ATL: ISAAC - Post-Tropical - Discussion

[KWT]

Yes that ULL does seem to be dropping down a little to the SW at the moment, thats got to be causing a certain amount of shear.

I wonder whether the ECM was right all along with very little strengthening happening till past Haiti/Cuba...

[tgenius]

Once the GIV data gets fed in then we can get a better understanding of what is going to happen. I'm not so sure it will affect SFL when all is said and done.

[cycloneye]

Here is Dr Jeff Masters discussion about Issac with the 11 AM Advisory data.

Posted by: Dr. Jeff Masters, 11:06 AM AST on August 22, 2012

Tropical Storm Isaac is lashing the entire Lesser Antilles chain of islands with heavy rains this morning, and the winds will be on the increase this afternoon as the storm heads west at 19 mph. Isaac is still a weak and disorganized tropical storm, but that is changing. The Hurricane Hunters are in the storm, and have thus far found no increase in the storm's top surface winds, which remain near 45 mph. In their 7:44 am EDT center fix, an Air Force Reserve aircraft measured a rather unimpressive center pressure of 1007 mb, and top winds at their 1000 foot flight altitude of 56 mph. The plane did not observe an eyewall trying to form, and recent microwave satellite images also show no signs of an eyewall forming. A large area of dry air to the north of the storm, as seen on water vapor satellite loops, continues to interfere with development, and it is unlikely Isaac will become a hurricane today. Satellite loops, however, show that Isaac has developed a Central Dense Overcast (CDO) of high cirrus clouds, the hallmark of a developing storm. These clouds have very cold cloud tops, indicating that the updrafts creating them are quite strong. An analysis of upper level winds from the University of Wisconsin CIMSS shows that upper-level outflow is becoming well-established to the southwest and northwest, but outflow is restricted on the southeast side. A large clump of heavy thunderstorms several hundred miles southeast of the center continues to compete for moisture, and is interfering with the low level inflow and upper level outflow of the storm. The intensification rate of Isaac will increase if the storm is able to integrate this clump of heavy thunderstorms into its circulation, which satellite loops this morning suggest is now beginning to happen. Radar imagery from Barbados and Martinique show plenty of heavy rain showers, mostly on the south side of Isaac where it is moister, but little spiral banding. Hurricane hunter missions are scheduled for Isaac every six hours, and a NOAA hurricane hunter research aircraft will also be in the storm, with missions scheduled every 12 hours. The NOAA jet is first scheduled to fly into the storm on Thursday afternoon, to do a large-scale dropsonde mission to aid model forecasts.

Intensity forecast for Isaac

The latest 8 am EDT run of the SHIPS model shows that wind shear is moderate, 10 - 15 knots, but is expected to relax by this evening to a low 5 - 10 knots. Ocean temperatures have increased to a very warm 29°C, and the storm is now over waters with a high total heat content. The lowering wind shear and warm waters should allow the storm to wall off the dry air that has been interfering with development, and also allow the storm to integrate the thunderstorm clump on its southeast side that has been interfering with low level inflow and upper level outflow. It will take some time for the increase in organization to result in an increase in Isaac's winds, and I still expect top winds of 45 - 60 mph in the Lesser Antilles Islands this evening when the core of the storm moves through. On Thursday, when Isaac will be in the Eastern Caribbean, conditions should be favorable enough to allow steady strengthening to a Category 1 hurricane. The 11 am EDT wind probability forecast from NHC predicted a 47% chance that Isaac will become a hurricane by Friday morning, and a 16% chance it will be a Category 2 or stronger hurricane then. This is a reduction in the odds given in the 5 am advisory.

Impact of Isaac on the Islands

The entire Lesser Antilles Islands chain will have a three-day period of heavy weather Wednesday through Friday. Sustained tropical storm-force winds extend out about 50 miles to the north of the center and 30 miles to the south, so an 80-mile wide swath of the Lesser Antilles will potentially see tropical storm-force winds of 45 - 60 mph this Wednesday evening. Guadaloupe, Antigua and Barbuda, Dominica, Montserrat, and St. Kitts and Nevis at highest risk of these winds.

Winds in St. Croix in the Virgin Islands will likely rise above tropical storm force on Thursday morning, and the south coast of Puerto Rico should see tropical storm-force winds by Thursday afternoon. The San Juan airport may be able to stay open Thursday afternoon and evening, but I think it is more likely they will be forced to shut down.

