Tropical Tidbits from the Tundra

Isaac Makes Landfall - Causing Flooding in Louisiana

By: Levi32, 3:40 PM GMT on August 29, 2012

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Couldn't get a video out today, so it is just a written post.

Hurricane Isaac finally moved ashore near Houma, Louisiana a couple of hours ago after dancing within a couple dozen miles of the coast for most of yesterday. Isaac is moving off to the northwest, but very slowly, and has not weakened significantly yet due to the center being over mostly marshes. The storm surge has been up to 11 feet high near New Orleans at Shell Beach due to the prolonged onshore flow, and the surge up the Mississippi River has caused a levee to be overtopped in Plaquemines Parish, which has put entire houses under water as of this morning. As we've talked about for the last few days, Isaac's slow movement due to a fragile steering pattern which brought him on a rare track into this area is going to result in 12-20 inches of rain in parts of southern Louisiana and Mississippi. Up to 10 inches have already fallen in New Orleans, with around a foot still forecasted on top of that by the HPC.



It may be another 24 hours before the rainfall starts to let up in the New Orleans area, though areas farther east such as Mobile, AL may see it let up sooner. In about 24 hours a ridge building to the east of the storm should accelerate it northward into Arkansas and Missouri, providing beneficial rains in drought-stricken areas there. Until then, inland flooding is going to be a huge problem.

Isaac is like a somewhat weaker version of Hurricane Ike in 2008, which was also a large storm with an unusually low pressure for its maximum winds. Isaac got down to 966mb before landfall, and is now up to 972mb, but as we also saw with Hurricane Irene last year, it doesn't take a major hurricane to cause life-threatening problems. A pressure this low means a lot of air is getting forced upward, and no matter how "light" the winds are compared to the pressure, the rainfall potential is massive, and flooding will be what Isaac is remembered for. Category 1 strength winds for an unusually long period of time are also capable of doing as much damage as Category 2 or 3 winds over a more typical short period of time. The effects of Isaac are only about halfway over for many people in Louisiana right now.

Elsewhere in the Atlantic...Tropical Storm Kirk has formed in the central Atlantic, and will be recurving out to sea well away from land, and is not a threat.

Invest 98L, another large area of monsoonal low pressure, is moving westward across the eastern Atlantic, and could be a threat for development in a few days. Currently all models agree this system should pass well north of the Antilles Islands, and may recurve harmlessly out to sea. The pattern remains active, and additional threats for development are expected over the next few weeks as we go through the peak period of the hurricane season in early September.

We shall see what happens!



Isaac 12-24 hours from Landfall in Louisiana

By: Levi32, 3:29 PM GMT on August 28, 2012

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Isaac remains a tropical storm per the NHC, despite a dropsonde observation of 70kt winds at the surface (hurricane force is 64kt), and 85kt flight-level winds in the NE eyewall. It is unclear why they have chosen not to upgrade Isaac, but he will be a hurricane at landfall, and strengthening. Isaac has continued to gradually deepen during the last 48 hour period at a fairly consistent pace, but dry air remains integrated with his circulation, making the eyewall ragged. This is what has prevented Isaac from becoming a major hurricane. The storm will be strengthening through landfall though, and Isaac will likely have upper-end Cat 1 winds coming to the surface at landfall due to turbulent mixing.

The biggest problem with Isaac will probably not be the wind but the extreme rainfall of 12-18 inches forecasted in the New Orleans and southern Mississippi area during the next 48 hours. This will cause flooding problems, along with a higher storm surge than one would typically expect from a Cat 1 hurricane due to its slow movement. Hopefully residents are not taking this storm lightly, as it will be a quite nasty one as it comes ashore in 12-24 hours, depending on exactly which point on the Mississippi Delta it crosses the coastline. Tropical storm force rain bands are already moving onshore. I will be posting short updates on Facebook throughout the day on Isaac.

We shall see what happens!

Isaac Struggling, but Will Still be a Dangerous Storm at Landfall

By: Levi32, 4:02 PM GMT on August 27, 2012

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Tropical Storm Isaac continues to struggle a bit this morning, and despite deepening 6mb in the last 24 hours, the maximum winds have not increased because the core is not tightening into an eyewall. The reason for this is dry air entrainment on the eastern side, which was expected to limit the system, but it has had a bit more potent effect than expected. Isaac has a large circulation and it can take a while to rid itself of this dry air, thus strengthening may stay only gradual today. A new source for the dry air affecting Isaac appears to be the upper-level low to his southwest, which is moving away from the storm and providing good ventilation aloft, but again, the long inflow fetch from Isaac is tapping into some of this dry air. Isaac should be able to become a hurricane as soon as the core tightens, as the current central pressure of 988mb would support Cat 1 winds if an eyewall develops. Good news for the north gulf coast is that Isaac now should not make landfall as a major hurricane, though with the environment aloft as good as it is and the favorable shape of the coastline near the Mississippie Delta, the storm should still be watched for rapid intensification just before landfall. It should also be noted that Isaac will be slowing down considerably near the coast, and if the models are underestimating this slowing, the storm could have more time over water to strengthen just before landfall. The new intensity forecast is adjusted downward, and makes Isaac an upper-end category 1 hurricane at landfall.

The track forecast is finally stable today, after requiring westward shifts for the past several days in a row. With Isaac 36-48 hours from landfall, the track up until the coast is getting better narrowed down. The current track takes Isaac into the Louisiana/Mississippi border. There is still some uncertainty in the track just before and after landfall due to how Isaac will be interacting with a trough to its northeast and a ridge to its northwest, both features trying to pull the storm in different directions. The American models continue to take Isaac WNW through Louisiana into eastern Texas, while the European models take Isaac more north up the Mississippi River into the center of the country. The forecast track is in better agreement with the latter scenario right now. This will be significant because Isaac should be slowing down considerably near landfall, which will allow copious amounts of rainfall to fall along the central gulf coast and areas inland along the track. The forecast track lies just to the right of the multi-model consensus.

We shall see what happens!



