25 - 35 mph, which may aid development. The UKMET, ECMWF, and NAVGEM model all show some support for this disturbance developing in the next 4 - 7 days, and the wave could spread heavy rains and gusty winds to the northern Lesser Antilles Islands as early as Sunday. There will be a trough of low pressure off the U.S. East Coast at the end of the week that will be capable of causing the wave to recurve and miss the Lesser Antilles, but if the disturbance stays relatively weak, it could stay on a more southerly track and impact the islands.

A tropical wave expected to emerge from the coast of Africa on Friday and track over the Cape Verde Islands is developed by the GFS and NAVGEM models. This wave would appear to have a high chance of recurvature, though, according to the latest run of the GFS model.

Jeff Masters

Hurricane

Fernand Hits Veracruz, Mexico; Active Atlantic Hurricane Pattern Setting Up

By: JeffMasters, 2:55 PM GMT on August 26, 2013

Tropical Storm Fernand's brief life as a tropical cyclone is almost complete, as the 35-mph tropical depression chugs inland over Mexico's mountainous terrain west of Veracruz. Fernand hit Veracruz at 12:45 am EDT Monday morning as a tropical storm with 50 mph winds. Two coastal stations in the city measured sustained winds of 50 mph at landfall, but there are no reports of any damage or injuries from the storm. Satellite loops show that Fernand is a small and weakening storm, and radar images from Alvarado, Mexico show the heavy rains of the storm have weakened considerably since landfall. The 4 - 8 inches of rain Fernand will dump along its track will be capable of creating flash flooding and dangerous mud slides, though.


Figure 1. Radar image of Tropical Fernand as it approached landfall in Veracuz, Mexico, taken at 9:45 pm EDT on August 25, 2013. Image credit: radar images from Alvarado, MexicoMexican weather service.

Fernand's place in history
Fernand is the 6th consecutive named storm in the Atlantic that has not reached hurricane strength. Only one season since record keeping began in 1851 has had a longer string of consecutive storms that did not reach hurricane strength--2011, when the season began with eight such storms. However, it is quite possible such an event occurred before the advent of reliable satellite data in 1966, when we were first able to identify weak tropical storms that stayed out to sea. Several other seasons have had six consecutive tropical storms without a hurricane, most recently in 2002. The air over the Tropical Atlantic has been more stable and drier than usual (and was so in 2011), making it difficult for storms to attain hurricane strength.

An active weather pattern coming to the Tropical Atlantic
It's been an unusually quiet August for hurricane activity in the Atlantic, and if we finish the month without a hurricane, it will mark the first year since 2002 without an August hurricane. However, the quiet weather pattern we've been blessed with is about to come to an end, as conditions favorable for hurricane formation move into place for the last few days of August and the first week of September. The big guns of the African Monsoon are firing off a salvo of African tropical waves over the next two weeks that will find the most favorable conditions for development that we've seen this year. While there is currently a new outbreak of dry air and dust from the Saharan Air Layer (SAL) over the Eastern Atlantic, the latest European model forecast calls for a reduction in dry air and dust over the Tropical Atlantic during the 7 - 14 day period, accompanied by low wind shear. The Madden Julian Oscillation (MJO), a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days, has begun a new active phase. The most active part of the MJO has not yet crossed into the Atlantic, but is expected to do so during the period 7 - 14 days from now. The MJO will bring rising air that will aid strong thunderstorm updrafts and thus tropical storms--and their subsequent intensification into hurricanes. According to Dr. Michael Ventrice, an MJO expert at WSI, Inc., the latest run of the GFS model predicts that this MJO event will be the 3rd strongest in the Western Hemisphere since 1989. During the last four comparable strong MJO events, 68% of all the tropical depressions that formed during these events (21 out of 31 storms) intensified into hurricanes. The MJO will likely continue to support Atlantic hurricane activity through September 15. The MJO is then expected to progress into the Western Pacific for the last half of September, which would likely bring sinking air over the Atlantic and a quieter portion of hurricane season.


Figure 2. Saharan Air Layer analysis at 8 am EDT on August 26, 2013. A burst of dust and dry air had emerged over the Eastern Atlantic, along with a new tropical wave to watch just south of the driest air. Image credit: University of Wisconsin CIMMS and NOAA/HRD.

The first tropical wave to watch is one that came off the coast of Africa on Sunday. This disturbance is moving westward at 10 - 15 mph, has a modest amount of spin, but is relatively thin on heavy thunderstorm activity. It has not yet earned status as an area of interest ("Invest") by NHC, but they are giving the wave a 30% chance of developing by Saturday. The wave will encounter an eastward-moving Convectively-Coupled Kelvin Wave (CCKW) that moved off the coast of South America on Monday. This atmospheric disturbance, moving eastwards across the tropical Atlantic at about 25 - 40 mph, has a great deal of upward-moving air, which may help the tropical wave develop when the two interact beginning on Wednesday. The UKMET model is predicting that the wave will develop into a tropical storm by Saturday, about 800 miles east of the Lesser Antilles Islands. The other models show limited or no development. There will be a trough of low pressure off the U.S. East Coast at the end of the week that will be capable of causing the wave to recurve and miss the Lesser Antilles, but it is too early to say how likely this is to occur.

There is much greater model consensus on developing a tropical wave expected to emerge from the coast of Africa on Friday. This wave would appear to have a high chance of recurvature, according to the latest run of the GFS model.

Jeff Masters

Hurricane

Tropical Depression Six Forms in the Gulf of Mexico

By: JeffMasters, 9:19 PM GMT on August 25, 2013

The tropical wave that crossed over Mexico's Yucatan Peninsula and entered the Gulf of Mexico's Bay of Campeche over the weekend has rapidly spun up into Tropical Depression Six, according to surface, satellite, and radar data. Satellite loops show that TD 6 is a small storm, and heavy rainfall will be the main threat. Radar images from Alvarado, Mexico show heavy rain bands from TD 6 are already over land, and the 4 - 8 inches of rain TD 6 will bring to the coast is capable of creating flash flooding and dangerous mud slides. The depression has precious little time to develop before moving inland over Mexico near Veracruz early Monday morning, but with low wind shear, warm waters of 30°C, and the topography of the Southern Gulf of Mexico to help it spin up, TD 6 will probably be Tropical Storm Fernand Sunday night. An Air Force hurricane hunter aircraft will investigate the depression Sunday evening.


Figure 1. MODIS satellite image of Tropical Depression Six as it was organizing, taken at 12:30 pm EDT on August 25, 2013. Image credit: NASA.

I'll have a more detailed look at the tropics on Monday. It looks like it might be knuckle-gnawing time for residents of the Atlantic's hurricane alley next week, as an active pattern moves into place for the climatological peak two-week period of the Atlantic hurricane season.

Jeff Masters

Hurricane

The Battle to Draw Down Lake Okeechobee

By: JeffMasters, 2:39 PM GMT on August 24, 2013

After the wettest July ever recorded in Florida, the Army Corps of Engineers is battling to draw down the level of Lake Okeechobee before the September peak of the rainy season. The huge lake represents an important source of fresh water to South Florida, but also poses a grave danger. The 25 - 30'-tall, 143-mile long Herbert Hoover Dike surrounding the lake was built in the 1930s out of gravel, rock, limestone, sand, and shell using old engineering methods. The dike is tall enough that it cannot be overtopped by a storm surge from anything but an extreme hurricane, but the dike is vulnerable to leaking and failure when heavy rains bring high water levels to the lake. The Army Corps of Engineers is scrambling to complete a $300 million upgrade to the dike to reduce the chances of such a failure. However, those repairs are not scheduled to be completed until 2018, and the Corps is warning that the Lake Okeechobee dike is in danger of failure this year should heavy rains from a tropical storm or hurricane raise the lake level and put high stresses on the old dike. A 2011 risk assessment estimated the dike's probable failure rate at every fourteen years. A 2008 Army Corp of Engineers study said this about the vulnerable dike:

"There is limited potential for a dike failure with lake levels as low as 18.5 feet. The likelihood of a failure increases at higher lake levels. At a lake level of 21 feet--a 1-in-100 year flood event--a dike failure would be likely at one or more locations. In the event of a dike failure, waters from Lake Okeechobee would pass through the breach--uncontrollably--and flood adjacent land. Flooding would be severe and warning time would be limited. And with 40,000 people living in the communities protected by the Herbert Hoover Dike, the potential for human suffering and loss of life is significant. Our engineering studies indicate the southern and eastern portions of the dike system are more likely to fail than the northern and western portions of the dike. In general, we would expect a warning time of 24 to 48 hours prior to a dike failure that releases water from the lake; however, under some conditions the warning time might be longer, and under others, a dike failure could occur with no warning."

The city most at risk from a dike failure may be Belle Glade (population 18,000) on the southeast shore. Belle Glade is at 16' elevation. If Lake Okeechobee is at 20' above mean sea level when the dike fails, this implies that at least three feet of water could flood Belle Glade. If a wide section of the dike breaks and there is a Cat 3+ hurricane driving a massive storm surge at the time, then the flood could be much higher. During the 1928 hurricane, which had 130 mph winds while over the lake, the water from the storm surge reached seven feet above ground level in Belle Glade.


Figure 1. Water level of Florida's Lake Okeechobee between January 2012 and August 23, 2013. The SFWMD reported that 2013 had the wettest start to the annual wet season in 45 years, with the district-wide average rainfall for July at 10.36". Heavy rains from Tropical Storm Isaac in late August of 2012 caused a 3.5' rise in lake water levels in six weeks; record rains in July 2013 caused a 3' rise in water levels to 16' in mid-August, just below the record high lake level for that time of year. The Army Corps tries to keep the lake level below 15.5'; the dike surrounding the lake is in danger of failure when the lake level hits 18.5'. As of August 23, 2013, the lake level was 15.6', after hitting a peak of 16.1' early in August. Lake Okeechobee reached an elevation of 18.6' and 18.5'--both 1-in-30-year events--in 1995 and 1998. Image credit: Army Corps of Engineers.


Figure 2. Aftermath of the 1928 Okeechobee Hurricane, showing damage to a cluster of Everglades scientific work stations in Belle Glade. The hurricane killed 2,500 people, mostly near Belle Glade. Image credit: University of Florida, via the historicpalmbeach.com.

The Great 1928 Lake Okeechobee Hurricane
The shores of Lake Okeechobee are the site of the second deadliest natural disaster in U.S. history--the 1928 Okeechobee Hurricane. This mighty hurricane caused catastrophic damage where it struck the Florida coast as a Category 4 storm near Palm Beach, and weakened only slightly to Category 3 strength with 130 mph winds when it passed over Lake Okeechobee. The powerful winds of the hurricane brought a 12' storm surge to the south end of the lake, which overwhelmed the 6' high levees protecting the farm lands to the south. The resulting flood covered an area of hundreds of square miles with water up to 20' deep, and killed at least 2,500 people--mostly black migrant farm workers. A mass grave at the Port Mayaca Cemetery east of Port Mayaca contains the bodies of 1,600 victims of the hurricane. The Herbert Hoover Dike was built in the 1930s around most of Lake Okeechobee in response to this disaster.


Figure 3. When Lake Okeechobee water levels exceed 15.5' above mean sea level, large amounts of lake water are released out of two canals that carry the storm water runoff to the ocean. The western drainage canal flows into the Caloosahtchee Estuary and into the Gulf of Mexico at Fort Myers. The eastern drainage canal flows into the St. Lucie River Estuary and into the Atlantic Ocean near Stuart. High levels of nutrients due to agricultural runoff in the Lake Okeechobee water have been causing toxic algae blooms in the areas marked in green during July and August of 2013.

Lake Okeechobee runoff contributing to toxic algae blooms
Heavy rains that began in early July raised the level of Lake Okeechobee by two feet, to sixteen feet. Torrential rains of 7+ inches from a tropical storm or hurricane are capable of raising the lake level by over three feet in a few weeks; this occurred in 2008, when Tropical Storm Fay took a leisurely romp across Florida, and again in 2012, when Tropical Storm Isaac lumbered past. Under ideal conditions, the Army Corps can only lower the lake at a rate of about 0.4" per day. The Corps has been dumping water out of the lake since May 9, and began dumping water out as fast as it could beginning on July 25, to keep the lake below 15.5'. Had these releases not occurred, the lake would have been two feet higher than it is now. Most of this excess water was sent out Lake Okeechobee's western drainage canal into the Caloosahtchee Estuary, which empties into the Gulf of Mexico at Fort Myers. The Lake Okeechobee water is full of agricultural runoff and laden with fertilizers, and this polluted water, combined with an even greater amount of polluted water coming from local runoff due to heavy rains, caused a toxic red tide bloom in the coastal waters of Fort Myers that is being blamed for a record 133 manatees deaths this year. A lesser amount of Lake Okeechobee water has been sent eastwards into the St. Lucie River, where it drains into the Atlantic Ocean near Stuart through the Indian River Lagoon. A similar level of discharge goes down the C-51 canal in into the estuary by West Palm Beach, the Lake Worth Lagoon. The polluted Lake Okeechobee water, which was also combined with large amounts of polluted local runoff water from heavy rains, has caused havoc in these coastal waters this summer, affecting oyster beds, sea grasses, mangroves, and wildlife. Numerous toxic algae blooms have created lime-green water unsafe for recreation in the Indian River Lagoon, leading to calls by local residents for immediate political action. Temporary relief is at hand, though. On Wednesday, the Corps announced that due to falling lake levels, an easing up of the summer rains, and a forecast for merely average rains over the next week, water releases from then lake would be cut almost in half. A further reduction in flow began on Saturday morning. Governor Rick Scott of Florida announced this week that the state of Florida was committing $40 million to a project to build a reservoir aimed at diverting storm water releases from Lake Okeechobee. However, a press release by a coalition of environmental groups labeled this solution as a "Band-aid", saying "It only addresses a tiny fraction of the sewage, manure, and fertilizer runoff--called ‘nutrient pollution’--that comes from within the St. Lucie watershed, and it does nothing to reduce the nutrient pollution sliming all the other, rivers, springs, lakes and bays all over Florida.”