On Thursday night, heavy rains and tropical storm-force winds should arrive on the southern coast of the Dominican Republic, and all airports in the D.R. will probably be closed on Friday. Rainfall amounts of 8 - 12 inches will likely affect the Dominican Republic Thursday through Saturday, creating dangerous flash floods and mudslides.

Isaac is potentially a very dangerous storm for Haiti, where 400,000 people still live outside underneath tarps in the wake of the 2010 earthquake. Heavy rains from Isaac will begin on Friday morning in Haiti, and last through Sunday. Rainfall amounts of 8 - 12 inches are possible, which will be capable of causing extreme flooding on the vegetation-denuded slopes of Haiti. It will be a major challenge to keep those Haitians living outside safe, if rainfall amounts of 5 - 10 inches occur.

Impact on Florida, Cuba, Jamaica, and the Bahamas

Florida, Cuba, Jamaica, and the Bahamas are all at high risk of receiving hurricane conditions from Isaac. The latest set of 06Z (2 am EDT) model runs for Isaac are fairly unified for the coming three days, showing a westward track to a point on the south coast of Hispaniola. All of the models then predict a more west-northwest track across the island and into eastern Cuba, as Isaac responds to a trough of low pressure over the Southeast U.S. Most of the models then predict a path for Isaac along the spine of Cuba, then into the Florida Straits off the coast of Miami by five days from now. A notable exception is our best-performing model, the ECMWF, which keeps Isaac just south of Cuba, and takes the storm more to the west between Jamaica and Cuba on Saturday, then into the Yucatan Channel between Mexico and Cuba by Monday. However, this model is keeping Isaac weaker than the other models, and thus predicts the storm will have a weaker response to the trough of low pressure over the Southeast U.S. If the official NHC intensity forecast is right and Isaac becomes a hurricane on Thursday, the more southerly track of the ECMWF is not going to verify, and Isaac will spend considerable time over Cuba on Saturday and Sunday. Where Isaac pops off the coast of Cuba will be critical in determining its future path and intensity. Some models predict a more easterly exit point, allowing Isaac to move up the east coast of Florida, and potentially make landfall in the Southeast U.S. The latest 06Z GFS model run predicts a more westerly track, which would potentially allow Isaac to move up the west coast of Florida towards Tampa. Keep in mind that the average error in a 5-day forecast is 260 miles. The two most recent runs of the GFS model, at 00Z and 06Z (8 pm and 2 am EDT), gave positions for Isaac that were about 250 miles apart--the earlier run putting the center near West Palm Beach, and the more recent run giving a location between Key West and Havana, Cuba. While passage over the high mountains of Hispaniola and then Cuba will substantially disrupt Isaac and probably reduce it below hurricane strength, the storm is quite large, and should be able to re-intensify once it emerges over the Florida Straits. Waters will be very warm, near 30°C, wind shear is predicted to be light, and forecasts of the upper-level winds show the possibility of an upper-level outflow pattern very favorable for intensification. If Isaac spends a day over water, that should be enough time for it to intensify into a Category 1 hurricane, and if the storm takes a longer 2-day track over water up either the east or west coast of Florida, a Category 2 or stronger storm is possible.

[Blown Away]

280 track from now on takes it just south of Hispaniola, but only just...290 takes it just north of the island.

Anything in between takes it through and I suspect a long term average it'll be inbetween that figure.

All it would take is a 75-100 mile shift north or a 50 mile shift south and Isaac skims or misses Hispaniola.

[KWT]

Does this look familiar to people:

http://img26.imageshack.us/img26/9180/230313w5nlsma.gif

Note the center of Irene was also a solid degree south of where Issac is, and that one made it north of Hispaniola.

It wouldn't shock me if the models shift northwards tonight, at least for the first 72hrs of the run.

[Aric Dunn]

it will be nice tomorrow. long range out of san juan should pick up on the center.

[ConvergenceZone]

Wow, the NHC just continues dropping down the expected strength of this... They seem to be forecasting a hurricane briefly and then dropping down to a tropical storm and then showing a hurricane briefly before it hits Florida...I guess the question
is, will it make it to a hurricane when it hits Florida? or will it still be a tropical storm....

[cycloneye]

Look at how Irene crossed Puerto Rico.