Isaac Taking the "Impossible" Path Towards the Central Gulf Coast

By: Levi32, 4:05 PM GMT on August 26, 2012

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Isaac remains a tropical storm this morning, and though he has restrengthened some since yesterday, his proximity to Cuba has filled his large circulation with lots of dry air, and the inner core remains rather ragged on satellite and radar imagery. Due to Isaac's large size it will take a while to mix out all of this dry air, but once he does, significant strengthening may be possible over the open waters of the Gulf of Mexico. Radar is showing spiral bands with tropical storm force winds already moving into the Florida keys and the southern Florida peninsula. The forecast intensifies Isaac into a hurricane tonight, and a category 2 hurricane over the central Gulf of Mexico. Currently an upper-end Cat 2 is forecasted at landfall on the gulf coast, but if Isaac mixes out the dry air and establishes an inner core more quickly, he could easily strengthen more than forecast, and it is not out of the question that Isaac becomes a major hurricane before landfall.

The track forecast has been a wreck over the last few days, and everyone is having to shift westward with the models, which now take Isaac into Louisiana or Mississippi. This is the path I called highly improbable, but it looks like it may happen. Isaac has hit the "sweet spot" between the trough over the eastern seaboard and the ridge over the Rockies, a situation similar to what we had with Debby earlier this year, where my forecast also had to flip. When a storm is in this kind of a position, it is very hard to predict whether the storm will get recurved by the trough or brought westward by the ridge. This is the time when we are very glad to have computer models that can catch on to which path the storm will ultimately take at least 2-3 days in advance of landfall. The track forecast now calls for a landfall on the Mississippi coastline, very close to the 11am NHC track, though it should be noted that the cone of uncertainty fans out considerably near the gulf coast, and it is still possible that Isaac could deviate significantly to the right or the left, and everyone on the north gulf coast should be prepared for a hurricane hit. The NHC has expressed the abnormally high uncertainty with this track forecast as well.

We shall see what happens!



Isaac Could be a Double Hurricane Hit for Florida

By: Levi32, 3:56 PM GMT on August 25, 2012

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Isaac is disorganized this morning, which is to be expected from interaction with the mountains of Haiti. The storm has taken a significant jog north of where it was "supposed" to be this morning, due to the frictional effects of the mountains which I warned about yesterday being a wildcard for the short-term track. It goes to show how the model cluster can be way off even at the 12-24 hour verification. The significance of this is that Isaac is now only moving over the eastern tip of Cuba instead of the entire eastern half of the island, and thus he will be spending more time over water before hitting the Florida keys or south Florida. With about 36 hours over the very warm Florida straights, Isaac should have enough time to regenerate a core and strengthen into a lower Cat 1 hurricane.

The models, although they have shifted east since yesterday, have not come over to the peninsula, and remain over the eastern Gulf of Mexico and into the Florida panhandle with Isaac's track. They have at least mostly dropped the improbable northwesterly track into the central gulf coast, and now show a more likely recurve northward into the coast farther east. With the models tightly clustered this close to the end of the forecast, my track has to shift westward to meet closer with the model consensus. There still, however, remains some uncertainty, and as we just saw last night, the models can be off even with a 24-hour forecast. With Isaac now coming west of Florida, further intensification is called for after he scrapes south Florida. However, intensification over the eastern gulf is expected to be slower than intensification over the Florida straights, due to a track close enough to the Florida peninsula that Isaac's main inflow channel, which is from the east, will be passing over land and bringing some drier air into the storm. Normally storms taking a track like this struggle to strengthen at all and often weaken. However, a very favorable upper pattern will be developing above the storm as a trough-split backs away to the southwest, ventilating the eastern gulf, and this should offset the normal trend and allow slow strengthening through a 2nd landfall in the panhandle. On the current track a low-end Cat 2 hurricane is expected near Apalachicola, Florida in about 3 days, though a track just a little farther west could result in a stronger storm, and a track closer to the Florida peninsula could result in a weaker storm.

We shall see what happens!



Isaac to Impact Hispaniola and Cuba, on Course for Florida After

By: Levi32, 4:59 PM GMT on August 24, 2012

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Tropical Storm Isaac continued to struggle yesterday as its circulation was still decoupled, with the surface center off to the north of the mid-level center. The biggest pressure falls remained in the vicinity of the mid-level center, however, and thus overnight the system has reformed with what looks like a much better vertically-stacked center farther to the south. Convection is now trying to wrap up the eastern side of the storm, though the northwest quadrant is void of deep thunderstorms. The last recon mission found a stronger storm with 60mph winds and a pressure of 1000mb. Outflow is healthy all around the system, and the only truly limiting factor left is dry air getting entrained from the western Caribbean. Isaac should strengthen on approach to Hispaniola today, and if he passes only over the western tip of Haiti, extra water time before hitting Cuba could allow him to make a run at hurricane status. If the mountains of Hispaniola draw him north into the island more quickly, however, he will have less time to strengthen. The mountains of Haiti and Cuba will weaken Isaac a bit during the crossing, but due to improving environmental conditions, only moderate weakening is expected. Once in the Florida Straights, strengthening should quickly resume. The current forecast track has Isaac interacting with the Florida Peninsula rather quickly after exiting Cuba, and thus keeps Isaac a tropical storm. If, however, Isaac passes just on either side of Florida, it will have much more time over water, and could easily become a hurricane.

The forecast track reasoning remains unchanged. A trough over the SE US is eroding the western periphery of the Bermuda High, and is allowing Isaac to begin moving northwestward. The average heading from the last 4 recon fixes was 305 degrees. This should take Isaac across the greater Antilles and in the general direction of Florida during the next 3 days. The forecast track has been shifted a bit westward due to the southward reformation of Isaac's center, which has placed him well southwest of where he was expected to be this morning. The track now takes Isaac up the Florida Penisula. This track is still slightly to the east of the model consensus. The models have been shifting slightly eastward with each run ever since recon G-IV data was injected into their routines last night, and they now show a much more reasonable-looking recurvature into the Florida panhandle and then Georgia and the Carolinas, as opposed to the nonsensical northwest track across the Mississippi River that they showed last night. I believe some eastward adjustments of the models are still likely to occur with time. It should be noted that there is still great uncertainty in the track due to the crossing of the Caribbean mountains, since they are known to jerk storms around in unpredictable fashions, and could cause am abrupt shift in the track at any time. The entire eastern gulf coast, Florida, and the other southeastern states should monitor Isaac closely, both for direct landfall impacts, and heavy rainfall afterwards.

Elsewhere...Joyce is sheared and weak, and is not an imminent threat to land. She will be moving northwest in the general direction of Bermuda during the next 4 days.

We shall see what happens!