Figure 4. A toxic algae bloom in the St. Lucie River in Stuart, FL on August 1, 2013. Image credit: Dick Miller.


Figure 5. This photo taken from Martin County Sheriffs Office Air 1 shows thousands of people spelling out “Save Our River” along the Florida shoreline from Stuart to Jensen Beach on August 11, 2013. The people were protesting water pollution due to storm water runoff from Lake Okeechobee and local sources that has caused toxic algae blooms. Source: Martin County Sheriffs Office, via the Martin County Times.

For further reading
Despite repairs, Lake Okeechobee dike remains a danger: August 16, 2013 Miami Herald article.

Jeff Masters

Hurricane Air and Water Pollution

Quiet in the Atlantic

By: JeffMasters, 12:29 PM GMT on August 23, 2013

A weak trough of low pressure over the Northern Gulf of Mexico has created a small area of heavy thunderstorms off the coast of Alabama, which are moving west at 10 - 15 mph. Surface pressures are high in the region, wind shear is a moderately high 15 - 20 knots, and an upper level low is centered over the Yucatan Channel is bringing some dry air to the Gulf of Mexico. These factors will discourage development, and in their 8 am EDT Friday Tropical Weather Outlook, NHC gave the disturbance just a 10% chance of development by Wednesday.


Figure 1. A weak trough of low pressure over the Northern Gulf of Mexico created a modest area of heavy thunderstorms on Thursday afternoon, August 22, 2013. Image crest: NASA.

A tropical wave that emerged form the coast of Africa on Thursday night could develop by Wednesday the 28th midway between Africa and the Lesser Antilles Islands, according to this morning's 00Z run of the Navy's NAVGEM model. However, this forecast is dubious, as none of the other models is calling for tropical cyclone formation in the Atlantic during the coming week. The tropical Atlantic is dominated by dry, stable air, and a new outbreak of dust and dry air is exiting the coast of Africa this weekend, which will keeping the Tropical Atlantic dry though at least Thursday, August 29. Another tropical wave will come off the coast of Africa on Monday August 26, and these two tropical waves may be able to prime the tropical Atlantic for higher chances of development by leaving a moister environment for a vigorous tropical wave predicted to move off the coast of Africa on Thursday, August 29.

In the Eastern Pacific, Tropical Depression 9-E is headed northwards along the coast of Baja California. Cold waters off the coast of Baja California will likely keep TD 9-E from strengthening into a hurricane, and the storm is not expected to make landfall in Mexico. However, moisture from TD 9-E will stream into Northwest Mexico over the weekend, increasing the odds of flash flooding. The models predict formation of a new tropical storm that will follow a similar path to TD 9-E along the west coast of the Baja Peninsula during the last few days of August.

I'll have a new post later today on the hurricane flood risk from Florida's Lake Okeechobee this year.

Jeff Masters

Hurricane

Chinese Floods Kill 250 During Past Week: Earth's 4th Deadliest Disaster of 2013

By: JeffMasters, 3:07 PM GMT on August 22, 2013

Typhoon Trami hit Southeast China on Wednesday as a Category 1 storm with 85 mph winds, after dumping torrential rains in the Philippines that killed 17 people. Trami's rains are creating new flooding flooding problems for a Chinese nation already reeling from a week of deadly floods that have left over 250 people dead or missing. Twenty-one of the deaths came on Tuesday evening, due to a flash flood in northwest China's Qinghai Province. The deadliest incident came earlier this week, when torrential rains caused the Nei River in northeastern Liaoning province to overflow near the city of Fushuan, killing 54 people and leaving 97 missing. It's been a disastrous summer for flooding in China. According to the July 2013 Catastrophe Report from insurance broker Aon Benfield, the 250 people killed in the past week of flooding in China make this disaster Earth's fourth deadliest natural disaster of 2013. The deadliest was the monsoon flood in India and Nepal in June that killed 6,500; the second deadliest was the severe winter weather that hit India, Bangladesh, and Nepal in January, killing 329; the third deadliest was flooding in China in mid-July that left 305 people dead or missing. China had three billion-dollar flooding disasters in July:

June 29 - July 3, Southwestern, Central, Eastern, and Northern China: $1.4 billion in damage, 4 deaths
July 7 - 17, nearly every section of China,: $4.5 billion in damage, 305 dead or missing
July 21 - 25, Jilin, Gansu, Heilongjiang, Xinjiang, and Sichuan provinces: $1.4 billion in damage, 36 deaths

One positive note: Trami's rains are falling over portions of China that are in moderate to exceptional drought. The 2013 drought in China has been that nation's most expensive natural disaster of the year, costing over $10 billion.


Figure 1. MODIS satellite image of Typhoon Trami taken at 02:45 UTC on August 21, 2013, as the storm approached the northern tip of Taiwan. At the time, Trami had maximum sustained winds of 75 mph. Image credit: NASA.


Figure 2. Workers use excavator to clean up mud after heavy rain hit on August 19, 2013 in Fushuan, in the Liaoning Province of China. The Nei River overflowed, killing 54 and leaving 97 missing in Fushuan. Photo by ChinaFotoPress/Getty Images.


Video 1. Shiman Dam in Northern Taiwan releases gargantuan fountains of flood waters on August 22, 2013, after Typhoon Trami dumped over 300 mm (11.81") of rain in 24 hours. Video taken by storm chaser James Reynolds, and brought to my attention by wunderground member Robert Speta.

Quiet in the Atlantic
There are no tropical disturbances of note in the Atlantic today. This morning's 00Z run of the Navy's NAVGEM model called for formation of a tropical storm early next week midway between Africa and the Lesser Antilles Islands, from a tropical wave expected to emerge from the coast of Africa on Friday. However, this forecast is dubious, as none of the other models is calling for tropical cyclone formation in the Atlantic during the coming week. The tropical Atlantic is dominated by dry, stable air, and a new outbreak of dust and dry air will exit the coast of Africa this weekend, keeping the Tropical Atlantic dry though at least Thursday, August 29. Dry air coming off the coast of Africa may abate some after that, as the long-range models call for the Azores High to shift more to the northeast. An upper level low pressure system is also likely to dig southwards from the Azores during the middle of next week, bringing high wind shear to the tropical Atlantic between Africa and the Lesser Antilles Islands. The NAVGEM also predicted that a tropical storm would form in the Southern Gulf of Mexico's Bay of Campeche early next week, but this also seems unlikely, due to high wind shear from from an upper level low, plus a dry northeasterly flow. The best chance of development this month in the Atlantic would appear to be from a tropical wave predicted to move off the coast of Africa on August 29.

In the Eastern Pacific, Tropical Depression 9-E has formed, about 390 miles SSW of Cabo San Lucas, Mexico. Cold waters off the coast of Baja California will likely keep TD 9-E from strengthening into a hurricane, and the storm is not expected to make landfall in Mexico. However, moisture from TD 9-E will stream into Northwest Mexico and possibly the Southwest U.S. 3 - 5 days from now, which could triggering flash flooding. The European and GFS models predict formation of a second tropical storm that will follow a similar path to TD 9-E late next week, along the west coast of the Baja Peninsula.

Jeff Masters

Hurricane

July 2013: Earth's 6th Warmest July on Record

By: JeffMasters, 3:20 PM GMT on August 21, 2013

July 2013 was the globe's 6th warmest July since records began in 1880, according to NOAA's National Climatic Data Center (NCDC). NASA rated it the 10th warmest July on record. The year-to-date period of January - July has been the 6th warmest such period on record. July 2013 global land temperatures were the 8th warmest on record, and global ocean temperatures were the 5th warmest on record. July 2013 was the 341st consecutive month with global temperatures warmer than the 20th century average. Global satellite-measured temperatures in July 2013 for the lowest 8 km of the atmosphere were 10th or 8th warmest in the 35-year record, according to Remote Sensing Systems and the University of Alabama Huntsville (UAH), respectively. Wunderground's weather historian, Christopher C. Burt, has a comprehensive post on the notable weather events of July 2013 in his July 2013 Global Weather Extremes Summary. The big stories that he highlights are the extraordinary heat waves in the central portion of Russia’s Arctic region and in eastern China. Both heat waves were unprecedented for their respective locations. Extreme heat has killed at least 40 people in China since July 1. Also, Greenland measured its hottest temperature on record July 30th when the mercury hit 25.9°C (78.6°F) at Maniitoq Mittarfia during an unusually strong local wind event called a foehn.


Figure 1. Departure of temperature from average for July 2013, the 6th warmest July for the globe since record keeping began in 1880. Most of the world's land surfaces were warmer than average during July. The United Kingdom and Australia had their 3rd warmest July temperatures on record, and South Korea and New Zealand, their 4th warmest July. Parts of the central and southeastern United States, small regions across northern Canada, eastern Greenland, and parts of Mongolia and eastern Russia were cooler than average. Far northwestern Canada and part of the eastern United States were much cooler than their long-term averages. Image credit: National Climatic Data Center (NCDC) .

Six billion-dollar weather disasters in July
Six billion-dollar weather disasters hit Earth during July. The most damaging of these were in China: the on-going drought in Central and Eastern China that has cost $6 billion this year, and significant flooding across nearly every section of China between the 7th and 17th, which left 305 people dead or missing, and cost $4.5 billion. The world-wide tally of billion-dollar weather disasters so far in 2013 is nineteen, and the U.S. total is five, according to the July 2013 Catastrophe Report from insurance broker Aon Benfield:

1) Flooding, Central Europe, 5/30 - 6/6, $22 billion
2) Drought, Brazil, 1/1 - 5/31, $8.3 billion
3) Drought, Central and Eastern China, 1/1 - 7/31, $6.0 billion
4) Flooding, Calgary, Alberta Canada, 6/19 - 6/24, $5.3 billion
5) Tornado, Moore, OK, and associated U.S. severe weather, 5/18 - 5/22, $4.5 billion
5) Flooding, China, 7/7 - 7/17, $4.5 billion
7) Flooding, Indonesia, 1/20 - 1/27, $3.31 billion
8) Flooding, Australia, 1/21 - 1/30, $2.5 billion
9) Tornadoes and severe weather, U.S., 5/26 - 6/2, $2 billion
9) Severe weather, Midwest U.S., 3/18 - 3/20, $2 billion
11) Winter weather, Europe, 3/12 - 3/31, $1.8 billion
12) Drought, New Zealand, 1/1 - 5/10, $1.6 billion
12) Severe weather, U.S., 4/7 - 4/11, $1.6 billion
14) Flooding, Toronto, Canada, 7/8, $1.45 billion
15) Flooding, China, 6/29 - 7/3, $1.4 billion
15) Flooding, China, 7/21 - 7/25, $1.4 billion
17) Flooding, Argentina, 4/2 - 4/4, $1.3 billion
18) Flooding, India and Nepal, 6/14 - 6/18, $1.1 billion
19) Winter weather, Plains, Midwest, Northeast U.S., 2/24 - 2/27, $1.0 billion


Figure 2. Heavy flood waters sweep through Beichuan in southwest China's Sichuan province on July 9, 2013. Rainfall amounts as high as 1,150 millimeters (45.3 inches) of rain fell in the Dujiangyan region, triggering Sichuan Province's worst floods in at least 50 years. Flooding in China from July 7 - 17, 2013 cost at least $4.5 billion. Image credit: AFP/Getty Images.

Neutral El Niño conditions continue in the equatorial Pacific
For the 16th month in row, neutral El Niño conditions existed in the equatorial Pacific during July 2013. NOAA's Climate Prediction Center (CPC) expects neutral El Niño conditions to last though the fall, and the large majority of the El Niño models also predict that neutral conditions will last through the fall of 2013. Temperatures in the equatorial Eastern Pacific need to be 0.5°C below average or cooler for three consecutive months for a La Niña episode to be declared; sea surface temperatures were 0.4°C below average as of August 19, and have been +0.1 to -0.4°C from average since April 1, 2013.

Arctic sea ice falls to 5th lowest July extent on record
Arctic sea ice extent during July was 5th lowest in the 35-year satellite record, according to the National Snow and Ice Data Center (NSIDC). Arctic sea ice extent maintained a steady, near-average pace of retreat through the first half of August, making it very unlikely that the record low minimum extent observed in September 2012 will be surpassed this year. Nevertheless, there are extensive areas of low-concentration ice, even in regions close to the North Pole.

Quiet in the Atlantic
There are no tropical disturbances of note in the Atlantic today. None of the computer models is calling for tropical cyclone formation in the Atlantic during the coming week. In the Eastern Pacific, there is a tropical disturbance (94E) about 600 miles southwest of Manzanillo, Mexico that is growing more organized. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC gave the disturbance a 60% of developing by Friday, and a 90% chance of developing by Monday. The GFS and European models predict that 94E will become Tropical Storm Ivo and pass close to the coast of Baja California over the weekend.


Video 1. Most spectacular weather video of July: a tornado in Milan, Italy on July 29 hurls huge amounts of debris against the office building the video was taken from. The photographer is lucky the building's windows didn't shatter and seriously injure him. Jason Samenow at the Capital Weather Gang has more videos and details on the event.

Jeff Masters

Climate Summaries

Unprecedented Cut in Colorado River Flow Ordered, Due to Drought

By: JeffMasters, 3:20 PM GMT on August 20, 2013

For the first time in history, the U.S. government has ordered that flow of Colorado River water from the 50-year-old Glen Canyon Dam be slashed, due to a water crisis brought about by the region's historic 14-year drought. On Friday, the Federal Bureau of Reclamation--a division of the Department of Interior that manages water and electric power in the West--announced that it would cut water released from Lake Powell's Glen Canyon Dam by 750,000 acre-feet in 2014. An acre-foot is the amount of water that will cover an acre of land one foot deep; 750,000 acre-feet is enough water to supply at least 750,000 homes for one year. The flow reduction will leave the Colorado River 9% below the 8.23 million acre feet that is supposed to be supplied downstream to Lake Mead for use in California, Nevada, Arizona and Mexico under the Colorado River Compact of 1922 and later agreements. "This is the worst 14-year drought period in the last hundred years," said Upper Colorado Regional Director Larry Walkoviak in a Bureau of Reclamation press release.

