Isaac reorganizing as it blows through the Lesser Antilles

By: Dr. Jeff Masters , 8:34 PM GMT on August 22, 2012

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Tropical Storm Isaac continues to maintain top winds of just 45 mph as its center prepares to move through the Lesser Antilles islands late this afternoon and early this evening. The entire Lesser Antilles chain of islands is receiving heavy rains from Isaac, with Martinique picking up 1.46" of rain as of 2 pm EDT, and St. Lucia receiving 1.49". However, Isaac is not yet generating much in the way of tropical storm-force winds, and none of the islands had received winds in excess of 30 mph as of 4 pm EDT. During their storm penetration to obtain their 2 pm EDT center fix, an Air Force Reserve aircraft measured top surface winds of just 40 mph, and a central pressure of 1004 mb. Top winds at their 5000 foot flight altitude were 49 mph. Isaac is undergoing significant changes to its structure. The plane found the center had become a broad, elongated oval that extended 40 miles from NW to SE. The old center, fixed at 2 pm near 16.1°N, and closer to the dry air to Isaac's north, is being challenged for dominance by a new center that is attempting to form near a burst of heavy thunderstorms at 15.5°N. The Hurricane Hunters' latest fix at 3:50 pm EDT put the center near 15.9°N, a southwards shift of about 17 miles. The resulting battle between centers is giving Isaac a rather odd spiral rectangular shape, as seen on visible satellite loops. The Hurricane Hunters 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 Isaac will not be able to begin strengthening until it resolves the battle between it two competing centers, and casts out the dry air infiltrating it. This is going to take at least a day, since Isaac is a very large storm, and it takes more time to spin up a big chunk of the atmosphere. Radar imagery from Barbados and Martinique show plenty of heavy rain showers, mostly on the south side of Isaac where it is moister. There has been a modest increase in spiral banding since this morning.


Figure 1. Afternoon satellite image of Isaac, showing its odd spiral rectangle shape.

Latest model runs for Isaac
The latest set of 12Z (8 am EDT) model runs are very similar to the previous set of runs, which I discussed in detail in this morning's post. The models show a westward track to a point on the south coast of Hispaniola. Isaac's center shift to the south may require some modest adjustments to the south and west for the models. This would result in the storm spending a few hours less time over Hispaniola, and more time over or just south of Cuba. This would slightly decrease the risk to the Dominican Republic, the east coast of Florida, and the Bahamas, but increase the risk to the west coast of Florida. The ECMWF--our best performing model over the past two years--continues to be an outlier among the models. It predicts that Isaac will track just south of Cuba, cross the western tip of Cuba on Monday, then head north towards an eventual landfall in Louisiana. 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, and at this point, we don't know it its more likely that Isaac will go up the east coast of Florida, the west coast, or straight up the peninsula over land. At this point, I'd put the odds at:

40% chance of a track through the Gulf of Mexico, west of Florida
25% chance of a landfall in South Florida, and a track mostly over the Florida Peninsula
35% chance of a track along the east coast of Florida

Which model should you trust?
Wunderground provides a web page with computer model forecasts for many of the best-performing models used to predict hurricane tracks. So which is the best? The best forecasts are made by combining the forecasts from three or more models into a "consensus" forecast. Over the past decade, NHC has greatly improved their forecasts by relying on consensus forecast models made using various combinations of the GFS, GFDL, NOGAPS, UKMET, HWRF, and ECMWF models. If you average together the track forecasts from these models, the NHC official forecast will rarely depart much from it, and the NHC forecast has been hard to beat over the past few years. The single best-performing model over the past two years has been the ECMWF (European Center model). The GFS model has been a close second. You can view 7-day ECMWF and 16-day GFS forecasts on our wundermap with the model layer turned on. Ten-day ECMWF forecasts are available from the ECMWF web site. The European Center does not permit public display of tropical storm positions from their hurricane tracking module of their model, so we are unable to put ECMWF forecasts on our computer model forecast page that plots positions from the other major models. As seen in Figure 2, the HWRF and GFDL were well behind the ECMWF and GFS in forecast accuracy in 2011, but were still respectable. The BAMM model did very well at 4 and 5-day forecasts. The UKMET, NOGAPS, and CMC models did quite poorly compared to the ECMWF, GFS, GFDL, and HWRF. For those interested in learning more about the models, NOAA has a great training video (updated for 2011.)