Isaac on Track to hit Hispaniola, then Florida

By: Levi32, 5:16 PM GMT on August 23, 2012

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Tropical Storm Isaac is starting to become better organized this morning after struggling a lot last night with decoupled centers in the low and mid levels, and a more consolidated, vertically-stacked center appears to be forming south of Puerto Rico. Forward motion has slowed from 21mph yesterday to 13mph now, indicating that Isaac is approaching the western periphery of the subtropical ridge to the north, and this slowing should help the storm strengthen on approach to Hispaniola during the next 24-48 hours. Intensification into a moderate-strong tropical storm seems likely, but hurricane strength before Hispaniola seems a little bit too much to ask for. The intensity forecast remains a tad lower than the NHC in the short range, a theme which has verified very well so far. Interaction with the mountains of Haiti and eastern Cuba should weaken Isaac, but restrengthening should be quick to ensue north of Cuba due to improving upper-level conditions and very warm water in the storm's path. Isaac could quickly become a hurricane very close to Florida if it gets any significant time over water on either side of the peninsula.

Isaac is beginning to move WNW around the periphery of the subtropical ridge, which currently extends into the Bahamas. Isaac will be gradually curving more towards the NW with time, taking it over Hispaniola and eastern Cuba, and eventually towards Florida. The models are in close agreement, and Florida is very likely to be significantly impacted by Isaac. The only exception is the ECMWF, which is still a western outlier, taking Isaac into Alabama in 7 days. This is a slight shift eastward from previous runs though, and may be the start of a correction trend eastward for the ECMWF towards the other models. The upper pattern ahead of Isaac features a weakness in the ridge that will be directly north of the Bahamas in 3 days, something that a strengthening storm coming off of Cuba may be more likely to take advantage of than some of the models show. The forecast track is still east of the model consensus at Days 4 and 5, very close to the eastern coast of Florida, similar to the 00z CMC and 06z GFDL solutions. The wildcard in the track forecast remains the mountains of Hispaniola and Cuba, which are notorious for jerking storms around, and could easily cause an unexpected shift in track at any time. A NOAA G-IV recon mission into Isaac tonight will hopefully transmit new data in time to be put into tonight's 00z model cycle, which should result in more accurate track forecasts. If the model consensus remains on the west coast of Florida after they have received the G-IV data, my track may have to shift a bit to the west.

Overall, Florida is likely to get impacted directly by Isaac no matter what adjustments are made during the next few days. The average NHC forecast error for 5 days is 260 miles, and there is lots of room for adjustment while we are still 4-5 days away from a Florida landfall. Until Isaac clears the mountains of the greater Antilles, great inherent uncertainty with such a track will make nothing certain about the long-range forecast.

Elsewhere....Joyce has formed in the central Atlantic, and will be recurving near Bermuda in 5 days or so. This is no imminent threat to land, and can be mostly ignored until Isaac is out of our hair.

We shall see what happens!



Isaac a Dangerous Storm for Hispaniola - U.S. to get Hit After

By: Levi32, 3:56 PM GMT on August 22, 2012

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Tropical Depression #10 has formed well east of Isaac in the central Atlantic, and is forecasted to pass north of the Antilles islands. This storm can be safely ignored for now while we deal with Isaac hitting land.

Tropical Storm Isaac was named yesterday, and has shown a healthy increase in thunderstorm activity, though the center remains under the northern edge of the main convective mass, and the northeast quadrant remains void of thunderstorms due to light wind shear and a lack of low-level convergence in that part of the storm. Isaac has not deepened during the last 12 hours, and is only up to 45mph winds right now. Only gradual strengthening should occur as Isaac crosses into the eastern Caribbean near Guadeloupe today, and the intensity forecast continues to keep Isaac under hurricane strength on approach to Hispaniola due to the time it will take for the large circulation to tighten, and the fast trade winds in the eastern Caribbean. This is still a less aggressive short-term intensification scheme than the NHC, though their forecast has come down several notches over the last couple of days, and is closer to my forecast now. Isaac should weaken while interacting with the high mountains of the Greater Antilles, but once clear of them, should restrengthen faster than it did in the Caribbean, and could quickly become a hurricane in the vicinity of Florida if it gets at least a couple of days of water time.

The track forecast philosophy remains generally unchanged. The trough currently over the eastern U.S. is lifting out to the northeast, but the southern part of this trough is being left behind as a trough-split over the SE U.S. during the next few days. This will be helping to erode the western periphery of the Bermuda High as it retreats eastward over the Atlantic, and the models generally agree that this will create an open weakness north of the Bahamas in 48-72 hours. The models then diverge on whether Isaac will move right into this weakness. The CMC continues to be the easterly outlier, taking Isaac well east of Florida. The GFS has Isaac approach the weakness but get entangled with the greater Antilles and miss it, not recurving until it reaches southwest Florida. The ECMWF remains persistent that the weakness will close off and a rebuilt ridge will direct Isaac into the central Gulf of Mexico as a major hurricane. Thus, there is still a large amount of uncertainty beyond 72 hours. For now, the forecast track remains close to, but just east of the consensus of the non-ECMWF models, and takes Isaac into the Bahamas just east of Florida with the assumption that Isaac's large circulation will be able to feel the weakness in the ridge north of the Bahamas. However, there is considerable inherent uncertainty to the west and south of the forecast track due to Isaac's interaction with the greater Antilles. The high mountains there have a tendency to jerk storms around in an unpredictable fashion, and could throw the track off at any time. Additionally, any further entanglement with the islands could reduce Isaac's intensity and prolong his recurvature, putting Florida and the eastern gulf coast at greater risk. The forecast cone reflects this, and interests from the Carolinas to the central gulf coast should monitor Isaac's progress closely.

We shall see what happens!



TD 9 Forms - Eyeing the Big Caribbean Islands and the United States

By: Levi32, 4:56 PM GMT on August 21, 2012

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Tropical Depression #9 has formed from Invest 94L east of the lesser Antilles. The system has displayed an increase in convection since last night, unsurprising since it has crossed 50W into an area of warmer water. Convective coverage remains low overall, and the northeastern semicircle of the storm is mostly void of thunderstorms. This should change as the system leaves its genesis area within the eastern Atlantic monsoon flow, and thunderstorm activity should gradually fill out during the next couple of days. TD 9 should steadily intensify as it enters the eastern Caribbean, bringing tropical storm conditions to the northern Antilles as far south as St. Lucia. By the time it nears Hispaniola in about 3 days, I expect it will be a moderate-strong tropical storm. This is not aggressive as the NHC forecast which takes TD 9 to a Cat 2 hurricane near Haiti. I am less bullish with the intensity through Day 3 due to the still less than ideal environment in the eastern Caribbean with continued fast trade winds and sinking air ahead of the storm.