In the winter of 2005, Lake Powell reached its lowest level since filling, an elevation 150' below full pool. Lake levels recovered some in during 2005 - 2011, but the resurgence of severe to extreme drought conditions have provoked a steep decline in 2012 and 2013, with the lake falling 35' over the past year. As of August 18, 2013, Lake Powell was 109' below full pool (45% of capacity), and was falling at a rate of one foot every six days.


Figure 1. Satellite comparisons of water levels in Arizona and Utah's Lake Powell between 1999 and 2013 show a huge reduction in the amount of water in the lake. Image credit: NASA Earth Observatory.


Figure 2. From October 1, 2012 - July 31, 2013, precipitation over the Colorado River Watershed was about 80% of average. Image credit: Colorado Basin River Forecast Center.

Las Vegas' Water Supply, Lake Mead, Near a Record Low
Downstream of Lake Powell lies Lake Mead, filled in 1936 when Hoover Dam was completed. Lake Mead supplies Las Vegas with ninety percent of its drinking water, and the water level of Lake Mead is expected to fall by eight feet in 2014 due to the lower water flow levels out of Lake Powell ordered on Friday. Lake Mead has fallen by 100 feet since the current 14-year drought began in 2000, and the higher of the two intake pipes used to supply Las Vegas with water from the lake is in danger of running dry. As a result, a seven-year, $800 million project is underway by the Southern Nevada Water Authority to build a third intake pipe that will tap the deepest part of the reservoir. This so-called "third straw" is scheduled to be available late in 2014, which may be cutting it close, if the Colorado River watershed experiences another year of drought as severe as in 2012 - 2013. Southern Nevada has done well to reduce water usage, though--the region's annual water consumption decreased by nearly 29 billion gallons between 2002 and 2012, despite a population increase of more than 400,000 during that span.


Figure 3. Lake Mead water levels from 1938 - 2013 in July show a precipitous drop since drought conditions gripped the Western U.S. in 2000. The Lake Mead photo was taken by wunderphotographer LAjoneson June 29, 2007, when the lake had a "bathtub ring" 109' tall. Water level data from The Bureau of Reclamation.


Figure 4. Workers handle the main drive sections of the tunnel boring machine that is drilling a 3-mile long tunnel through solid rock to supply Las Vegas with water from Lake Mead. The new intake tunnel is designed to maintain the ability to draw upon Colorado River water at lake elevations as low as 1,000 feet above sea level. The lake already has two intake pipes, and the higher of these will go dry when the lake level hits 1050' - 1075'. As of August 2013, the Lake Mead water level was 1106' above sea level, which is 114' below full pool, but 24' above the record low water level of 1081' set in November 2010. Image credit: Southern Nevada Water Authority.

Drought conditions worsen over Southwest U.S. in August
According to the U.S. Drought Monitor, the Western U.S. drought peaked in July 2002, when 79% of the West was in at least severe drought, and 45% of the region was in the two highest categories of drought--extreme to exceptional. However, drought conditions have been steadily intensifying this summer. The August 13, 2013 Drought Monitor report showed that drought conditions in the Western U.S. are now the worst since 2004, with 78% of the West in at least severe drought, and 20% in the two highest categories of drought, extreme and exceptional. The latest U.S. Seasonal Drought Outlook, issued on August 15, calls for drought to remain entrenched over the large majority of the Western U.S. through the end of November.


Figure 5. As of August 13, 2013, severe to exceptional drought gripped nearly all of the Colorado RIver's watershed in Arizona, Utah, New Mexico, Wyoming, California, and Colorado. Image credit: NOAA/NESDIS/NCDC.

Causes of the great Western U.S. drought
It is well-known that natural variations in sea surface temperature patterns, such as seen from the El Niño/La Niña oscillation, can influence storm tracks and can cause prolonged periods of drought. These natural variations likely had a hand in causing the great 2000 - 2013 Western U.S. drought. However, changes in the amount of sea ice covering the Arctic can also have a major impact on Northern Hemisphere atmospheric circulation patterns. We must consider if global warming, which has led to a 50% decline in summer Arctic sea ice extent since 1979, may be altering storm tracks and contributing to drought. In 2004, Lisa Sloan, professor of Earth sciences at UC Santa Cruz, and her graduate student Jacob Sewall published an article in Geophysical Research Letters, Disappearing Arctic sea ice reduces available water in the American west. An accompanying news release explained that their climate models found "a significant reduction in rain and snowfall in the American West” as a result of Arctic sea ice loss:

What they found was a change in atmospheric circulation patterns that caused a small northward shift in the paths of winter storms over western North America. This shift in winter storm tracks resulted in significantly reduced winter precipitation from southern British Columbia to the Gulf of California. In some areas, average annual precipitation dropped by as much as 30 percent. The reductions were greatest along the West Coast, with lesser changes further inland. But even as far inland as the Rocky Mountains, winter precipitation fell by 17 percent.

The sea ice acts like a lid over the ocean surface during the winter, blocking the transfer of heat from the ocean to the atmosphere, Sewall explained. Where the sea ice is reduced, heat transfer from the ocean warms the atmosphere, resulting in a rising column of relatively warm air. The shift in storm tracks over North America was linked to the formation of these columns of warmer air over areas of reduced sea ice in the Greenland Sea and a few other locations.


A follow-up paper by Dr. Sewall in 2005, "Precipitation Shifts over Western North America as a Result of Declining Arctic Sea Ice Cover: The Coupled System Response", used a more sophisticated modeling technique but confirmed the results of the 2004 paper. In a June 2013 interview with climateprogress.org, Dr. Sewall commented:

"I think the hypothesis from 2004 and 2005 is being borne out by current changes. The only real difference is that reality is moving faster than we thought/hoped it would almost a decade ago."


Figure 6. The area of the Western U.S. in drought peaked during 2002 - 2004, but during 2013 has been approaching levels not seen since 2004. Image credit: U.S. Drought Portal.

Western North America drought of 2000 - 2004 the worst in over 800 years
The Colorado River's water woes are due to an extraordinary 14-year drought that began in 2000, which peaked during 2000 - 2004. A 2012 study titled, Reduction in carbon uptake during turn of the century drought in western North America, found that the 2000 - 2004 drought was the most severe Western North America event of its kind since the last mega drought over 800 years ago, during the years 1146 - 1151. The paper analyzed the latest generation of climate models used for the 2013 IPCC report, which project that the weather conditions that spawned the 2000 - 2004 drought will be the new normal in the Western U.S. by 2030, and will be considered extremely wet by the year 2100. If these dire predictions of a coming "megadrought" are anywhere close to correct, it will be extremely challenging for the Southwest U.S. to support a growing population in the coming decades.


Figure 7. Normalized precipitation over Western North America (five-year mean) from 22 climate models used to formulate the 2013 IPCC report, as summarized by Schwalm et al., 2012, Reduction in carbon uptake during turn of the century drought in western North America. The horizontal line marks the precipitation level of the 2000 - 2004 drought, the worst of the past 800 years. Droughts of this intensity are predicted to be the new normal by 2030, and will be considered an outlier of extreme wetness by 2100. The paper states: "This impending drydown of western North America is consistent with present trends in snowpack decline as well as expected in-creases in aridity and extreme climate events,including drought, and is driven by anthropogenically forced increases in temperature with coincident increases in evapotranspiration and decreases in soil moisture. Although regional precipitation patterns are difficult to forecast, climate models tend to underestimate the extent and severity of drought relative to available observations. As such, actual reductions in precipitation may be greater than shown. Forecasted precipitation patterns are consistent with a probable twenty-first century megadrought." Image credit: Schwalm et al., 2012, Reduction in carbon uptake during turn of the century drought in western North America, Nature Geoscience 5, 551-555, Published online 29 JULY 2012, DOI: 10.1038/NGEO1529, www.nature.com/naturegeoscience.

Related posts
Lessons from 2012: Droughts, not Hurricanes, are the Greater Danger, my November 2012 post.

National Geographic has an excellent interactive satellite image that shows the difference in Lake Powell water levels between 1999 and 2013.

How Two Reservoirs Have Become Billboards For What Climate Change Is Doing To The American West, August 12, 2013 climateprogress.org post by Tom Kenworthy.

Scientists Predicted A Decade Ago Arctic Ice Loss Would Worsen Western Droughts. Is That Happening Already?, June 2013 post by Joe Romm at climateprogress.org.

Twenty Cities At Risk of Water Shortages, August 14, 2013 wunderground news post by Nick Wiltgen

References
Sewall, Jacob O., 2005, Precipitation Shifts over Western North America as a Result of Declining Arctic Sea Ice Cover: The Coupled System Response, Earth Interact., 9, 1–23. doi: http://dx.doi.org/10.1175/EI171.1

Sewall, J.O., and L.C. Sloan, 2004, Disappearing Arctic sea ice reduces available water in the American west, Geophys. Res. Lett., 31, L06209, doi:10.1029/2003GL019133. Accompanying news release.

Jeff Masters

Drought Climate Change

The Atlantic Goes Quiet

By: JeffMasters, 2:48 PM GMT on August 19, 2013

There are no tropical disturbances of note in the Atlantic today. The tropical wave off the coast of Africa (94L) that we were watching on Sunday has become too disorganized to be a threat to develop. None of the reliable computer models is calling for tropical cyclone formation in the Atlantic this week. In the Eastern Pacific, there is a tropical disturbance several hundred miles south of Manzanillo, Mexico that most of the models predict will develop by Thursday. In their 8 am EDT Monday Tropical Weather Outlook, NHC gave the disturbance a 20% of developing by Tuesday, and an 80% chance of developing by Saturday. The GFS and European models predict that this will become a tropical storm that will pass close to the tip of Baja on Friday or Saturday, respectively.


Figure 1. The third week of August is when Atlantic hurricane activity traditionally takes a large upswing, but that will not be the case in 2013.

We've had five named storms so far in the Atlantic this year, which is more that average for this point in the season. Usually, the fifth named storm does not arrive until August 31. However, we are well behind average for the arrival of the season's first hurricane, which usually occurs by August 10. The season's second hurricane usually arrives by August 28. It is questionable if we will see the season's first hurricane by that date, given the current lack of activity, the dry air moving across the Tropical Atlantic, and the lack of model predictions for tropical storm formation this week. Still, I'm not willing to downgrade the seasonal forecasts for above-average activity yet, as we are still three weeks away from the usual September 10 peak in activity, and the Atlantic is capable of getting very active in a hurry.

Jeff Masters

Hurricane

African Wave 94L Has Potential to Develop; 92L Dead; Erin Dying

By: JeffMasters, 4:22 PM GMT on August 18, 2013

It's time to turn our attention to the coast of Africa, where a new tropical wave (94L) has just emerged over the Tropical Atlantic. Wind shear is a moderate 10 - 20 knots over 94L, and water temperatures are warm enough for development, 27.5°C. Satellite loops show that 94L has a modest amount of spin, but the storm's heavy thunderstorms are not very intense, and are poorly organized. The 12Z Sunday run of the SHIPS model predicted that during the next five days, wind shear for 94L will be mostly in the moderate range, and ocean temperatures will slowly cool to 26.5°C. These conditions should allow for some slow development. As usual, dry air from the Saharan Air Layer will likely be an impediment to development, as the 11 am EDT Sunday SAL analysis showed a large pulse of dry air and dust exiting from the Sahara just to the north of 94L. The Sunday 06Z run of the GFS model and 00Z run of the ECMWF model did not calling for 94L to develop. In their 8 am EDT Sunday Tropical Weather Outlook, NHC gave 94L a 10% of developing by Tuesday, and a 30% chance of developing by Friday.


Figure 1. Saharan Air Layer (SAL) analysis from 11 am EDT Sunday August 18, 2013. A large pulse of dust and dry air was exiting the coast of Africa just north of tropical wave 94L. Note the swirl of dry air marking the center of Tropical Depression Erin, near 20°N 40°W. Image credit: University of Wisconsin/NOAA-HRD.

Gulf of Mexico disturbance 92L dies
The tropical disturbance (92L) that was over the Gulf of Mexico the past few days has now degenerated into a trough of low pressure with little heavy thunderstorm activity, and is no longer a threat to develop. However, a flow of moist tropical air will take some of the remnants of 92L northwards over the Southeast U.S. over the next few days, bringing a swath of 3+ inches of rain to the Gulf Coast from Louisiana to the Florida Panhandle.


Figure 2. MODIS satellite image of Tropical Depression Erin taken at 10:30 am EDT Sunday August 18, 2013. At the time, Erin had maximum sustained winds of 35 mph. Image credit: NASA.

Tropical Storm Erin dying
Tropical Storm Erin over the Eastern Atlantic is small and weak and has lost nearly all of its heavy thunderstorms, as seen on satellite loops. Dry air, moderate wind shear, and marginal water temperatures will likely destroy the storm by Monday, as predicted by all of the reliable global computer models and the NHC.

Have a great weekend, everyone!

Jeff Masters

Hurricane

Gulf of Mexico Disturbance 92L Less Organized; Erin No Threat to Land

By: JeffMasters, 3:30 PM GMT on August 17, 2013

Tropical disturbance 92L over the South-Central Gulf of Mexico is looking less organized today due to dry air from an upper-level low pressure system over the Gulf. Satellite loops show that 92L has lost the well-developed surface circulation it had Friday, and its heavy thunderstorms are elongated and not well-organized, even though wind shear is a moderate 10 - 20 knots over the the wave, and ocean temperatures are a very warm 29 - 30°C. The hurricane hunter flight scheduled for today has been cancelled and re-scheduled for Sunday afternoon.


Figure 1. Visible satellite image of 92L taken at 10:30 am EDT Saturday August 17, 2013. Image credit: NASA/GSFC.