Figure 2. Skill of computer model forecasts of Atlantic named storms 2011, compared to a "no skill" model called "CLIPER5" that uses just climatology and persistence to make a hurricane track forecast (persistence=a storm will tend to keep going in the direction it's current going.) OFCL=Official NHC forecast; GFS=Global Forecast System model; GFDL=Geophysical Fluid Dynamic Laboratory model; HWRF=Hurricane Weather Research Forecasting model; NOGAPS=Navy Operational Global Prediction System model; UKMET=United Kingdom Met Office model; ECMWF=European Center for Medium Range Weather Forecasting model; TVCA=one of the consensus models that lends together several of the above models; CMC=Canadian Meteorological Center (GEM) model; BAMM=Beta Advection Model (Medium depth.) Image credit: National Hurricane Center 2011 verification report.

I'll have a new post in the morning.

Jeff Masters

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Quoting NYCyclone86:


That is more than enough to be influenced by the upper air pattern. The only thing I can think of is possibly land friction causing him to hug the coast and end up in the central gulf... But models do not really have a grasp on the friction factor so I really do not agree with it.


Do you know that for sure? Do you have proof? By the time Isaac reaches the weakness in the ridge, he should be a tropical storm again. NOAA will be sampling the upper pattern starting tomorrow and we will know for sure how strong the ridge will be.

Just because you think Gulf bloggers are wishing this their way doesn't mean there isn't a big chance of it making it into the Gulf. In fact, that seems to be the consensus at the moment. NHC has the correct track for now.
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Quoting GTcooliebai:
Convection flaring up again and white dots on the Rainbow.



its doing its best to consolidate an elongated center. can you see how it looks like it jumps from one part to the other, like a slinky going down a stair, like a worm from tremors?

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1497. pottery
Quoting Levi32:
Really interesting. Dropsonde due west of St. Lucia just now reported northeast winds at flight-level as the plane has been observing, which implies no closed circulation at flight level, but as soon as the sonde drops 5000 feet the winds switch to westerly as they should be. One heck of a vertically misaligned circulation.


Good catch there.
A storm of Perplexity.
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its like the llc over 5000 is 200 miles wide while a the surface its about 50....this is one crazy storm, kinda pacific like except its not blowing up into a cat 5 like the pacific would have already done
Member Since: August 4, 2010 Posts: 0 Comments: 572
1495. SLU
Quoting pottery:

Thanks.
Rain won't bother me. I live on a bump.
The feeder bands are often more destructive that anything else here.


Oh great. I hope it doesn't rain too much.
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Quoting TropicalAnalystwx13:

Uhm...recon?


Recon did not show a closed low. I watched the entire time they were in there, anyone else that did should have seen the same thing.
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Quoting allancalderini:
I have seen that td 10 and td 2 are the two numbers of depression that almost always fail to become a name storm.
Well, I wouldn't say "almost always fail", though where TD10s are concerned, only four of the past eight have failed to mature:

hurricanes
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developmental.stage..favorite
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1490. yoboi
Quoting Tazmanian:



TD 10 better get name


i hate TD 10s evere year this about do not get named



it will get named.......
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Quoting JLPR2:
Using Google Earth's ruler, I got that Isaac is 509miles from N to S and 850miles in its widest area from E to W.