The track forecast reasoning remains unchanged. The subtropical ridge directly north of TD 9 right now should continue to direct it westward or WNW for the next 48 hours or so. Thereafter, this ridge will be shifting eastward and a weakness will develop north of the Bahamas, inducing a more northwesterly motion near or over the island of Hispaniola. This motion should then continue, bringing TD 9 in the vicinity of the Bahamas and Florida within 5 days. The 0z ECMWF takes TD 9 south of Cuba and into the eastern Gulf of Mexico as a major hurricane. This is the western outlier of the model guidance envelope, and is being discounted as too far west at this time. The track has been nudged slightly to the left, and is in best agreement with the 06z GFS ensemble mean.

Overall, this storm is now developing, and we should start to get a better handle on the track as the models become better able to resolve the situation. The northern Antilles, Puerto Rico, Hispaniola, and eastern Cuba could all face impacts from this storm during the next few days. The Bahamas and the southeast United States are very likely to be impacted as well, but there is still uncertainty on the details given that a potential U.S. landfall is still at least 6-7 days away.

Elsewhere....Invest 95L in the western Gulf of Mexico has not developed significantly since yesterday, mainly due to the frontal boundary to the northeast being too strong to allow much consolidation. 95L still has another day or so over water and could still wind up just before moving ashore, but should not be more than a rain maker for northern Mexico and extreme south Texas.

We shall see what happens!



95L Trying to Develop in Western Gulf; 94L a Threat to Caribbean Islands and U.S.

By: Levi32, 5:40 PM GMT on August 20, 2012

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Invest 95L, a spin-off from Tropical Storm Helene, is back over the water in the western Gulf of Mexico where we expected it to be today, and is starting to rotate and generate thunderstorms as the tail-end of the front driving into the northern gulf helps promote low-level convergence in the area. As we've been warning since Ernesto made landfall, these remnants are a threat for development off of the west gulf coast, and they have probably about 48 hours before they are forced inland over northern Mexico or extreme southern Texas by high pressure building to the north of the front. A recon plane is going to be investigating the system shortly. 95L should be mostly a rain-maker, but small, quick spin-ups like this in the gulf can surprise you, and with Humberto still haunting the recent past, these kinds of things have to be watched closely.

Invest 94L will likely be the bigger threat as it moves into the eastern Caribbean islands and potentially other land masses during the next week or so. 94L has a well-defined circulation on satellite imagery this morning, but is still lacking convection. We have seen many times big swirls like this move across the central Atlantic and struggle with the dry air there, waiting until sea surface temperature increase sharply west of 50W to finally generate thunderstorms. 94L will be approaching 50W today, and tonight and tomorrow I expect it will start to develop, and should be a tropical storm by the time it reaches the lesser Antilles. Land interaction with the greater Antilles will then modulate the intensity, but 94L will likely not have a chance to become a hurricane until it clears the Caribbean island chain.

The forecast philosophy has not changed today. A deep-layer ridge to the north of 94L should steer the system on the same W to WNW track that it has been on, bringing it into the northern Antilles in 48-60 hours. Thereafter, the upper-level pattern over eastern North America favors a weakness developing in the ridge off of the southeast United States in response to strong upper blocking over Canada and an exiting mid-latitude trough entering the NW Atlantic and eroding the subtropical ridge west of Bermuda in 48-72 hours. This should result in 94L making a gradual turn towards the northwest, crossing the Caribbean island chain and ending up near the Bahamas in about 5 days. Due to 94L developing slightly slower than expected, the forecast track has been shifted southwestward slightly after 48 hours, and now takes 94L over Hispaniola. This part of the forecast is important, because a pass over Hispaniola would be a wildcard that determines how strong 94L becomes north of the Caribbean, and could cause a jerky shift in its track. Beyond this point, uncertainty increases markedly due to an unknown amount of land interaction, and although the odds of a U.S. landfall are increasing, such a potential landfall would be 7+ days away, too far away to get specific about that part of the forecast.

We shall see what happens!



94L to Gradually Develop, Lesser Antilles Threatened First

By: Levi32, 3:11 PM GMT on August 19, 2012

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Hurricane Gordon continues to move northeastward and will scrape the southern Azores as it weakens below hurricane status during the next few days.

The remnants of Helene officially dissipated well inland over Mexico as per the NHC yesterday. However, this morning we see the remnant rotation in the low-mid levels directly above Tampico, Mexico, right on the coastline of the western gulf. This small disturbance still poses a threat to try to redevelop over the water as it gets drawn north by an upper-level trough, represented by a frontal boundary that is pushing towards the northern gulf waters. Model support for redevelopment has waned since yesterday, but Texas and North Mexico should not ignore Helene's remnants yet. Even if redevelopment does occur, the only change in the weather for the TX/MX coastlines will be slightly heavier rainfall than they are already getting. Louisiana will also be getting in on pulses of tropical rain over the next few days.

The big story continues to be Invest 94L in the eastern Atlantic. The system's organization has changed little since yesterday, and as expected it should take its time in developing during the next few days. Dry air to the north is getting entrained and is hampering the development of deep convection, and the main center is competing with a rather large envelope of low pressure to the south and southwest within the ITCZ/monsoon trough. The global models, specifically the GFS and ECMWF, are starting to pick up on the slower rate of development of 94L due to the time needed to consolidate a circulation within such a large area of low pressure, and both of these models have actually ceased to show significant strengthening of 94L before reaching the Antilles islands, at which point it becomes entangled with Hispaniola and Cuba on both models and never strengthens again. These jumpy changes in intensity forecasts are typical of models when they are struggling with resolving the development of a tropical system within a large area of low pressure, and until solid development occurs, I think they will continue to struggle a bit with how to handle 94L. My intensity forecast is unchanged from yesterday, and I think 94L should get named Isaac before reaching the northern Antilles Islands in about 4 days. Folks there and in Puerto Rico should expect a strengthening tropical storm on their doorstep as a good possibility by that time. Thereafter, land interaction will largely determine the intensity, and the track is not nailed down well enough yet to know exactly how that will unfold.