Forecast for 92L
The 12Z Saturday SHIPS model forecast predicts that 92L will remain in an area of low to moderate wind shear through Monday, and ocean temperatures will be a favorable 29 - 30°C. However, the dry air due to the upper-level low over the Gulf will continue to keep any development slow. A trough of low pressure is over the northern Gulf of Mexico and is bringing high wind shear values of 20 - 30 knots just to the north of 92L. This trough may also be able to pull the storm northwestwards to a landfall in Louisiana, Texas, or Northern Mexico on Monday of Tuesday, as suggested by all of our top models. The farther north 92L penetrates, the more difficult intensification becomes, due to higher wind shear values to the north. Regardless of 92L's track, a flow of moist tropical air along the storm's eastern flank will form an atmospheric river of moisture that will bring a swath of 5+ inches of rain to the Gulf Coast from Louisiana to the Florida Panhandle over the next few days. In their 8 am EDT Saturday Tropical Weather Outlook, NHC gave 92L a 40% of developing by Monday, and a 50% chance of developing by Thursday.


Figure 2. Predicted precipitation for the 7-day period ending on Saturday, August 24, 2013. Tropical moisture flowing north and northeastwards from 92L over the Southeast U.S. is expected to create a swath of 5+ inches of rain along the coast. Image credit: NOAA/HPC.

Tropical Storm Erin
Tropical Storm Erin over the Eastern Atlantic is small and weak and has lost nearly all of its heavy thunderstorms, as seen on satellite loops. This is due, in part, to the 25.5 - 26°C waters the storm is traversing, which is a marginal water temperature for tropical cyclones. Erin is also having trouble with dry air from the Saharan Air Layer (SAL), and the storm's northwest motion has begun to cut Erin off from a moist source of air to its south--the semi-permanent band of tropical thunderstorms called the ITCZ (Inter-Tropical Convergence Zone.) Several of the major global computer models call for Erin to dissipate over the next few days, and it is unlikely that the storm will ever threaten any land areas.


Figure 3. MODIS satellite image of Tropical Storm Erin taken at 10:30 am EDT Saturday August 17, 2013. At the time, Erin had maximum sustained winds of 40 mph. Image credit: NASA.

The tropical wave that spawned Erin began over East Africa on August 9, dumping 0.83" of rain on Khartoum, Sudan, worsening a devastating flooding situation there. Periodic torrential rains in Sudan that began on August 2 have triggered flash flooding that has killed 53 people. More rain is on the way for the waterlogged nation, as a series of strong tropical waves associated with an unusually active African monsoon parade across the continent. Several of these tropical waves have the potential to grow into tropical storms once they exit the coast, but the latest 06Z run of the GFS model and 00Z run of the ECMWF model are not calling for any new tropical waves to develop off of the coast of Africa over the next seven days.

Wunderblogger Lee Grenci just posted this excellent analysis of Typhoon Utor's eyewall replacement cycle before the storm made landfall in the Philippines last week.


Video 1. Debris flies as Typhoon Utor hits Zhapo, China on 14th August 2013. Video taken by storm chaser James Reynolds, and brought to my attention by wunderground member Robert Speta.

Have a great weekend, everyone!

Jeff Masters

Hurricane

92L Poised to Develop in Gulf of Mexico; Erin Struggling in Far Eastern Atlantic

By: JeffMasters, 2:09 PM GMT on August 16, 2013

Tropical wave 92L crossed over Mexico's Yucatan Peninsula overnight, and the center of the disturbance is now located in the Gulf of Mexico along the west coast of the Yucatan Peninsula. Satellite loops show that 92L has a well-developed surface circulation, but there are no heavy thunderstorms near the center. A moderate-sized region of heavy thunderstorms does lie to the northeast and east of the center, over Cancun, Cozumel, and southwards to Belize. An upper-level low pressure system over the Gulf of Mexico is pumping dry air into 92L, slowing development. Wind shear is a moderate 10 - 20 knots over the the wave, which should allow slow development today. The hurricane hunter flight scheduled for today has been cancelled.


Figure 1. MODIS satellite image of 92L taken at 1:30 pm EDT Friday August 16, 2013. Image credit: NASA.

Forecast for 92L
The 12Z Friday SHIPS model forecast predicts that 92L will remain in an area of low to moderate wind shear through Saturday, and ocean temperatures will be a favorable 29 - 30°C. The topography of the Southern Gulf of Mexico's Bay of Campeche can aid in getting a storm spinning more readily, as well. Given these favorable conditions for intensification, 92L should be able to become a tropical depression by Saturday, and a tropical storm by Sunday. A trough of low pressure over the northern Gulf of Mexico will dip down by Sunday over the Central Gulf of Mexico, increasing the wind shear to a high 20 - 30 knots just to the north of 92L. This trough may also be able to pull the storm northwestwards to a landfall in Texas on Monday or Tuesday, as the 00Z Friday runs of UKMET and NAVGEM model predict. If 92L does follow this more northwesterly path, intensification into a strong tropical storm would be difficult, due to the high wind shear. An alternate scenario is presented by our two top-performing models, the European and GFS. They predict that 92L will take a nearly due west track, resulting in a landfall south of Tampico, Mexico on Monday. The storm would have more of an opportunity to strengthen in this scenario, since wind shear would be lower. Either scenario is reasonable, and residents of the Mexican and Texas Gulf Coast should anticipate the possibility of a tropical storm hitting the coast as early as Sunday night. Regardless of 92L's track, a flow of moist tropical air along the storm's eastern flank will form an atmospheric river of moisture that will bring a wide swath of 4+ inches of rain to the Gulf Coast from Louisiana to the Florida Panhandle over the next few days. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC gave 92L a 50% of developing by Sunday, and a 60% chance of developing by Wednesday. I put these odds higher, at 70% and 80%, respectively.


Figure 2. MODIS satellite image of Tropical Depression Erin taken at 10:30 am EDT Friday August 16, 2013. At the time, Erin had maximum sustained winds of 35 mph. Image credit: NASA.

Tropical Storm Erin
Tropical Storm Erin is over the far Eastern Atlantic off the coast of Africa, and continues west-northwest at 15 mph. Erin is small and weak and has lost nearly all of its heavy thunderstorms, as seen on satellite loops. This is probably due, in part, to the fact the storm is over waters of 25.5 - 26°C, which is a marginal temperature for tropical cyclones. Erin is also having trouble with dry air from the Saharan Air Layer (SAL), and the storm's west-northwest motion is beginning to cut Erin off from a moist source of air to its south--the semi-permanent band of tropical thunderstorms called the ITCZ (Inter-Tropical Convergence Zone.) The latest 00Z runs of the major global computer models, except for the GFS, call for Erin to dissipate by early next week. Given Erin's struggles today, I expect the storm will be dead by Sunday.

Jeff Masters

Hurricane

Caribbean Disturbance 92L Moving Over the Yucatan; Erin Forms Off of Africa

By: JeffMasters, 2:40 PM GMT on August 15, 2013

The tropical wave in the Western Caribbean near the Yucatan Peninsula (92L) is growing more organized this morning, after an evening when it lost most of its heavy thunderstorm activity. Satellite loops show a modest-sized area of heavy thunderstorms that are increasing in intensity and areal coverage, but there are a no signs of a surface circulation. Winds at surface stations in the Western Caribbean also do not show a surface circulation. The highest surface wind reports this Thursday morning were at Western Caribbean buoy 42056 about 140 miles east southeast of Cozumel, which had east winds of 25 mph, gusting to 34 mph, at 10 am EDT. Wind shear is a moderate 10 - 20 knots over the the wave, which should allow slow development today until its west-northwest movement at 10 - 15 mph carries it over the Yucatan Peninsula Thursday afternoon. Four hurricane hunter flights were scheduled to fly into 92L today--an Air Force mission tasked to provide a center fix early this afternoon, two NOAA P-3 missions aimed at collecting real-time radar data to feed into the HWRF model, and a flight by the NOAA jet to collect dropsonde data around the periphery of 92L. However, all of these flights were cancelled, given that 92L did not organize as much as much as it could have.


Figure 1. MODIS satellite image of Invest 92L taken at 12:30 pm EDT Thursday August 15, 2013. Image credit: NASA.

Forecast for 92L
92L will trek across the Yucatan Peninsula Thursday evening and arrive in the Southern Gulf of Mexico on Friday, when it will have the opportunity to strengthen. The 06Z Thursday SHIPS model forecast predicts that 92L will remain in an area of low to moderate wind shear through Saturday, and ocean temperatures will be a favorable 29 - 30°C. The topography of the Southern Gulf of Mexico's Bay of Campeche can aid in getting a storm spinning more readily, but 92L may be far enough north that this influence will be negligible. Given all these factors, 92L should be able to become at least a tropical depression by Saturday, A trough of low pressure over the northern Gulf of Mexico will dip down by Saturday evening over the Central Gulf of Mexico, potentially increasing wind shear to a high 20 - 30 knots, stalling any further intensification. This trough may also be able to pull the storm northwards to a landfall between Eastern Louisiana and the Florida Panhandle, as the 00Z Thursday run of the European model is suggesting. This would bring a plenty of tropical moisture into the Southeast U.S., resulting in a large area of 4+" of rain. However, the other models show a more westerly track for 92L, with landfalls possible in Texas or Mexico south of the Texas border, and there is high uncertainty where 92L may go once it enters the Gulf of Mexico. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC gave 92L a 50% of developing by Saturday, and a 60% chance of developing by Tuesday.


Figure 2. MODIS satellite image of Tropical Storm Erin taken at 10:30 am EDT Thursday August 15, 2013. At the time, Erin had maximum sustained winds of 40 mph. Image credit: NASA.

Tropical Storm Erin
The season's fifth named storm of the year, Tropical Storm Erin, has formed over the far Eastern Atlantic off the coast of Africa. Erin is over warm waters of 27°C and is under a moderate 10 - 15 knots of wind shear, which should allow continued development today and Friday. Erin is a small storm, as seen on satellite loops. The 12Z Thursday SHIPS model forecast predicts that wind shear will be low to moderate for the next five days, which favors development. However, the waters beneath Erin will steadily cool to a marginal 26°C by Friday, and the atmosphere will steadily get drier, as the storm encounters the Saharan Air Layer (SAL), discouraging development. Erin's west-northwest motion will cut the storm off on Sunday from a moist source of air to its south--the semi-permanent band of tropical thunderstorms called the ITCZ (Inter-Tropical Convergence Zone.) The storm should weaken beginning on Sunday, which would result in Erin turning more to the west as the east-to-west blowing surface trade winds begin to dominate the steering of the shallower storm. We may see a situation like occurred for Tropical Storm Dorian in late July--intensification to a 60 mph tropical storm, followed by a slow decay and dissipation. The latest run of the GFS model calls for Erin to dissipate well before reaching the Lesser Antilles Islands.

Typhoon Utor dissipates
Typhoon Utor has dissipated after hitting Southeast China about 150 miles southwest of Hong Kong on Wednesday as a Category 1 storm with 90 mph winds. The typhoon is being blamed for 1 death in China, and sank a 21-person cargo ship off the coast. In the Philippines, where Utor hit as a Category 4 storm with 140 mph winds on Monday, 8 deaths are being blamed on the storm, and damage is estimated at $20 million.

Jeff Masters

Hurricane

Caribbean Disturbance 92L Organizing; Typhoon Utor Hits China

By: JeffMasters, 12:40 PM GMT on August 14, 2013

Pressures are falling in the Western Caribbean where a tropical wave (92L) is headed northwest at 10 - 15 mph. Cayman Islands radar shows that the thunderstorm activity is disorganized, and satellite loops show only a modest area of heavy thunderstorms, and no signs of a surface circulation. Wind shear is a moderate 10 - 20 knots over the the wave, which should allow slow development today. The 00Z SHIPS model forecast predicts that 92L will remain in an area of low to moderate wind shear through Friday, but then shear will rise on Saturday. If a tropical depression or tropical storm does form, and its circulation extends high above the surface, a trough of low pressure over the northern Gulf of Mexico would likely steer the storm northwards to a landfall between Eastern Louisiana and the Florida Panhandle. This is the solution presented by the 06Z run of the GFS model. It shows a landfall on Saturday of a weak tropical storm between Eastern Louisiana and the Florida Panhandle. The Navy's 00Z run of the NAVGEM model shows a more westerly path for 92L, with the storm eventually coming ashore near the Texas/Mexico border. The European model keeps 92L weak and does not develop it. The more northwards path advertised by the GFS model would bring a large amount of moisture into the Southeast U.S., resulting in a large area of 4+" of rain. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC gave the system a 50% of developing by Friday, and a 60% chance of developing by Monday. The Hurricane Hunters are on call to investigate the system on Thursday, if necessary.


Figure 1. Morning satellite image of Invest 92L over the Western Caribbean, taken at 10:31 am EDT August 14, 2013. Image credit: NASA/GSFC.

New African Tropical Wave 93L Organizing
A tropical wave that pushed off the coast of Africa on Tuesday (93L) is showing a modest amount of heavy thunderstorm activity and spin on satellite loops as it heads west-northwest at 10 mph. The wave is over warm waters of 28°C and is under a moderate 10 - 15 knots of wind shear, which should allow continued development today and Thursday. The 00Z SHIPS model forecast predicts that wind shear will be low to moderate for the next five days, which favors development. However, the waters beneath 93L will steadily cool to a marginal 26°C by Friday, and the atmosphere will steadily get drier, as 93L encounters the Saharan Air Layer (SAL), discouraging development. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC gave the system a 60% of developing by Monday, and a 60% chance of developing by Friday. The expected west-northwest track of 93L over the next five days will carry it into a region of ocean where it is uncommon for tropical cyclones located there to eventually impact any land areas except Bermuda.


Figure 2. Radar image of Typhoon Utor as it was closing in on the coast of Southeast China at 10:42 am local time on August 14, 2013. At the time, Utor had top winds of 100 mph. Image credit: Meteorological Bureau of Shenzhen Municipality.


Figure 3. MODIS satellite image of Typhoon Utor taken at 05:45 UTC on Wednesday, August 14. At the time, Utor was a Category 1 storm with 90 mph winds, and was making landfall in China, 150 miles southwest of Hong Kong. Image credit: NASA.