Go see if you can find out the size of Typhoon Bolaven. it's HUUUUUUGGGGEEEEE.
Member Since: September 30, 2007 Posts: 9 Comments: 15962
1488. scott39
Quoting StormJunkie:


Not at all, I am not asking them to declassify it, nor would I if I worked at the NHC. I completely agree that declassification at this time would be imprudent. I also think there is a chance it could pull together more over time. This is more of a strictly scientific debate on its current state and I would be interested in hearing expert opinions on.
The NHC are the experts.
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I must say it is quite surprising that we are already on our 10th cyclone of the 2012 season. On this exact date in 2010, we were still only by number 6, and on this date in 2005, the 11th tropical cyclone just developed. Not bad at all so far for what was supposed to be just an "average" season.
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Quoting CHANGESALOT:
will the east central coast of florida still getting anything from issac? or it issac moving to far away now?
all of florida will receive very very heavy rain and super bennificial for lake okeechobee it appears southern florida the florida keys and west central florida will have the worst of the weather
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1485. icmoore
Quoting LargoFl:
where he goes, no one knows

#1445
This (white line) looks like it would be awful for the Tampa area with the east side being worse right?
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Quoting StormJunkie:
So no one can provide on piece of evidence that this is not an open wave at the moment?

So, those talking about RI, or even significant strengthening...Open waves of this size do not just go through RI. It's just the way it is. I will gladly listen to any evidence that supports otherwise.
I hope your joking. In now way shape or form is Issac a open wave. All that has happened is a center relocation that happens all the time in weak systems. Not to mention Isaac is really tilted. That being said it appears that Isaac has consolidated. Have to wait for HH pass (Next 20 min) to confirm exact details. Plus NHC say TS not an open wave and that is really all you need.
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997. Very true, took my kayak down bayshore blvd after debbie. Be prepared...
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1482. scott39
Quoting StormJunkie:
So no one can provide on piece of evidence that this is not an open wave at the moment?

So, those talking about RI, or even significant strengthening...Open waves of this size do not just go through RI. It's just the way it is. I will gladly listen to any evidence that supports otherwise.

This is a dangerous rain maker for many, but for now, that is all it is.
Are you refering to Isaac?
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Quoting NYCyclone86:


You think they will classify this with it being on top of the islands and threatening more land? Not a smart idea.


Not at all, I am not asking them to declassify it, nor would I if I worked at the NHC. I completely agree that declassification at this time would be imprudent. I also think there is a chance it could pull together more over time. This is more of a strictly scientific debate on its current state and I would be interested in hearing expert opinions on.
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1480. guygee
Latest GFS pressure fields. Biggest difference from last run has system weaker and less organized south of Hispaniola. No big track differences.

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Quoting StormJunkie:
So no one can provide on piece of evidence that this is not an open wave at the moment?

So, those talking about RI, or even significant strengthening...Open waves of this size do not just go through RI. It's just the way it is. I will gladly listen to any evidence that supports otherwise.

This is a dangerous rain maker for many, but for now, that is all it is.

Uhm...recon?
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1478. hydrus
Quoting MAweatherboy1:
The blob on the right ate/is eating the blob on the left:

It almost looks like the center is just south of 15 degrees north.
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will the east central coast of florida still getting anything from issac? or it issac moving to far away now?
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1476. klew136
Quoting Levi32:
I still think the Euro is more likely to be bogus. The last time I can remember a storm running parallel along Hispaniola and Cuba, then continuing the shortcut track into Louisiana, was Georges in 1998, and it took this strong of a ridge to keep him from recurving into the Florida area:





Here the ridge is going to be broken (GFS 84 hour):


Levi I was here in the Keys for Georges and it was not very nice. What I am looking at now is the system is going to be right on top of the Keys as it marches thru the Straits or makes a curve hopefully
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1475. Levi32
Really interesting. Dropsonde due west of St. Lucia just now reported northeast winds at flight-level as the plane has been observing, which implies no closed circulation at flight level, but as soon as the sonde drops 5000 feet the winds switch to westerly as they should be. One heck of a vertically misaligned circulation.

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This is not a wish casting statement. The center is relocating further north and the intensity is under done, the waters are very warm, the storm is slowing down and finally the shear will be lower to the W/NW below Eastern Cuba, any takers?
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Quoting 1900hurricane:

And in 2005. That one is probably the most famous 10L because of its role in the development of 12L/Katrina that year.
Yeah... but td 10 is almost always curse to not be name.
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1472. Drakoen
Quoting StormJunkie:
So no one can provide on piece of evidence that this is not an open wave at the moment?