The track forecast continues to look threatening for not only the northeastern Caribbean Islands but also the eastern United States. The pattern itself over North America and the north Atlantic is a classic for an east coast hurricane threat. The question is just whether a storm will be in the correct position to take advantage of it. 94L may very well be, as long as it doesn't bomb out into a hurricane quickly, in which case it would recurve out to sea, likely between the eastern seaboard and Bermuda. The slow development of the storm now is bad news for the U.S. later, as slower development means a farther west and south track. While the global model solutions are currently erratic, due to their recognition of the struggles that 94L will face during its development, my long-range track has been nudged slightly to the southwest, but the forecast is largely an update of yesterday's.

We shall see what happens!



Watching Helene; 94L Could Threaten the Northern Antilles Next Week

By: Levi32, 5:28 PM GMT on August 18, 2012

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Tropical Storm Helene formed from the remnants of TD 7 in the western Gulf of Mexico yesterday. The mid-level center of this system quickly went inland over Mexico, and this morning the surface center has also crossed the coastline. However, as we have talked about for the last several days, a portion of this system is going to be drawn northward along the coastline in response to a trough digging into the southern U.S., and with the trough being reluctant to pull out quickly, the remnants of Helene could fester near the coast of Mexico over the course of the next several days, possibly redeveloping near the tail-end of the old frontal boundary that will be pushing out over the northern gulf ahead of the upper trough. The models are less supportive of this solution than they were a couple of days ago due to a splitting of Helene's remnants that is now shown on several models, where a piece gets pulled northeast along the front into Louisiana, and another piece gets left behind off of Mexico, but without enough energy left to redevelop. Some models like the CMC and FIM still show redevelopment, and given the stagnant situation and how Helene has already shown she can develop without model support (they models did not foresee her getting named so soon), she should be monitored over the next 5 days or so.

As Helene dances with death over the next few days, attention will quickly shift away from her and towards Invest 94L in the eastern Atlantic, which is quickly stealing the show in the tropics. This system will be traveling westward and may threaten the northern Antilles islands in 5-6 days as what I think will be a tropical storm. Dry air to the north of the system should keep it from developing quickly over the next few days, and I think it will take until 94L is a couple days away from the islands to earn the name Isaac. As usual in years like this, significant strengthening will likely wait until west of 50W or so, and right now I don't think the Antilles will be facing a hurricane threat, but a strengthening tropical storm is fairly likely.

While the Antilles may be threatened first, 94L has the potential to impact land farther west. The models today have shifted much farther west than yesterday, showing what I think is a more reasonable track possibility with a recurving storm somewhere between 65W (Bermuda's longitude) and 80W. Any recurve west of 70W has the potential to impact the U.S. eastern seaboard. The current pattern favors a recurve in this corridor for a strengthening storm coming out of the central Atlantic, and I illustrate this on the ECMWF ensembles in the video.

Overall, we have a very long time to watch 94L, as it is still 5 days or so away from the Antilles islands, and over a week away from any potential impacts on the United States. The pattern favors this system getting uncomfortably close, so we will be monitoring it closely. The Antilles Islands should not be surprised to see a strengthening tropical storm near their doorstep in about 5 days.

We shall see what happens!



Threat for Home-grown Development in the Western Gulf next Week

By: Levi32, 5:13 PM GMT on August 16, 2012

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The deep tropics are quiet today. Tropical Storm Gordon has formed way up in the northern Atlantic, and may threaten the Azores as likely a tropical storm in a few days, but could become a hurricane before that time. Overall, he is not a significant threat, but illustrates again how storms this year are waiting to develop and strengthen until they are well north of the tropics.

Of interest in the near-term are the remnants of tropical depression 7, which are currently over central America. Last week I flagged these remnants as a potential redevelopment issue in the western Gulf of Mexico due to the robust mid-level wave signature that has remained with the system despite the dissipation of the surface circulation. A weakness in the ridge over the western gulf due to a trough digging into the south-central U.S. over the next few days will be drawing TD7's remnants northwestward along the coastline of the western gulf. This trough is forecasted to take its sweet time in leaving the scene, and will cause the pattern to remain rather stagnant for the next 6-8 days or so. An old frontal boundary will be dropping down over the northern gulf during this time in association with the upper trough, and could provide a focus of convergence to allow redevelopment of low pressure very near the Mexican coastline.

The evolution of a pattern where an upper trough is moving eastward to the northeast of a tropical disturbance can be favorable for development because of the high pressure that develops on the back (western) side of the upper trough, promoting surface convergence and piling up of air near the tropical disturbance. Dry air off the continent can initially be an issue for the disturbance in this situation, but as the upper trough gradually moves on, the surface high pressure also moves eastward, and eventually opens up a moist southeasterly flow into the disturbance, making dry air less of an issue and igniting significant convection due to the still existing low-level convergence. I illustrate in the video above how this happens on last night's 0z ECMWF run.

Overall, development chances will largely be modulated by proximity to land, whether TD 7's remnants bury themselves in Mexico or end up 50-100 miles off the coast. With this system having the opportunity to spend several days near the coast with 30C SSTs in the western gulf, tropical development could eventually occur. The models are only showing hints of development right now due to the uncertainty of land interaction, but they were showing nothing last week, and have slowly started coming around to the idea that this could potentially develop in our back yard next week. Regardless of development, this pattern favors a rain event coming to the northern Mexican and Texas coastlines, possibly Louisiana as well, as this tropical moisture surge interacts with the old frontal boundary that will be draping itself across the northern gulf.

We shall see what happens!

TD 7 not a Significant Threat for Now; Atlantic Becoming Active

By: Levi32, 2:15 PM GMT on August 10, 2012

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TD 7 has formed out in the central Atlantic, a bit farther north than Ernesto, but will be similar to Ernesto in many ways. Right now thunderstorm activity is limited to mostly SW of the center, and the center itself is mostly exposed. Dry air is impinging upon the system from the north, and the circulation is a bit elongated SW to NE. TD 7 is also embedded in a large-scale inverted-V wave signature, which is partially responsible for the elongation, and is resulting in a less robust circulation than what Ernesto had at this longitude. The short-term track will follow in Ernesto's footsteps into the Caribbean, possibly a bit farther north, and the system will likely struggle for the next several days with dry air, wind shear, and fast trade winds making it difficult to maintain a closed circulation. Such conditions are prevalent often in the central Atlantic and central-eastern Caribbean during El Nino years such as this one.