Typhoon Utor hits China
Typhoon Utor hit Southeast China about 150 miles southwest of Hong Kong near 3 pm local time this Wednesday, as a Category 1 storm with 90 mph winds. The typhoon brought sustained wind of 34 mph, gusting to 44 mph to Macao, and wind gusts as high as 54 mph to Hong Kong. Widespread heavy rains are falling across much of Southeast China, as seen on Hong Kong radar and China radar. Utor will continue to dump torrential rains capable of causing deadly flash floods and mudslides over much of Southeast China and Northern Vietnam over the next three days. Satellite imagery shows that Utor is a large but weakening typhoon, with the eye no longer visible. In the Philippines, where Utor hit as a Category 4 storm with 140 mph winds on Monday, 4 deaths are being blamed on the storm, and 11 people are missing.

Jeff Masters

Hurricane

July 2013 the 30th Warmest July on Record for the U.S.

By: JeffMasters, 9:40 PM GMT on August 13, 2013

July 2013 was the 30th warmest July in the contiguous U.S. since record keeping began in 1895, said NOAA's National Climatic Data Center (NCDC) in their latest State of the Climate report. A persistent trough of low pressure over the middle of the country brought unusually cool conditions there, and strong ridges of high pressure over both the East Coast and West Coast brought unusually warm conditions to the West and Northeast. Massachusetts had its warmest July on record, and ten other states in the Northeast and West had top-ten warmest Julys on record. Four southern states had top-ten coolest Julys on record. The number of record warm highs and lows was roughly the same as the number of record cold highs and lows during July. For the year-to-date period January - July, both temperature and precipitation over the contiguous U.S. have been above normal, ranking in the upper 35% and 21% of years, respectively.

According to NOAA's U.S. Climate Extremes Index (CEI), which tracks the percentage area of the contiguous U.S. experiencing top-10% and bottom-10% extremes in temperature, precipitation, and drought, July extremes were slightly above average, and the year-to-date period January - July 2013 was also slightly above average.


Figure 1. Historical temperature ranking for the U.S. for July 2013. Massachusetts had its warmest July on record, and nine other states in the Northeast and West had top-ten warmest Julys on record. Five southern states had top-ten coolest Julys on record. Image credit: National Climatic Data Center (NCDC).

Wet July increases tropical storm flood risk
It was a very wet July for the contiguous U.S., ranking as the 5th wettest July since 1895. Florida had its wettest July on record, and twelve other states, ten of them in the Eastern U.S., had top-ten wettest Julys on record. As a result, soil moisture levels are high over portions of the Eastern U.S., increasing the chances of damaging flooding should a tropical storm or hurricane hit. The latest 2-week forecast from the GFS model keeps the Southeast U.S. under a very wet weather pattern through August, and the latest 1-month and 3-month precipitation outlooks from NOAA's Climate Prediction Center also give the Southeast above-average chances of wetter than average conditions. Lake Okeechobee in Florida is 2.1' above average for this time of year, and nearly 6' higher than two years ago. This puts the lake just 4" below what is considered high water, and Lake Okeechobee water levels are a concern as we head into the peak part of hurricane season.


Figure 2. Historical precipitation ranking for the U.S. for July 2013. Florida had its wettest July on record, and twelve other states had top-ten wettest Julys on record. Oregon had its driest July on record, and Washington and Oregon also had a top-ten driest July. Image credit: National Climatic Data Center (NCDC).


Figure 3. Soil moisture for August 12, 2013, expressed as percent average of the soil moisture observed between 1916 - 2004. Portions of the Eastern U.S. have high soil moisture levels, increasing the odds of damaging flooding should a tropical storm or hurricane hit this year. Texas and Western Louisiana are dry, and could use the rains from a tropical storm. Image credit: University of Washington Variable Infiltration Capacity Macro-scale Hydrological Model, which includes soil moisture, snow water equivalent, and runoff.

Drought conditions remained relatively unchanged during June and July. According to the August 6 Drought Monitor report, about 45% of the contiguous U.S. is still in moderate or greater drought, compared to 44% at the beginning of June. The U.S. Seasonal Drought Outlook issued on July 18 calls for little overall change in the U.S. area covered by drought conditions through October. Despite the drought, the year-to-date total acreage burned by wildfires is the second lowest in the past ten years.

Jeff Masters

Climate Summaries

Utor Pounds China; Japan Breaks All-Time Heat Record; Caribbean Disturbance

By: JeffMasters, 3:23 PM GMT on August 13, 2013

Category 2 Typhoon Utor is closing in on Southeast China, where it is expected to come ashore near 08 UTC (4 am EDT) on Wednesday, about 150 miles southwest of Hong Kong. Widespread heavy rains are already falling across much of Southeast China, as seen on Hong Kong radar and China radar. Satellite imagery shows that Utor is a large typhoon, and will dump torrential rains capable of causing deadly flash floods and mudslides over much of Southeast China and Northern Vietnam over the next three days; a wide swath of 6+ inches of rain is predicted over a 24-hour period for Southeast China using satellite estimates of the typhoon's current rainfall intensity. Unfortunately, the heaviest rains will fall just south of an area of extreme drought responsible for $6 billion in damages so far in 2013 (Figure 2.) Utor has drawn in some dry air and is slowly weakening, and should make landfall as a Category 2 storm.


Figure 1. Radar image of Typhoon Utor from August 13, 2013 taken at 17:12 local time (10:12 am EDT.) Image credit: Meteorological Bureau of Shenzhen Municipality.

A rough summer for extreme weather in China
China has already experienced five billion-dollar weather disasters so far in 2013. This is the most of any nation, according to insurance broker Aon Benfield. Utor is likely to the be sixth such disaster. The five Chinese billion-dollar weather disasters have all hit this summer:

1) Drought, Central and Eastern China, 1/1 - 7/31: $6.0 billion
2) Flooding, nationwide, 7/7 - 7/17: $4.5 billion
3) Flooding, Sichuan Province, China, 7/7 - 7/11: $1.6 billion
4) Flooding, China, 6/29 - 7/3: $1.4 billion
5) Flooding, China, 7/21 - 7/25: $1.4 billion

The most expensive of the these disasters, the $6 billion drought that hit Eastern China, helped intensify a remarkable and historic heat wave that assaulted Eastern China in July and August. In his latest post, wunderground's weather historian Christopher C. Burt writes:

Virtually every possible heat statistic has been broken for most sites in eastern China (as well as central and southern Japan, and South Korea). I cannot think of any other heat event that has affected so many people for so long (including those that plagued the U.S. in the mid 1930s, Russia in 2010, and Western Europe in August 2003). Obviously, the Chinese authorities are keeping the fatalities from this ongoing event under wraps.

The Eastern China heat wave moved northwards and eastwards over Korea and Japan over the past few days, and brought Japan its all-time national heat record on August 12, 2013, when the temperature peaked at 41.0°C (105.8°F) at the Ekawasaki site in Shimanto. The previous record of 40.9°C (105.6°F) was recorded at Tajima and Kumagaya on August 16, 2007. the record heat wave also brought stiflingly hot weather to Tokyo, which on August 11 endured its warmest daily minimum temperature ever recorded: 30.4°C (86.7°F). This was also the 2nd warmest minimum on record for Japan.


Figure 2. Widespread drought over Eastern and Southeast China has caused at least $6 billion in damage, according to Aon Benfield. Image credit: Beijing Climate Center.

The Philippines clean up after Utor
The Philippines are cleaning up after Typhoon Utor powered ashore on the northern Philippine Island of Luzon on Monday near 3 am local time (3 pm EDT Sunday), as a Category 4 storm with 140 mph winds. At least 3 deaths are being blamed on the storm, and 54 people are missing, mostly fishermen. Damage was heavy in Casiguran (population 24,000) near where the typhoon made landfall, with 80% of the infrastructure of the town reportedly destroyed.

Utor is a Marshallese word for squall line, and has been used for three tropical cyclones in the Western Pacific--in 2001, 2006, and 2013. Utor reached super typhoon status with 150 mph winds on Sunday, making it the strongest tropical cyclone globally so far in 2013. Earth's previous most powerful tropical cyclone of 2013 was Typhoon Soulik, which reached Category 4 strength with 145 mph winds on July 10. Soulik weakened to a Category 2 storm before hitting Taiwan on July 12.


Figure 2. Typhoon Utor approaches the Philippines in this 375 meter-resolution IR image taken by the VIIRS instrument on the Suomi satellite at 04:34 UTC August 11, 2013. At the time, Utor was a Category 4 storm with 135 mph winds. Image credit: Dan Lindsey, NOAA Center for Satellite Applications and Research, Fort Collins.

Caribbean tropical wave may develop when it reaches the Gulf of Mexico
A tropical wave in the Central Caribbean is kicking up disorganized heavy thunderstorms over Jamaica today, and this activity will spread westwards into the Cayman Islands by Wednesday, and into Mexico's Yucatan Peninsula and Western Cuba by Thursday. Wind shear is a very high 30 - 40 knots over the the wave, making development very unlikely through Wednesday. However, once the wave reaches the Western Caribbean on Thursday and pushes into the Gulf of Mexico on Friday, the wave will find a region with lower wind shear, and a strong tropical disturbance capable of becoming a tropical storm could form. If a tropical depression or tropical storm does form, and its circulation extends high above the surface, a trough of low pressure over the northern Gulf of Mexico would likely steer the storm northwards to a landfall between Eastern Louisiana and the Florida Panhandle. This is the solution presented by the Navy's 00Z run of the NAVGEM model, which shows a landfall on Saturday of a moderate-strength tropical storm. The other reliable models for genesis--the GFS, European, and UKMET--do not develop the system, or show very weak development. The European model takes much of the wave's moisture west-northwest across the Gulf of Mexico over the weekend, but the other models show the main moisture heading northwards into the Southeast U.S. Soils across the Southeast U.S. are already saturated, and tropical moisture from this storm system will be capable of dumping a large area of 4+" of rain, potentially causing significant flooding over the weekend. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC gave the system a 30% of developing by Sunday, and a 10% chance of developing by Thursday. The Hurricane Hunters are on call to investigate the system on Thursday.


Figure 4. Predicted precipitation for the 7-day period ending on Tuesday, August 20, 2013. Tropical moisture flowing north and northeastwards over the Southeast U.S. is expected to create a broad swath of 4+ inches of rain, capable of triggering damaging flooding. Image credit: NOAA/HPC.

Resilience to Extreme Weather panel discussion being livestreamed today (Tuesday)
The 6th annual National Clean Energy Summit is today, Tuesday, August 13, and will be livestreamed here. Of particular interest may be the 6pm EDT panel on Resilience to Extreme Weather, featuring:

- Dr. Kathryn Sullivan, Acting Under Secretary of Commerce, Oceans and Atmosphere and Acting Administrator, National Oceanic and Atmospheric Administration (NOAA)
- Maria LaRosa, Meteorologist, The Weather Channel
- Patricia Mulroy, General Manager, Southern Nevada Water Authority
- Chris Taylor, Executive Director, West Coast Infrastructure Exchange

Jeff Masters

Hurricane Extreme Weather Drought Heat

Typhoon Utor Pounds Phiippines, Heads for China

By: JeffMasters, 1:51 PM GMT on August 12, 2013

Typhoon Utor powered ashore on the northern Philippine Island of Luzon on Monday near 3 am local time (3 pm EDT Sunday) as a Category 4 storm with 140 mph winds. Damage was heavy in Casiguran (population 24,000) near where the typhoon made landfall, with 80% of the infrastructure of the town reportedly destroyed, and all roads into the city blocked. Utor is being blamed for two deaths so far, and 44 fishermen are reported as missing.


Figure 1. MODIS satellite image of Typhoon Utor taken at approximately 02:30 UTC on Monday, August 12. At the time, Utor was a Category 2 storm with 100 mph winds. Image credit: NASA.

Passage over Luzon disrupted the inner core of Utor, reducing the storm to Category 2 strength with winds of 100 mph. Satellite imagery shows that the typhoon is re-organizing, and a new eyewall is forming. Ocean temperatures are very warm, about 30°C (86°F), which is approximately 0.5 - 1.0°C above average. These warm waters extend to tremendous depth, giving Utor a huge source of energy to tap into. Wind shear is low, 5 - 10 knots. These favorable conditions for intensification will last until the typhoon gets midway between the Philippines and China, where wind shear will rise to the moderate range and ocean waters will cool to 29°C with a much lower heat content. I expect Utor will intensify into a Category 3 storm today, and make landfall in China as a Category 2 or 3 storm about 200 hundred miles southwest of Hong Kong about 06 UTC on Wednesday. Utor is a very wet storm, and will likely bring a large swath of 8+ inches of rain across Southeast China on Wednesday. These rains will cause dangerous flash flooding and mudslides.

Utor is a Marshallese word for squall line, and has been used for three tropical cyclones in the Western Pacific--in 2001, 2006, and 2013. Utor reached super typhoon status with 150 mph winds on Sunday, making it the strongest tropical cyclone globally so far in 2013. Earth's previous most powerful tropical cyclone of 2013 was Typhoon Soulik, which reached Category 4 strength with 145 mph winds on July 10. Soulik weakened to a Category 2 storm before hitting Taiwan on July 12.


Video 1. News video of the damage from Typhoon Utor in Casiguran in the Philippines. Utor is being called Typhoon Labuyo locally in the Philippines. Thanks to wunderground member AussieStorm for posting this in my blog comments.

The Philippines no stranger to powerful typhoons
The Philippines lie in the most tropical cyclone-prone waters on Earth, and rarely escape a year without experiencing a devastating typhoon. Usually, these storms impact the northern Philippine island of Luzon, but last year, Earth's deadliest weather disaster of 2012 occurred on the southern Philippine island of Mindanao, where Super Typhoon Bopha struck as a Category 5 super typhoon with winds of 160 mph (260 km/h), on December 3. Bopha made two additional landfalls in the Philippines, on central Visayas and on Palawan, on December 4. The typhoon left 1901 people dead, mostly on the island of Mindanao, making Bopha the 2nd deadliest typhoon in Philippine history. Bopha affected over 5.4 million people and left over 700,000 people homeless. With damages estimated at $1.7 billion, Bopha was the costliest natural disaster in Philippines history.