[KWT]

Wow, the NHC just continues dropping down the expected strength of this... They seem to be forecasting a hurricane briefly and then dropping down to a tropical storm and then showing a hurricane briefly before it hits Florida...I guess the question
is, will it make it to a hurricane when it hits Florida? or will it still be a tropical storm....

If it takes the perfect land track, it'll probably stay a TS, if it heads north or south of that 'perfect land track' then its got a solid shot of being a hurricane (from here the PLT would be 285 lifting to 290-295 once past Haiti then NW up Florida.)

[wxman76]

12Z GFS just started coming in. At 48 hours it has a slowly developing storm 50 or so miles south of hispanola, basically a 270 or 280 degree trajectory. keeps it south of Hispanola at 60 hours.

[CrazyC83]

Does this look familiar to people:

http://img26.imageshack.us/img26/9180/230313w5nlsma.gif

Note the center of Irene was also a solid degree south of where Issac is, and that one made it north of Hispaniola.

It wouldn't shock me if the models shift northwards tonight, at least for the first 72hrs of the run.

In the case of Irene, the first couple advisories had an incorrect center location and it relocated (or formed) well north of there. But there is a good point...

[KWT]

With a track like this one, even wobbles of say 30-40 miles to the north could mean the difference between a hurricae just offshore, or a weakening TS/TD over Haiti's mountions.

Even a 6-12hr stint of WNW motion (recon showed a 3hr motion around that earlier) pretty much makes it super unlikely this goes south of Haiti at this point.

We shall see though!!

[Ptarmigan]

Does this look familiar to people:

http://img26.imageshack.us/img26/9180/230313w5nlsma.gif

Note the center of Irene was also a solid degree south of where Issac is, and that one made it north of Hispaniola.

It wouldn't shock me if the models shift northwards tonight, at least for the first 72hrs of the run.

The forecast did not show Irene as a major hurricane. Shows that intensity forecast has a long way to go. That could get interesting for Isaac as it is the "I" storm.

[Aric Dunn]

looking at radar loops. appears the center is just east of Guadalupe. Slightly north of the forecast point.

[wxman57]

Satellite imagery and recon indicate that Isaac is really struggling with ENE wind shear and dry air into the core. There are no squalls within 50nm of the center. Soundings from the islands indicate a good bit of mid-level dry air that's likely being driven into Isaac's core by the persistent ENE wind shear. Good news for the Lesser Antilles is that any 45 mph sustained winds probably cover only a small area, and significant strengthening is unlikely before Isaac crosses into the eastern Caribbean.

I'm not really so sure Isaac will become a hurricane in the NE Caribbean. All other systems have struggled there this year, and I don't think the conditions there are so different than when Ernesto or Seven/Helene passed through. Down the road, I'm still not sure if the center will pass west, over, or east of the FL Peninsula. However, I am starting to doubt whether it will be a hurricane Monday morning.

[CourierPR]

Correct me if I'm wrong but Isaac seems to be developing better banding features.

[jdray]

I still think the EURO is a bit too far WEST and the GFS is a bit too far EAST. I think this RIDES along the WEST coast of FL or just into the EGOM and makes landfall near the big bend of FL. *moving North*

Yay, just what us in parts of north Florida need, more rain....

We are already getting close to the yearly normal amounts, and most of it has been in the past two months.
I've turned my sprinkler system off completely, and the water level in the pool never lowers unless I drain the rainfall addition.

[Aric Dunn]

its very possible there are multiple vorts spinning around each other right now.

[wxman76]

12Z GFS 114 hours West of NHC Just finishing crossing central Cuba. Little slower than NHC as well.

[ConvergenceZone]

Satellite imagery and recon indicate that Isaac is really struggling with ENE wind shear and dry air into the core. There are no squalls within 50nm of the center. Soundings from the islands indicate a good bit of mid-level dry air that's likely being driven into Isaac's core by the persistent ENE wind shear. Good news for the Lesser Antilles is that any 45 mph sustained winds probably cover only a small area, and significant strengthening is unlikely before Isaac crosses into the eastern Caribbean.

I'm not really so sure Isaac will become a hurricane in the NE Caribbean. All other systems have struggled there this year, and I don't think the conditions there are so different than when Ernesto or Seven/Helene passed through. Down the road, I'm still not sure if the center will pass west, over, or east of the FL Peninsula. However, I am starting to doubt whether it will be a hurricane Monday morning.

2012 is becoming is quickly becomming the most frustrating hurricane season
I can ever remember.