I've decided to experiment with drawing my own forecast tracks to make my ideas more coherent. Below is my first attempt. My track closely follows the current NHC track on the southern side of the model guidance envelope, based on the previous poleward bias that the models had with Ernesto, and tend to have with weak central Atlantic systems in general. This track is also based on the premise that TD 7 will remain weak and get whisked along quickly westward by the trade winds, with little opportunity to strengthen and move poleward. While TD 7 may get named Gordon before reaching the Caribbean, strengthening of any kind will be hard to come by, and TD 7 could dissipate in the central Caribbean.



In the long-range, TD 7's remnants may retain a mid-level signature and move towards central America and the western Gulf of Mexico into a potential opening in the ridge to the north. The CMC and GFS ensembles show TD 7's mid-level wave structure taking this path in 7-10 days. Given the pattern this year favoring development farther west and north, it is possible that a former TD 7 getting into this region could pose a development threat down the road, though it is too far away to know at this time. In general, a southeasterly 500mb flow out of the Caribbean towards Texas indicates that tropical moisture will be advecting northward, and could be something to watch in the long range.

Elsewhere, a vigorous tropical wave has moved off of Africa and is moving across the Cape Verde islands bringing rare tropical rains to that area. Due to its northerly latitude, recurvature in to the middle of the Atlantic seems to be the likely future for this system, with no threat to land. Also due to its latitude, dry air is quickly choking the circulation, and development, if any, will be slow to occur, and the system will have to get significantly farther west and north if it is to have a chance at development.

We shall see what happens!

Ernesto to Make Landfall Tonight as a Hurricane

By: Levi32, 2:12 PM GMT on August 07, 2012

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Ernesto is now deepening again after a delay yesterday when its small, fragile core collapsed and allowed some dry air into the circulation, forcing Ernesto to rebuild from scratch. The pattern is very conducive in the western Caribbean for intensification, but Ernesto got a little too excited too fast, and had to reset. The pressure is now falling again, down to 988mb, and Ernesto should easily attain hurricane-force winds before landfall near Chetumel, Mexico late tonight. Ernesto's core is still small, but his spiral bands expanded yesterday and have broadened the wind field of the storm as a whole, so a large area of rain and TS-force winds will be slamming Mexico and Belize as the center comes ashore. Ernesto will be weakened by the crossing of the Yucatan, but will likely restrengthen some on the other side in the Bay of Campeche, possibly back to hurricane strength if it moves far enough north to get more time over water. However, the 2nd landfall will occur relatively quickly, not allowing much time for intensification. This will be a classic double hit for Mexico, with flooding rains probably being a bigger problem for them than the winds.

In the rest of the Atlantic, we have ex-Florence and Invest 92L in the central Atlantic. The models are not excited about either system, and develop nothing in the Atlantic during the next 10 days. The GFS and CMC hint that the Gulf of Mexico will remain somewhat active after Ernesto leaves, with upper ridging and some low-level disturbances possibly making some noise during the next 8 days, but right now no immediate or significant threats are apparent.

The MJO is forecasted to move deeply into phases 1 and 2 over the next two weeks, supporting an active Atlantic overall, and the GFS has been bullish on many Cape Verde waves that it has moving off of Africa during that time. As promised pre-season, the African wave train is strong this year, but will have trouble generating storms in the deep tropics, and these waves are more likely to develop after they get north of 20N and farther west. We will have to watch for some of these to sneak up on us closer to our back yard. Overall, there are no immediate development threats after Ernesto leaves.

We shall see what happens!

Ernesto Rapidly Intensifying, Will Likely be a Hurricane Very Soon

By: Levi32, 2:23 PM GMT on August 06, 2012

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Ernesto, after having an exposed surface center yesterday, is now rapidly intensifying as the recon plane currently in the storm has found that the center has jumped northeastward into an area of deep thunderstorms, and the pressure has fallen to 994mb. The plane has also found a closed eye, very small, with a diameter of only 8 nautical miles. Flight-level winds of 77kt and an uncontaminated SFMR surface wind report of 62kt puts Ernesto at barely below hurricane strength, and another 3-4mb pressure drop will likely make this a hurricane. The jump in Ernesto's position will be significant for the short-term track and exact landfall point along the Yucatan in about 36 hours. With a small core like Ernesto has, a 50-mile change in landfall location can shift the entire area of maximum winds away from one location and on top of another. For Honduras this shift is good news since Ernesto will be farther off their coastline, and the southern side of the storm is the weakest side, making hurricane conditions unlikely there.

This significant strengthening was warned about in this dangerously conducive pattern in the western Caribbean, and Ernesto now seems to be taking full advantage of it. There was some doubt yesterday due to the unknowns of how fast Ernesto would reorganize, but this will now likely be a hurricane landfall for the Yucatan, possibly stronger than a Cat 1 if current trends continue. Regardless of first landfall intensity, Ernesto will be knocked down significantly by a full crossing of the Yucatan, but still may restrengthen significantly in the Bay of Campeche if it is far enough north to get quality time over the water. It is not outside the realm of possibility for Ernesto to regain hurricane status for a second landfall in Mexico depending on the exact track.

The long-term track since yesterday has shifted south yet again, farther south than expected, now well south of Tampico as the models have become tightly clustered around Veracruz, Mexico. The forecast philosophy overall has not changed. Height falls are now being observed over the SE US as the weakness in the ridge begins to become a little bit more pronounced, but not strong enough to bring Ernesto very far north. However, latitude gains are likely before first landfall in the Yucatan, followed by a turn back to the west as the storm is caught south of the big Texas ridge. Ernesto's chances of affecting anyone north of Tampico, Mexico are now quite small given the tight and confident clustering of the models as we get closer to the end of Ernesto's journey.

Right now though, the Yucatan portion of Mexico and Belize are getting hit first in the next 36 hours or so, likely by a hurricane, and hopefully nobody took their eyes off of this storm that fell apart but is now roaring back.

We shall see what happens!

Ernesto Weaker, Restrengthening Likely Before Yucatan Landfall

By: Levi32, 3:54 PM GMT on August 05, 2012

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Ernesto has weakened considerably in satellite presentation since yesterday, and recon and Oceansat passes have confirmed that this is now an open wave. It goes to show that satellite appearance can be deceiving in the central-eastern Caribbean with storms like this, and this kind of weakening before 75W was expected. The NHC will likely continue advisories on Ernesto even though it is undeserving of its name, simply because it is expected to regain a circulation in the near future.