Figure 2. December 7, 2012: rescuers and residents look for missing victims amongst toppled tree trunks and coconut shells after flash floods caused by Super Typhoon Bopha hit Compostela Valley on Mindanao Island in the Philippines on December 3 - 4, 2012. (AP Photo/Jay Morales, Malacanang Photo Bureau, HO)

Activity possible late this week in the Gulf of Mexico
A tropical wave in the Central to Eastern Caribbean is kicking up disorganized heavy thunderstorms as it heads westwards. Wind shear is a very high 40 knots over the region, and the wave is not a threat to develop for the next two days. However, once the wave reaches the Gulf of Mexico on Saturday, some of the models are suggesting that the wave will find a region with lower wind shear, and a strong tropical disturbance capable of becoming a tropical storm could form near Mexico's Yucatan Peninsula or farther north in the Gulf of Mexico. If it penetrates far enough into the northern Gulf of Mexico, the tropical wave could interact with a stalled cold front expected to push off the Southeast U.S. coast late this week. This interaction could produce a hybrid low pressure system that might be partially tropical, and capable of bringing heavy rains to the Southeast U.S. on Saturday and Sunday. In their 8 am EDT Tropical Weather Outlook, NHC gave the system a 20% of developing by Saturday, and a 0% chance of developing by Wednesday.

Jeff Masters

Hurricane

Category 4 Super Typhoon Utor Bearing Down on the Philippines

By: JeffMasters, 11:52 AM GMT on August 11, 2013

Earth's strongest and most dangerous tropical cyclone so far in 2013 is Category 4 Super Typhoon Utor, which is closing in on the northern Philippine Island of Luzon with 150 mph sustained winds. Landfall is expected at approximately 20 UTC (4 pm EDT) Sunday near Casigran. Satellite imagery shows a formidable storm with well-organized spiral bands, a prominent 15-mile diameter eye, and good (but not excellent) upper-level outflow. Ocean temperatures are very warm, about 30°C (86°F), which is approximately 0.5 - 1.0°C above average. These warm waters extend to tremendous depth, giving Utor a huge source of energy to tap into. Wind shear is low, 5 - 10 knots. Theoretically, the Maximum Potential Intensity (MPI) that Utor can achieve under these conditions is sustained winds of 185 mph. However, Utor will not have time to reach that strength before encountering Luzon. Utor is a very wet storm, and will likely bring a large swath of 8+ inches of rain across Luzon. These rains will cause dangerous flash flooding and mudslides. Utor will likely weaken to a Category 1 storm as it passes over Luzon, but is expected to re-intensify to a Category 2 storm before hitting China a few hundred miles south of Hong Kong about 20 UTC on Tuesday.

Utor is a Marshallese word for squall line, and has been used for three tropical cyclones in the Western Pacific--in 2001, 2006, and 2013. Typhoon Utor is called Typhoon Labuyo in the Philippines. Utor's 150 mph winds make it the strongest tropical cyclone globally so far in 2013. Earth's previous most powerful tropical cyclone of 2013 was Typhoon Soulik, which reached Category 4 strength with 145 mph winds on July 10. Soulik weakened to a Category 2 storm before hitting Taiwan on July 12.


Figure 1. MODIS satellite image of Typhoon Utor taken at 04:30 UTC on Sunday, August 11. Image credit: NASA.

The Philippines no stranger to powerful typhoons
The Philippines lie in the most tropical cyclone-prone waters on Earth, and rarely escape a year without experiencing a devastating typhoon. Usually, these storms impact the northern Philippine island of Luzon, but last year, Earth's deadliest weather disaster of 2012 occurred on the southern Philippine island of Mindanao, where Super Typhoon Bopha struck as a Category 5 super typhoon with winds of 160 mph (260 km/h), on December 3. Bopha made two additional landfalls in the Philippines, on central Visayas and on Palawan, on December 4. The typhoon left 1901 people dead, mostly on the island of Mindanao, making Bopha the 2nd deadliest typhoon in Philippine history. Bopha affected over 5.4 million people and left over 700,000 people homeless. With damages estimated at $1.7 billion, Bopha was the costliest natural disaster in Philippines history.


Figure 2. December 7, 2012: rescuers and residents look for missing victims amongst toppled tree trunks and coconut shells after flash floods caused by Super Typhoon Bopha hit Compostela Valley on Mindanao Island in the Philippines on December 3 - 4, 2012. (AP Photo/Jay Morales, Malacanang Photo Bureau, HO)

Quiet in the Atlantic
There are no tropical cyclone threat areas in the Atlantic to discuss today. Some of the models are suggesting a strong tropical disturbance capable of becoming a tropical storm could form by Saturday in the Gulf of Mexico near Mexico's Yucatan Peninsula, in association with a stalled cold front expected to push off the Southeast U.S. coast late this week.

Jeff Masters

Hurricane

An Active Atlantic Hurricane Season Still Predicted by NOAA, CSU, and TSR

By: JeffMasters, 4:07 PM GMT on August 09, 2013

As we stand on the cusp of the peak part of hurricane season, all of the major groups that perform long-range seasonal hurricane forecasts are still calling for an active 2013 Atlantic hurricane season. NOAA forecasts an above-normal and possibly very active Atlantic hurricane season in 2013, in their August 8 outlook. They give a 70% chance of an above-normal season, a 25% chance of an near-normal season, and 5% chance of a below-normal season. They predict a 70% chance that there will be 13 - 19 named storms, 6 - 9 hurricanes, and 3 - 5 major hurricanes, with an Accumulated Cyclone Energy (ACE) 120% - 190% of the median. If we take the midpoint of these numbers, NOAA is calling for 16 named storms, 7.5 hurricanes, 4 major hurricanes, and an ACE index 155% of normal. This is well above the 1981 - 2010 average of 12 named storms, 6 hurricanes, and 3 major hurricanes. Hurricane seasons during the active hurricane period 1995 - 2012 have averaged 15 named storms, 8 hurricanes, and 4 major hurricanes, with an ACE index 151% of the median.


Figure 1. Tropical Storm Dorian on July 25, 2013, when the storm reached peak intensity--sustained winds of 60 mph. Formation of early-season tropical storms like Chantal and Dorian in June and July in the deep tropics is usually a harbinger of an active Atlantic hurricane season. Image credit: NASA.

NOAA cites five main reasons to expect an active remainder of hurricane season:

1) Sea Surface Temperatures (SSTs) are above average in the Main Development Region (MDR) for hurricanes, from the coast of Africa to the Caribbean. As of August 9, SST were 0.4°C (0.8°F) above average.
2) Trade winds are weaker than average across the MDR, which has caused the African Monsoon to grow wetter and stronger, the amount of spin over the MDR to increase, and the amount of vertical wind shear to decrease.
3) No El Niño event is present or expected this fall.
4) There have been two early-season tropical storms in the deep tropics (Tropical Storms Chantal and Dorian), which is generally a harbinger of an above-normal season.
5) We are in an active hurricane period that began in 1995.

Colorado State predicts a much above-average hurricane season
A much above-average Atlantic hurricane season is on tap for 2013, according to the seasonal hurricane forecast issued August 2 by Dr. Phil Klotzbach and Dr. Bill Gray of Colorado State University (CSU). The CSU team is calling for 18 named storms, 8 hurricanes, and 3 intense hurricanes, and an Accumulated Cyclone Energy (ACE) of 142. The forecast calls for an above-average chance of a major hurricane hitting the U.S., both along the East Coast (40% chance, 31% chance is average) and the Gulf Coast (40% chance, 30% chance is average). The risk of a major hurricane in the Caribbean is also above average, at 53% (42% is average.)

Analogue years
The CSU team picked five previous years when atmospheric and oceanic conditions were similar to what we are seeing this year: cool neutral ENSO conditions and slightly above-average tropical Atlantic sea surface temperatures. Those five years were 2008, a very active year with 16 named storms and 4 major hurricanes--Gustav, Ike, Paloma, and Omar; 2007, an active year with 15 named storms and two Category 5 storms--Dean and Felix; 1996, an above average year with 13 named storms and 6 major hurricanes--Edouard, Hortense, Fran, Bertha, Isidore, and Lili; 1966, an average year with 11 named storms and 3 major hurricanes--Inez, Alma, and Faith; and 1952, a below average year with 7 named storms and 3 major hurricanes. The average activity during these five analogue years was 12.4 named storms, 7.2 hurricanes, and 3.8 major hurricanes.

TSR predicts an above-average hurricane season: 14.8 named storms
The August 6 forecast for the 2013 Atlantic hurricane season made by British private forecasting firm Tropical Storm Risk, Inc. (TSR) calls for an active season with 14.8 named storms, 6.9 hurricanes, 3 intense hurricanes, and an Accumulated Cyclone Energy (ACE) of 121. The long-term averages for the past 63 years are 11 named storms, 6 hurricanes, 3 intense hurricanes, and an ACE of 103. TSR rates their skill level as good for these August forecasts--47% - 59% higher than a "no-skill" forecast made using climatology. TSR predicts a 58% chance that U.S. land falling activity will be above average, a 26% chance it will be near average, and a 16% chance it will be below average. They project that 4 named storms will hit the U.S., with 1.8 of these being hurricanes. The averages from the 1950-2012 climatology are 3.1 named storms and 1.4 hurricanes. They rate their skill at making these August forecasts for U.S. landfalls just 9% - 18% higher than a "no-skill" forecast made using climatology. In the Lesser Antilles Islands of the Caribbean, TSR projects 1.4 named storms, 0.6 of these being hurricanes. Climatology is 1.1 named storms and 0.5 hurricanes.

TSR's two predictors for their statistical model are the forecast July - September trade wind speed over the Caribbean and tropical North Atlantic, and the forecast August - September 2013 sea surface temperatures in the tropical North Atlantic. Their model is calling for warmer than average SSTs and near average trade winds during these periods, and both of these factors should act to increase hurricane and tropical storm activity.


Figure 2. Comparison of the percent improvement over climatology for May and August seasonal hurricane forecasts for the Atlantic from NOAA, CSU and TSR from 1999-2009 (May) and 1998-2009 (August), using the Mean Squared Error. Image credit: Verification of 12 years of NOAA seasonal hurricane forecasts, National Hurricane Center.


Figure 3. Comparison of the percent improvement in mean square error over climatology for seasonal hurricane forecasts for the Atlantic from NOAA, CSU and TSR from 2003-2012, using the Mean Square Skill Score (MSSS). The figure shows the results using two different climatologies: a fixed 50-year (1950 - 1999) climatology, and a 2003 - 2012 climatology. Skill is poor for forecasts issued in December and April, moderate for June forecasts, and good for August forecasts. Image credit: Tropical Storm Risk, Inc.

FSU predicts an above-average hurricane season: 15 named storms
The Florida State University (FSU) Center for Ocean-Atmospheric Prediction Studies (COAPS) issued their fifth annual Atlantic hurricane season forecast on May 30, calling for a 70% probability of 12 - 17 named storms and 5 - 10 hurricanes. The mid-point forecast is for 15 named storms, 8 hurricanes, and an accumulated cyclone energy (ACE) of 135. The scientists use a numerical atmospheric model developed at COAPS to understand seasonal predictability of hurricane activity. The model is one of only a handful of numerical models in the world being used to study seasonal hurricane activity and is different from the statistical methods used by other seasonal hurricane forecasters such as Colorado State, TSR, and PSU (NOAA uses a hybrid statistical-dynamical model technique.) The FSU forecast has been one of the best ones over the past four years:

2009 prediction: 8 named storms, 4 hurricanes. Actual: 9 named storms, 3 hurricanes
2010 prediction: 17 named storms, 10 hurricanes. Actual: 19 named storms, 12 hurricanes
2011 prediction: 17 named storms, 9 hurricanes. Actual: 19 named storms, 7 hurricanes
2012 prediction: 13 named storms, 7 hurricanes. Actual: 19 named storms, 10 hurricanes

Penn State predicts an above-average hurricane season: 16 named storms
A statistical model by Penn State's Michael Mann and alumnus Michael Kozar is calling for an active Atlantic hurricane season with 16 named storms, plus or minus 4 storms. Their prediction was made using statistics of how past hurricane seasons have behaved in response to sea surface temperatures (SSTs), the El Niño/La Niña oscillation, the North Atlantic Oscillation (NAO), and other factors. The statistic model assumes that in 2013 the May 0.87°C above average temperatures in the MDR will persist throughout hurricane season, the El Niño phase will be neutral to slightly warm, and the North Atlantic Oscillation (NAO) will be near average.

The PSU team has been making Atlantic hurricane season forecasts since 2007, and these predictions have done pretty well, except for in 2012, when an expected El Niño did not materialize:

2007 prediction: 15 named storms, Actual: 15
2009 prediction: 12.5, named storms, Actual: 9
2010 prediction: 23 named storms, Actual: 19
2011 prediction: 16 named storms, Actual: 19
2012 prediction: 10.5 named storms, Actual: 19

UK Met Office predicts a slightly above-average hurricane season: 14 named storms
The UKMET office forecast for the 2013 Atlantic hurricane season, issued May 13, calls for slightly above normal activity, with 14 named storms, 9 hurricanes, and an ACE index of 130. In contrast to the statistical models relied upon by CSU, TSR, and NOAA, the UKMET model is done strictly using two dynamical global seasonal prediction systems: the Met Office GloSea5 system and ECMWF system 4. In 2012, the Met Office forecast was for 10 tropical storms and an ACE index of 90. The actual numbers were 19 named storms and an ACE index of 123.


Figure 4. Total 2013 Atlantic hurricane season activity as predicted by twelve different groups.