So, those talking about RI, or even significant strengthening...Open waves of this size do not just go through RI. It's just the way it is. I will gladly listen to any evidence that supports otherwise.


Bunch of westerlies being reported across the southern islands. The circulations is being strung out due to the speed of the trades in the eastern Caribbean.
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Quoting hydrus:
That looks more impressive and tropical storm force winds extend 140 miles out.

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1470. LargoFl
Quoting hydrus:
its got a nice spin on it but it does not look so good
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Quoting GetReal:
There is a distinct weakness being shown over MS/AL/GA on the HWRF.
Member Since: August 31, 2009 Posts: 0 Comments: 5628
1468. pottery
Quoting SLU:


Batten down the hatches. Beware of the potent tail of Isaac.

Thanks.
Rain won't bother me. I live on a bump.
The feeder bands are often more destructive that anything else here.
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Quoting hurricanehunter27:
This convection keeps growing and getting cooler. U can see a little white pixel now.


Just one or two tiny white pixels but they are there. Isaac is trying to organize/ wrap-up now.
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Quoting StormJunkie:
So no one can provide on piece of evidence that this is not an open wave at the moment?

So, those talking about RI, or even significant strengthening...Open waves of this size do not just go through RI. It's just the way it is. I will gladly listen to any evidence that supports otherwise.


You think they will classify this with it being on top of the islands and threatening more land? Not a smart idea.
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1464. Dakster
Quoting aislinnpaps:
I have figured out the solution of the two blobs of Issac. It's simple and keeps the majority of people happy. The northern blob splits off and goes to Florida, the southern one goes into the Gulf to keep everyone trying to figure out where...


And td10 and the blob behind it can take care of the carolinas and northeast us...
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The blob on the right ate/is eating the blob on the left:

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Quoting allancalderini:
I agree last year didn`t get name either nor 2007.

And in 2005. That one is probably the most famous 10L because of its role in the development of 12L/Katrina that year.
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Quoting MississippiWx:


Georges was also a major hurricane before he started interacting with the islands. Isaac should not be more than a Cat 1 before making landfall or land interaction with an island.


That is more than enough to be influenced by the upper air pattern. The only thing I can think of is possibly land friction causing him to hug the coast and end up in the central gulf... But models do not really have a grasp on the friction factor so I really do not agree with it.
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So no one can provide on piece of evidence that this is not an open wave at the moment?

So, those talking about RI, or even significant strengthening...Open waves of this size do not just go through RI. It's just the way it is. I will gladly listen to any evidence that supports otherwise.

This is a dangerous rain maker for many, but for now, that is all it is.
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1459. JLPR2
I'm guessing the Pacman looking area west of the gap between Guadeloupe and Dominica could be very close to the center.

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Convection flaring up again and white dots on the Rainbow.

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1457. pottery
Looks like we are about to get some real heavy rain coming in from the SW.
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Quoting hurricanehunter27:
This convection keeps growing and getting cooler. U can see a little white pixel now.


This would be a great time for a polar orbiter pass so we could really see it in great detail.
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According to the OFFICIAL NATIONAL HURRICANE CENTER! track as of 8:00PM,Isaac is not going to get closer to the Cayman Island!!! at all.
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1454. hydrus
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1453. luigi18
Quoting serialteg:


georges was a good one. last cat2 to really hit PR. last real good cane we've had.

Good ???It was Horrible!
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Quoting hurricanehunter27:
This convection keeps growing and getting cooler. U can see a little white pixel now.



best outflow to the north yet! hopefully will continue! becoming symetrical at least with the outflow
Member Since: August 4, 2010 Posts: 0 Comments: 572
Quoting pottery:

Yeah, I am watching that too.
Still rumbling down south.... you must be seeing the flashes.
Try RAdar doppler, is working for San Juan, but the center is not showing yet..if there is one that is..
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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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