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 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!
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 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 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!
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!
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!
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!
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!
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!
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