NOAA predicts a below-average Eastern Pacific hurricane season
NOAA's pre-season prediction for the Eastern Pacific hurricane season, issued on May 23, calls for a below-average season, with 11 - 16 named storms, 5 - 8 hurricanes, 1 - 4 major hurricanes, and an ACE index 60% - 105% of the median. The mid-point of these ranges gives us a forecast for 13.5 named storms, 6.5 hurricanes, and 2.5 major hurricanes, with an ACE index 82% of average. The 1981 - 2010 averages for the Eastern Pacific hurricane season are 15 named storms, 8 hurricanes, and 4 major hurricanes.

NOAA predicts a below-average Central Pacific hurricane season
NOAA's pre-season prediction for the Central Pacific hurricane season, issued on May 22, calls for a below-average season, with 1 - 3 tropical cyclones. An average season has 4 - 5 tropical cyclones, which include tropical depressions, tropical storms, and hurricanes. Hawaii is the primary land area affected by Central Pacific tropical cyclones.

West Pacific typhoon season forecast not available this year
Dr. Johnny Chan of the City University of Hong Kong usually issues a seasonal forecast of typhoon season in the Western Pacific, but did not do so in 2012 or 2013. An average typhoon season has 27 named storms and 17 typhoons. Typhoon seasons immediately following a La Niña year typically see higher levels of activity in the South China Sea, especially between months of May and July. Also, the jet stream tends to dip farther south than usual to the south of Japan, helping steer more tropical cyclones towards Japan and Korea.

Quiet in the Atlantic this weekend
There are no Atlantic threat areas to discuss today, and none of the reliable models for tropical cyclone formation is predicting development during the coming seven days. However, there are some indications that the atmosphere over the tropical Atlantic will become more conducive for tropical storm formation beginning around August 15. The Madden Julian Oscillation (MJO), a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days, may move into the Atlantic then, increasing tropical storm formation odds. At the same time, the computer models are indicating an increase in moisture over the tropical Atlantic, due to a series of tropical waves expected to push off of the coast of Africa. There will also be several eastward-moving Convectively-Coupled Kelvin Waves (CCKWs) traversing the Atlantic during that period. These atmospheric disturbances have a great deal of upward-moving air, which helps strengthen the updrafts of tropical disturbances. Formation of the Eastern Pacific's Hurricane Gil and Henriette were aided by CCKWs. These same CCKWs will cross into the Atlantic and increase the odds of tropical storm formation during the period August 15 - 20.

Have a great weekend, everyone!

Jeff Masters

Hurricane

Austria and Slovenia Set All-time Heat Records; Record Heat in Shanghai, China

By: JeffMasters, 11:09 PM GMT on August 08, 2013

A historic heat wave is underway in Central Europe, where both both Austria and Slovenia set all-time national heat records on August 8. Three locations in Austria passed the 40°C (104°F) mark, beating the former national record of 39.9°C (103.8°F) set just last week, on August 3rd at Dellach im Drautal. According to the Austrian met service, ZAMG, the hottest spot was Bad Deutsch-Altenburg, with a 40.5°C (104.9°F) reading. Slovenia also surpassed its national heat record on August 8, with a 40.8°C (105.4°F) reading at Cerklje ob Krki (former record: 40.6°C (105.1°F) at Crnomelj on July 5, 1950.) Ljubljana, the capital of Slovenia, has broken its all-time heat record five of the past six days, with each day hotter than the previous record. The newest record is the 40.2°C (104.4°) recorded on August 8th. Records go back 150 years at this station.


Figure 1. South Korean children cool themselves off by playing in a fountain in downtown Seoul, South Korea, Thursday, Aug. 8, 2013. A heat wave warning was issued in South Korea as Ulsan reached 38.8°C (101.8°F), just 1.2°C short of South Korea’s national all-time record high of 40.0°C (104.0°F). (AP Photo/Lee Jin-man)

Remarkable heat in East Asia
China's most populous city, Shanghai, broke its all-time record for hottest temperature on record for the second time this summer on August 7, when the mercury topped out at 40.8°C (105.4°F). The previous record was set just the day before (40.6°C/105.1°F), and also on July 26th. Prior to this summer, the record for Shanghai was 40.2°C (104.4°F) during the summer of 1934. Records in Shanghai date back to 1872. Today (August 8th), the temperature peaked at 40.2°C (104.4°F), so Shanghai has had its four hottest days in its history this summer. Extreme heat was also experienced over South Korea and Japan today, and wunderground's weather historian Christopher C. Burt has the details in his latest post. Many more all-time heat records may have fallen in both Central Europe and East Asia, and Chris plans to update his post with all the latest records on Friday.

Extensive credit for researching these records goes to weather records researcher Maximiliano Herrera, who maintains a comprehensive set of extreme temperature records on his web site.

Jeff Masters

Heat

Which Hurricane Forecast Model Should You Trust?

By: JeffMasters, 1:05 PM GMT on August 07, 2013

The National Hurricane Center (NHC) set a new record in 2012 for accuracy of their 1, 2, 3, and 4-day Atlantic tropical cyclone track forecasts, but had almost no skill making intensity forecasts, according to the 2012 National Hurricane Center Forecast Verification Report, issued in March 2013. The new records for track accuracy were set despite the fact that the season’s storms were harder than average to forecast. The average error in a 1-day forecast was 46 miles, and was 79 miles for 2 days, 116 miles for 3 days, 164 miles for 4 days, and 224 miles for 5 days. The official track forecast had a westward bias of 10 - 17 miles for 1 - 3 day forecasts (i.e., the official forecast tended to fall to the west of the verifying position), and was 38 and 75 miles too far to the northeast for the 4- and 5-day forecasts, respectively.


Figure 1. Verification of official NHC hurricane track forecasts for the Atlantic, 1990 - 2012. Over the past 15 - 20 years, 1 - 3 day track forecast errors have been reduced by about 60%. Track forecast error reductions of about 50% have occurred over the past ten years for 4- and 5-day forecasts. Image credit: 2012 National Hurricane Center Forecast Verification Report.

NHC Intensity Forecasts: Little Improvement Since 1990
Official NHC intensity forecasts did better than usual in 2012, and had errors lower than the 5-year average error for 1, 2, 3, 4, and 5-day forecasts. However, 2012's storms were easier to predict than usual, due to due to a lack of rapidly intensifying hurricanes. These rapid intensifiers are typically the source of the largest forecast errors. The skill of official NHC 24-hour intensity forecasts made in 2012 for the Atlantic basin were only about 15% better than a "no-skill" forecast; 2, 3, 4, and 5-day intensity forecasts had no skill.


Figure 2. Verification of official NHC hurricane intensity forecasts for the Atlantic, 1990 - 2012. Intensity forecasts have shown little to no improvement since 1990. Image credit: 2012 National Hurricane Center Forecast Verification Report.

Which Track Model Should You Trust?
As usual, in 2012 the official NHC forecast for Atlantic storms was almost as good as or better than any individual computer models--though NOAA's GFS model did slightly better than the NHC official forecast at 12, 24, and 48-hour periods, and the European model forecast was slightly better at 12-hour forecasts. Despite all the attention given to how the European Center (ECMWF) model outperformed the GFS model for Hurricane Sandy's track at long ranges, the GFS model actually outperformed the European model in 2012 when summing up all track forecasts made for all Atlantic named storms. This occurred, in part, because the European model made a few disastrously bad forecasts for Tropical Storm Debby when it was in the Gulf of Mexico and steering currents were weak. For several runs, the model predicted a Texas landfall, but Debby ended up moving east-northeast to make a Northwest Florida landfall, like the GFS model had predicted. However, the best-performing model averaged over the past three years has been the European Center model, with the GFS model a close second. Wunderground provides a web page with computer model forecasts for many of the best-performing track models used to predict hurricane tracks. The European Center does not permit public display of tropical storm positions from their hurricane tracking module of their model, so we are unable to put ECMWF forecasts on this page. Here are some of the better models NHC regularly looks at:

ECMWF: The European Center's global forecast model
GFS: NOAA's global forecast model
NOGAPS: The Navy's global forecast model (now defunct, replaced by the NAVGEM model in 2013)
UKMET: The United Kingdom Met Office's global forecast model
GFDL: The Geophysical Fluid Dynamics Laboratory's hurricane model, initialized using GFS data
HWRF: The intended successor for the the GFDL hurricane model, also initialized using GFS data
CMC: The Canadian GEM model
BAMM: The very old Beta and Advection Model (Medium layer), which is still useful at longer ranges

If one averages together the track forecasts from the first six of these models, the NHC official forecast will rarely depart much from it. As seen in Figure 3, the HWRF and UKMET were well behind the ECMWF and GFS in forecast accuracy in 2012, but were still respectable. The simple BAMM model did well at 3, 4, and 5-day forecasts. The GFDL and CMC models did quite poorly compared to the ECMWF, GFS, UKMET, and HWRF. The Navy's NOGAPS model also did poorly in 2012, and has been retired. Its replacement for 2013 is called the NAVGEM model.


Figure 3. Skill of computer model forecasts of Atlantic named storms in 2012, compared to a "no skill" model called "CLIPER5" that uses just climatology and persistence to make a hurricane track forecast (persistence means that a storm will tend to keep going in the direction it's current going.) OFCL=Official NHC forecast; GFS=Global Forecast System model; GFDL=Geophysical Fluid Dynamic Laboratory model; HWRF=Hurricane Weather Research Forecasting model; UKMET=United Kingdom Met Office model; ECMWF=European Center for Medium Range Weather Forecasting model; TVCA=one of the consensus models that lends together several of the above models; CMC=Canadian Meteorological Center (GEM) model; BAMM=Beta Advection Model (Medium depth.) Image credit: National Hurricane Center 2012 verification report.

Which Intensity Model Should You Trust?
Don't trust any of them. NHC has two main statistical intensity models, LGEM and DSHP (the SHIPS model with inland decay of a storm factored in.) In addition, four dynamical models that are also use to track hurricanes--the GFS, ECMWF, HWRF, and GFDL models--all offer intensity forecasts. With the exception of the GFS model, which had a skill just 5% better than a "no-skill" intensity forecast for predictions going out 36 hours, all of these models had no skill in their intensity forecasts during 2012. The ECMWF and HWRF models were the worst models for intensity forecasts of 3, 4, and 5 days, with a skill of 20% - 60% lower than a "no-skill" forecast. The LGEM model, which was a decent intensity model in 2011, tanked badly in 2012 and had near-zero skill. The only model that was any good in 2012 was the IVCN "consensus" model, which averages together the intensity forecasts of two or more of the intensity models such as LGEM, GFDL, HWRF, and DSHP.

Some Promising Models From the Hurricane Forecast Improvement Project (HFIP)
Last year was the fourth year of a ten-year project, called the Hurricane Forecast Improvement Project (HFIP), aimed at reducing hurricane track and intensity errors by 50%. The new experimental models from HFIP generally performed poorly in 2012. However, the new FIM9 15-km global model was competitive with the ECMWF and GFS models for track, and the new CIRA Statistical Intensity Consensus (SPC3) model for intensity performed better than many of the traditional intensity models.

For those interested in learning more about the hurricane forecast models, NOAA has a 1-hour training video (updated for 2011.) Additional information about the guidance models used at the NHC can be found at NHC and the NOAA/HRD Hurricane FAQ.

Sources of Model Data
You can view 7-day ECMWF and 16-day GFS forecasts on wunderground's wundermap with the model layer turned on.
Longer ten-day ECMWF forecasts are available from the ECMWF web site.
FSU's experimental hurricane forecast page (CMC, ECMWF, GFDL, GFS, HWRF, and NAVGEM models)
NOAA's HFIP model comparison page (GFS, ECMWF, FIM, FIM9, UKMET, and CMC models.)
Experimental HFIP models

Quiet in the Atlantic
There are no tropical cyclone threat areas in the Atlantic to discuss today, and none of the reliable models for tropical cyclone formation is predicting development during the coming seven days. I plan on having a detailed update on Friday to discuss the latest long-range forecasts for the coming peak part of hurricane season.

Jeff Masters

Hurricane

Hurricanes and Climate Change: Huge Dangers, Huge Unknowns

By: JeffMasters, 9:57 AM GMT on August 05, 2013

Hurricane Sandy's enormous $65 billion price tag put that great storm in third place for the most expensive weather-related disaster in U.S. (and world) history, and six of the ten most expensive U.S. weather-related disasters since 1980 have been hurricanes. Thus, how the strongest hurricanes may be affected due a changing climate is a topic of critical concern. Since hurricanes are heat engines that extract heat energy from the oceans to power themselves, hurricane scientists are confident that the very strongest storms will get stronger by the end of the century, when Earth's land and ocean temperatures are expected to warm 2 - 3°C, to levels unmatched since the Eemian Era, 115,000 years ago. Computer modeling work consistently indicates that greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift towards stronger storms, with intensity increases of 2–11% by 2100. But hurricanes are fussy creations, and are sensitive to wind shear and dry air. Although the strongest storms should get stronger when "perfect storm" conditions are present, these "perfect storm" conditions may become less frequent in the future, due to the presence of higher wind shear, altered atmospheric circulation patterns, or more dry air at mid levels of the atmosphere. Indeed, the climate models used to formulate the 2007 IPCC report suggested that we might see the strongest hurricanes getting stronger, but a decrease in the total number of hurricanes in the Atlantic (and worldwide) later this century. However, the latest set of models used to formulate the 2013 IPCC report left open the possibility that we might see in increase in the total number of hurricanes, and and increase in their intensity. Given the conflicting model results, we really don't know how global warming will affect the number of hurricanes and their intensity, but we run the risk of making one of humanity's greatest scourges worse.


Figure 1. The list of most expensive U.S. weather-related disasters since 1980 is dominated by hurricanes.