Now phase 2 of Ernesto's life begins, as later tonight and tomorrow it will enter conditions more conducive for regeneration and further development. The upper low over the Gulf of Mexico is making its way WSW out of the way and starting to ventilate the NW Caribbean, and Ernesto should have room to expand an upper ridge once convection starts firing again. The global models support this upper pattern developing within the next 72 hours. The ECMWF is finally onboard with restrengthening, something it has refused to show until the last couple of runs. While hurricane intensity is not yet impossible before reaching the Yucatan, Ernesto is very beat up, and how much it can restrengthen before landfall will heavily depend on how soon and how fast it starts regenerating. The longer it hesitates, the better the news for the Yucatan. A strengthening tropical storm near Chetumel seems the most likely scenario now.

Once across the Yucatan, Ernesto will be weakened again, but again should restrengthen, likely to its maximum intensity while in the western Gulf of Mexico, and here I think it will have enough time over water to become a hurricane. The ECMWF now supports this, though the other global models have just become weaker and farther south with time. My target area from the last couple of days of the northern half of Mexico to extreme south Texas still looks good. At this time we can basically discount Ernesto getting any farther north than that, and the north gulf coast can rest easy now. The farther north Ernesto tracks in the gulf, the more time over water it will have with favorable conditions aloft, and the stronger it will get. Interests along the west gulf coast and the Yucatan Peninsula should monitor Ernesto closely. He's coming back, and is not done just yet.

We shall see what happens!

Ernesto Still Weak but Better Organized, Likely to Become a Hurricane Threat

By: Levi32, 7:08 PM GMT on August 04, 2012

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Ernesto has become better organized since yesterday with a persistent CDO and expansion of the spiral bands and upper-level outflow. However, recon this morning found that Ernesto has not strengthened since yesterday, and the central pressure is actually up a few millibars. The impressive organization of Ernesto with this high of a pressure came as a surprise to me, but the idea that this storm would halt intensification upon entering the Caribbean until about 75W has verified nicely so far. I explain in the video my theory as to why Ernesto has been firing all of this convection without strengthening.

Once past 75W, or about the longitude of Jamaica, conditions will rapidly improve for Ernesto as the upper-level pattern allows expansion of upper ridging above the storm, and the trade winds slow down in the western Caribbean, both of which should allow pressures to fall and the storm to strengthen. We could see Ernesto quickly become a hurricane in the western Caribbean, and a big problem for the Yucatan Peninsula. If Ernesto crosses a portion of the Yucatan it will knock it down, but Ernesto may reach his peak intensity afterwards in the Gulf of Mexico.

Ernesto's track is still the toughest part of this forecast. It will largely be a function of his intensity, since a stronger storm will tend to move more northward into the weakness in the steering ridge that will be over the north gulf coast over the next several days. The forecasted pattern consists of not a wide open trough, but rather a large area of sprawling, light, erratic winds over the Gulf of Mexico, indicative of a weak steering pattern that a strengthening Ernesto will likely start to turn more northwest into while moving slower. I believe this will eventually take Ernesto near the northeastern Yucatan, and then into the western Gulf of Mexico. The largely uncertain part of the forecast is where the second landfall in the Gulf of Mexico will be. Right now I am leaning towards the idea that Ernesto will be guided back towards the WNW by the Texas ridge into northern Mexico or southern Texas, a solution supported by last night's ECMWF ensembles. However, this is still 6-8 days out, and if we have a particularly powerful hurricane moving far enough north in the NW Caribbean, the door could open for a track into the north gulf coast instead, so the entire north and west gulf coasts and the Yucatan Peninsula should closely monitor Ernesto, as he could become a big problem down the road.

We shall see what happens!

Ernesto to Remain Weak for Now, then Strengthen Later; Long-term Track Still Uncertai

By: Levi32, 6:20 PM GMT on August 03, 2012

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Ernesto has formed, and is now moving into the eastern Caribbean after passing the Antilles Islands last night. Overall organization of the system has not changed, with limited thunderstorm coverage over a tight but weak circulation. Dry air being entrained from the west is inhibiting Ernesto, and the storm is also now embedded in a fast 20mph+ trade wind flow that is unfavorable for the intensification of the circulation. This will likely limit Ernesto to weak-moderate tropical storm intensity until it makes it to the longitude of Jamaica, at which point the trade winds slow down, and upper-level ridging will likely make the environment quite conducive for intensification. Ernesto may even open up into an open wave without westerlies on the southern side for a time before then, but should remain robust and redevelop a closed circulation in the western Caribbean if it does.

The future track of Ernesto largely depends on his intensity in the western Caribbean, as the weakness in the ridge over the Gulf of Mexico will not be strong enough to pick up a weakling system, but would likely pick up a hurricane and bring it north of 25N. While Ernesto should start strengthening once to 75W, if he starts off very weak then he may not have the time to strengthen enough to move into the weakness and encroach upon the northwest Gulf of Mexico, but instead could get re-caught by the Texas ridge and move WNW across the Yucatan and then into the northern half of Mexico in the western gulf. This is the solution adopted by the ECMWF over the last couple days, and more recently the latest GFS and CMC runs. The other models still go to the north with a stronger system, along with the NHC. It is starting to seem a bit more likely that Ernesto hits the Yucatan and then moves into the Bay of Campeche, given that the environment doesn't seem to support strengthening until west of Jamaica. The NHC shows Ernesto strengthening into a hurricane near Jamaica, a solution I find less likely given the strong trade winds and dry air ahead of Ernesto right now.

Overall, Ernesto's intensity in the western Caribbean will largely determine whether he becomes a problem for the United States down the road. The favorable conditions in the western Caribbean mean that Ernesto will likely become a significant problem for central America even if the shorter, southerly route is taken with less time over water. These possibilities are still 5-8 days out so nothing can be guaranteed yet, and interests in the western Caribbean and along the gulf coast should monitor Ernesto over the coming days.

We shall see what happens!