Climate models and hurricane frequency
The database we have on historical hurricanes does not extend far enough into the past and is not of high enough quality to make many judgements on how human-caused climate change may be affecting these great storms. A landmark 2010 review paper, "Tropical Cyclones and Climate Change", authored by ten top hurricane scientists concluded that the U.S. has not seen any long-term increase in landfalling tropical storms and hurricanes, and that "it remains uncertain whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes" (tropical cyclone is the generic term which encompasses tropical depressions, tropical storms, hurricanes, and typhoons.) Based in part on modeling studies using climate models run for the 2007 IPCC report, the scientists concluded that "it is likely that global mean tropical cyclone frequency will either decrease or remain unchanged owing to greenhouse warming." For example, one of the modeling studies the review paper quoted, Knutson et al. (2008), "Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions", projected a decrease in Atlantic tropical storms by 27% and hurricanes by 18% by the end of the century. An important reason that their model predicted these decreases was due to a predicted increase in wind shear. As I explain in my wind shear tutorial, a large change of wind speed with height over a hurricane creates a shearing force that tends to tear the storm apart. The amount of wind shear is critical in determining whether a hurricane can form or survive.

But a July 2013 study by MIT's Dr. Kerry Emanuel, "Downscaling CMIP5 climate models shows increased tropical cyclone activity over the 21st century", challenged this result. Dr. Emanuel argued that tropical cyclones are likely to become both stronger and more frequent as the climate continues to warm. This increase is most likely to occur in Western North Pacific, with smaller increases in the Atlantic. Dr. Emanuel took output from six newer higher-resolution climate models used to formulate the 2013 IPCC report, and used the output to drive a high-resolution hurricane model. The simulations found that the global frequency of tropical cyclones would increase by 11% to 40% by 2100, with intensity increases as well. The combined effects produced a global increase in Category 3 and stronger hurricanes of 40%. The behavior of these strongest hurricanes is critical, since they do most of the damage we observe. Over the past century, Category 3 - 5 hurricanes accounted for 85% of US hurricane damage, despite representing only 24% of U.S. landfalling storms. Category 4 and 5 hurricanes made up only 6% of all U.S. landfalls, but accounted for 48% of all U.S. damage (if normalized to account for increases in U.S. population and wealth, see Pielke et al., 2008.)


Figure 2. Projected changes in tropical cyclone track density during the 2006-2100 period compared to the 1950-2005 period, using output from six climate models included in the 2013 IPCC report. The global frequency of tropical cyclones is predicted to increase by 11% to 40%, with the largest changes occurring in the Northwest Pacific off the coast of Japan. Smaller increases are predicted for the Atlantic and near Australia. Image credit: Kerry Emanuel, "Downscaling CMIP5 climate models shows increased tropical cyclone activity over the 21st century", Proceedings of the National Academy of Sciences, July 8, 2013, doi: 10.1073/pnas.1301293110.

However, a study by Knutson et al. (2013), using the same latest-generation climate models as used by Emanuel (2013), but using the output from the models to drive a different high-resolution hurricane model, found a 20% decrease in Atlantic tropical storms and hurricanes by 2100. Two other 2013 studies by Villarini et al. and Camargo, also using output from the 2013 IPCC models, found essentially no change in Atlantic tropical cyclones. The reason for the differences, lies, in part, with how much global warming is assumed in the studies. Dr. Emanuel's study, which found an increase in tropical cyclone activity, assumed a worst-case warming situation (RCP 8.5), following the "business as usual" emissions path humanity is currently on. The Knutson et al. study, which found a decrease of 20% in Atlantic tropical cyclones, used a scenario (RCP 4.5) where it was assumed humans will wise up and cause about half of the worst-case greenhouse warming. The study found found "marginally significant" increases in Atlantic Category 4 and 5 hurricanes of 39% - 45% by 2100. These dramatically different results give credence to Dr. Emanuel statement at the end of his paper, "the response of tropical cyclones to projected climate change will remain uncertain for some time to come." The 2013 IPCC report also emphasized the high amount of uncertainty in how climate change might affect hurricanes, stating that there was "low confidence" that we have observed any increases in intense tropical cyclones due to human causes. However, since the 1970s, it is virtually certain (99 - 100% chance) that the frequency and intensity of hurricanes and tropical storms in the North Atlantic has increased, and there is medium confidence that a reduction in small air pollution particles (aerosols) over the North Atlantic caused part of this effect. The report's forecast for the future stated that it is "more likely than not" (50 - 100% chance) that human-caused climate change will cause a substantial increase in intense tropical cyclones in some ocean basins by 2100, with the Western North Pacific and Atlantic being at particular risk. Also, there will likely (66 - 100% chance) be an increase in both global mean tropical cyclone maximum wind speed and rain rates by 2100, and more likely than not (50 - 100% chance) that the increase in the most intense tropical cyclones will be larger than 10% in some basins.


Figure 3. Expected change in Atlantic Category 4 and 5 hurricanes per decade expected by the year 2100, according to Knutson et al. (2013), "Dynamical Downscaling Projections of 21st Century Atlantic Hurricane Activity: CMIP3 and CMIP5 Model-based Scenarios." This research used the latest generation of climate models from the 2013 IPCC report, and found "marginally significant" increases in Atlantic Category 4 and 5 hurricanes of 39% - 45% by 2100.

Commentary
Hurricane damages are currently doubling every ten years without the effect of climate change, according to Pielke et al., 2008. This is primarily due to the increasing population along the coast and increased wealth of the population. The authors theorize that the Great Miami Hurricane of 1926, a Category 4 monster that made a direct hit on Miami Beach, would have caused about $150 billion in damage had it hit in 2005. Thus, by 2015, the same hurricane would do $300 billion in damage, and $600 billion by 2025. This is without considering the impact that accelerating sea level rise will have on storm surge damages. Global sea level rise over the past decade has been about double what it was in the 20th century, and the rate of sea level rise is expected to increase further in the coming decades. Storm surge does the majority of damage in major hurricanes, and storm surges riding on top of higher sea levels are going to do a lot more damage in the coming decades. If we toss in the (controversial) increases in Category 3 and stronger storms like Dr. Emanuel suggests may occur, the hurricane damage math gets very impressive. We can also add onto that the relatively non-controversial increase in tropical cyclone rainfall of 20% expected by 2100, which will sharply increase damages due to fresh water river flooding. It is controversial whether or not we are already be seeing an increase in heavy precipitation events associated with tropical cyclones in the U.S., though. The total number of daily rainfall events exceeding 2" associated with tropical cyclones in the Southeast U.S. on a century time scale has not changed significantly, according to Groisman et al., 2004. But a 2010 study by Kunkel et al., "Recent increases in U.S. heavy precipitation associated with tropical cyclones", found that the number of Southeast U.S. tropical cyclone heavy precipitation events, defined as 1-in-5-year events, more than doubled between 1994 - 2008, compared to the long-term average from 1895 - 2008.


Figure 4. Time series of the 15-year running average (plotted at the end point of the 15-yr blocks) of a Tropical Cyclone Heavy Precipitation Index (red) and 15-year running average of U.S. landfalling hurricanes (blue). Note that there has been no long-term increase in U.S. landfalling hurricanes, but there has been a sharp increase in extreme rainfall events associated with landfalling tropical cyclones--the kind of rainfall events most likely to cause damaging flooding. Image credit: Kunkel et al. (2010), "Recent increases in U.S. heavy precipitation associated with tropical cyclones", Geophysical Research Letters.

It is essential that we limit coastal development in vulnerable coastal areas, particularly along barrier islands, to reduce some of the astronomical price tags hurricanes are going to be causing in the future. Adoption and enforcement of strict building standards is also a must, as well as more reforms to the government's National Flood Insurance Program (NFIP), which subsidizes development in high-risk coastal regions that private insurers won't touch. NFIP is now $25 - 30 billion in the red, thanks to Hurricane Katrina and Hurricane Sandy. Reform of NFIP is already underway. In 2012, before Sandy hit, Congress passed the Biggert-Waters Flood Insurance Reform Act, which requires people with NFIP policies to pay large premium increases of about 25% per year over the next five years. Naturally, this move has caused major controversy.

References
Camargo, S., (2013), "Global and regional aspects of tropical cyclone activity in the CMIP5 models," J. Climate.

Emanuel, K.A., 2013, "Downscaling CMIP5 climate models shows increased tropical cyclone activity over the 21st century", PNAS, July 8, 2013, doi: 10.1073/pnas.1301293110

Groisman, Pavel Ya, et al., "Contemporary changes of the hydrological cycle over the contiguous United States: Trends derived from in situ observations," Journal of Hydrometeorology 5.1 (2004): 64-85.

Knutson et al., 2010, "Tropical Cyclones and Climate Change", Nature Geoscience 3, 157 - 163, Published online: 21 February 2010 | doi:10.1038/ngeo779

Knutson et al., 2013, Dynamical Downscaling Projections of 21st Century Atlantic Hurricane Activity: CMIP3 and CMIP5 Model-based Scenarios, Journal of Climate 2013 ; e-View
doi: http://dx.doi.org/10.1175/JCLI-D-12-00539.1

Pielke, R.A, et al., 2008, "Normalized Hurricane Damage in the United States: 1900 - 2005," Natural Hazards Review, DOI:10.1061/ASCE1527-6988(2008)9:1(29)

Villarini, G, and G.A. Vecchi, 2012, "Twenty-first-century projections of North Atlantic tropical storms from CMIP5 models," Nature Clim. Change 2:604–607.

Related posts
Global warming and the frequency of intense Atlantic hurricanes: model results, my 2010 blog post.

Climate Central's analysis of the new 2013 Kerry Emanuel paper.

Goodbye, Miami: Jeff Goodell's sobering 2013 article in Rolling Stone on the challenges Miami faces due to sea level rise and hurricanes.


What the official climate assessments say about climate change and hurricanes
The 2013 IPCC report gives “low confidence”--a 20% chance--that we have observed a human-caused increase in intense hurricanes in some parts of the world. This is a reduction in odds from the 2007 report, which said that it was more likely than not (greater than 50% chance.) The IPCC likely took note of a landmark 2010 review paper, "Tropical Cyclones and Climate Change", authored by ten top hurricane scientists, which concluded that the U.S. had not seen any long-term increase in landfalling tropical storms and hurricanes, and that "it remains uncertain whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes." The 2013 IPCC report predicts that there is a greater than 50% chance (more likely than not) that we will see a human-caused increase in intense hurricanes by 2100 in some regions; this is also a reduction from the 2007 report, which said this would be likely (66% chance or higher.)

The May 2014 United States National Climate Assessment found that “The intensity, frequency, and duration of North Atlantic hurricanes, as well as the frequency of the strongest (Category 4 and 5) hurricanes, have all increased since the early 1980s. The relative contributions of human and natural causes to these increases are still uncertain. Hurricane-associated storm intensity and rainfall rates are projected to increase as the climate continues to warm.”

Jeff Masters

Hurricane Climate Change

Dorian Makes a Short-Lived Comeback as a Tropical Depression

By: JeffMasters, 3:37 PM GMT on August 03, 2013

Like an annoying fly buzzing around your head that just won't go away, Dorian is back. Dorian began as a tropical wave that moved off the coast of Africa on July 22. Although initially given only a 10% of developing, Dorian surprised forecasters by becoming the 2nd farthest east-forming tropical storm for so early in the year on July 24. Dorian peaked in strength on July 25, when its top winds reached 60 mph. On July 26, Dorian encountered a dryer, more stable atmosphere, and the storm dissipated to a tropical wave on July 27. The remnants of Dorian continued tracking west-northwest, passing north of the Lesser Antilles Islands and into the Bahamas this week. Finally, at 5 AM EDT this Saturday morning, the remnants of Dorian reorganized enough to prompt the National Hurricane Center to upgrade Dorian's remnants to be Tropical Depression Dorian.


Figure 1. Morning satellite image of Tropical Depression Dorian, showing the surface circulation exposed to view as a naked swirl of low clouds, and a modest area of heavy thunderstorms well to the south of the center.

Dorian does not have long to live, due to high wind shear of 20 - 30 knots that has removed all of the storm's heavy thunderstorms far from the center, leaving the surface circulation exposed to view as a naked swirl of low clouds. With wind shear expected to rise even further this weekend, Dorian will likely be dead by Sunday, and get absorbed into the cold front lying off of the Southeast U.S. coast. Dorian is not a threat to any land areas.

Elsewhere in the Atlantic, none of the reliable computer models is predicting tropical cyclone formation over the coming week.


Video 1. With Discovery Channel's Shark Week approaching, Veronica of the web video series, "Fact or Fictional?" talks with me and a marine biologist about sharknadoes. Is it possible for a ferocious tornado to rip through Southern California, raining man eating sharks? My bit starts about halfway through the video.

Have a great weekend, everyone!

Jeff Masters

Hurricane

Ex-Dorian Attempting a Comeback off the Florida Coast

By: JeffMasters, 1:44 PM GMT on August 02, 2013

After a long trek over the Atlantic Ocean from the coast of Africa, the remains of Tropical Storm Dorian (now called Invest 91L) have finally arrived at the shores of North America. Ex-Dorian is nearly stationary, and is situated over the Northwestern Bahama Islands, just off the coast of Southeast Florida. Satellite loops and Melbourne, Florida radar images show that ex-Dorian has only a limited amount of heavy thunderstorms, which are not well-organized. There does appear to be a surface circulation center trying to form just north of the storm's heaviest thunderstorms, about 70 miles east of Vero Beach, Florida. However, dry air to the northwest, as seen on water vapor satellite loops, is inhibiting development. WInd shear is moderate, 10 - 20 knots, but is expected to rise to the high range, 20 - 30 knots, by Saturday morning. Ex-Dorian is expected to move slowly northwards and then north-northeastwards on Saturday. This motion will get ex-Dorian tangled up with a cold front that extends from Northern Florida northeastwards, just offshore from the Southeast U.S. coast. Before it merges with the front, ex-Dorian has some potential for regeneration into a tropical depression, and in their 8 am Friday Tropical Weather Outlook, NHC gave ex-Dorian a 30% chance of becoming a tropical cyclone by Sunday. Ex-Dorian will likely bring heavy rains to the Northwest Bahamas on Friday, and these heavy rains may also clip the coast of Southeast Florida. However, the bulk of ex-Dorian's rains should stay offshore.


Figure 1. Morning radar image of ex-Dorian from the Miami radar.

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™

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