TD 5 Forms, not a Big Deal for Now, but Could Become One Later

By: Levi32, 2:21 PM GMT on August 02, 2012

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Invest 99L developed into Tropical Depression #5 yesterday, and has maintained about the same intensity since then. Thunderstorm activity partially covers the center, but is still clearly being limited by wind shear being imposed on the northern side of the circulation by an upper low over the central Atlantic. This wind shear will continue to be a moderate issue for TD 5 over the next few days as the upper trough expands westward a bit to the north of the big Caribbean islands, following TD 5 as it too moves westward. The bigger issue for TD 5 though will be the strong trade wind flow that it is entering. The depression is now moving at 21mph, indicating the increasing flow that it is becoming embedded in, which will make it difficult for its circulation to survive and for thunderstorms to develop over it. This is a typical pattern for the eastern Caribbean that makes this region unfavorable most of the time for developing tropical systems, but especially during El Nino years like this one. Due to this pattern, I don't expect TD 5 to really strengthen at all for the next 3-4 days. It may get named Ernesto, but should remain a very weak tropical storm for the time being, and could even degenerate back into an open wave at some point if it loses the westerlies on the southern side of its circulation. The lesser Antilles will get minimal tropical storm conditions from this starting tomorrow, but not a big deal there.

However, TD 5's best days may lie well ahead of it. If TD 5's circulation survives as a defined entity until it reaches Jamaica, the pattern starts to turn in its favor. Right now there is an upper trough over the northwest Caribbean that won't be moving much over the next 7 days, and neither will the upper trough over the central Atlantic. As TD 5 moves westward, the NW Caribbean upper trough will start backing away to the southwest in front of TD 5, a situation that is almost always favorable, since it allows upper-level ridging to expand over the system and cause light wind shear and divergence aloft that promotes convection and lowering of pressures. The positioning of the TUTT-like trough to the northeast of TD 5 at that time would further improve ventilation of the area in general. The GFS ensemble mean supports this, as I show in the video. Such a pattern could rapidly become conducive for significant strengthening if TD 5 is organized enough to take advantage down the road in the northwest Caribbean and quite possibly the Gulf of Mexico.

As far as the future track goes, much will still depend on how strong TD 5 can get west of Jamaica if it is still alive, but some general ideas can be discussed. The GFS specifically and now the ECMWF have been leaning northwestward with TD 5's track in the long term, and it is worth investigating whether this could make sense. I show in the video how the typhoon pattern in the western Pacific looks like it is connected with this idea, causing amplification in the pattern that ripples downstream and strengthens a trough over the eastern U.S. in 7-10 days that may break down the ridge over the Gulf of Mexico and allow TD 5 to turn northwestward towards the gulf. The strengthening of the eastern trough also looks like it could force the Texas heat ridge far enough back into the Rockies that Texas itself could be open to a hurricane hit in this kind of a situation, but such a pattern leaves everywhere from central America to Louisiana open to a hit, and specifics cannot be known this far in advance.

Overall, TD 5 is no big deal in the near term, and will be directly affecting only the Lesser Antilles as a tropical depression or minimal tropical storm tomorrow and Saturday. Jamaica and the rest of the NW Caribbean may have to deal with TD 5 in 4-6 days, possibly as a restrengthening system of higher caliber than it is now if it survives the trip through the Caribbean and is able to take advantage of what should be a much more favorable pattern for intensification in that area. Interests in the NW Caribbean, central America, and the Gulf of Mexico should keep a wary eye on this situation due to its possible long-range implications.

We shall see what happens!

99L Organizing, Will Likely Bring TS Conditions to Lesser Antilles

By: Levi32, 2:37 PM GMT on August 01, 2012

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The main feature of interest in the Atlantic continues to be Invest 99L east of the Antilles islands The system has gradually become better organized over the last couple of days, with moderate-deep convection persisting over a developing area of surface low pressure. This convection is not yet well-organized, however, and 99L is still largely attached to the ITCZ, and it will need to detach from this in order to close off its own surface circulation. This could happen within the next day or two, and would likely result in the NHC designating this a tropical depression. The system has not exploded with strengthening, and thus has not been able to gain enough latitude to pass north of the big Caribbean islands, and will instead be trekking through the Caribbean itself. 99L, depending on exactly where the surface center develops, looks like it will pass into the Caribbean through the southern half of the Lesser Antilles, and we should get a good look at 99L through the new Barbados doppler radar system.

There will be numerous struggles for 99L in the central-eastern Caribbean. The biggest one will be strong trade winds redeveloping in the Caribbean behind the tropical wave that is running in front of 99L, that will make it difficult for 99L's circulation to hold itself together. There is also still large-scale sinking in the Caribbean as the MJO is still in the Pacific, and is not supporting the Atlantic with upward motion yet. There will also be some wind shear imparted by a TUTT-like upper trough currently situated north of 99L that will be expanding westward as 99L moves into the Caribbean. 99L's eventual separation from the ITCZ will also open it up to entrainment of the dry air to its north and to its west in the Caribbean. Due to all of these things, it seems likely that any strengthening of 99L will halt after it passes the Antilles Islands, and the system will likely struggle to survive after that point, possibly even dissipating.

However, 99L's story may not be that short. If 99L becomes sufficiently organized to survive a trip through the central Caribbean as a defined entity, conditions may improve in 7-10 days in the western Caribbean and Gulf of Mexico. This region will be west of the TUTT axis, in a good position for being ventilated by upper-level ridging that the GFS ensembles are now showing should develop west of the TUTT late next week. In addition, the MJO will have moved farther into the eastern Pacific by that time, and may decrease the large-scale sinking over the area, making the environment more hospitable for a tropical system. Such a situation could result in 99L restrengthening west of 80W, as long as it can avoid hitting Nicaragua or Honduras straight away. No global model currently strengthens 99L into anything significant, but most models do keep it as a defined enough entity throughout its journey that once into the western Caribbean, it may be able to cause problems, even though the models do not show it yet. It is too soon to know whether 99L will curve northwestward into the Gulf of Mexico in the long range or move into Central America. That will depend on how well 99L handles the Caribbean, and whether it survives to make it that far west.

Overall, we still have a lot of time to monitor this system. The Lesser Antilles, especially the southern half, will deal with 99L first, likely receiving tropical storm-like conditions late Friday well into the weekend, but not a big deal for them. The rest of the Caribbean farther west along with central America and the southeastern United States should keep a wary eye on the situation due to its long-term potential.

We shall see what happens!


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

Tropical Tidbits from the Tundra

About Levi32

Masters student in tropical meteorology at FSU. Raised in Alaskan blizzards, but drawn toward tropical cyclones by their superior PGF.

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