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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1601. Gearsts
Quoting Tropicsweatherpr:


So far no power outage here in Santurce with those bands moving thru.
I was with no power for 1 hour here in Aguadilla lol
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Quoting Tropicsweatherpr:


So far no power outage here in Santurce with those bands moving thru.


so quiet and tranquil in ponce :( (((((

even in south storms

im moving from here
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Quoting allancalderini:
Unlikely but at the pace we are going there is a possibility.
Nothing will surprise me anymore...
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1598. poikoo
Quoting yoboi:



and
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Quoting Thing342:
Am I the only one who thinks it is rather ironic that when Isaac first formed, it was well organized but lacked convection, but now it has very strong convection and is a hot mess of a storm?


Not at all, I am right there with you.
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1596. trey33
Quoting LargoFl:
supposedly tomorrow is supposed to be drier, about 40% chance of rain if i remember right


thx :)
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Quoting sunlinepr:
Bolaven...

What a bola de viento....



has some of those white things that isaac has
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1594. pottery
Quoting bajelayman2:

Looks heavy on Rainbow!
Stay dry !
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1593. poikoo
Quoting LargoFl:
ok this is looking real Bad again for us
always does!: i'm in largo too!:) oh and thank's for the images!
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1591. LargoFl
TREY......
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Quoting JLPR2:
I know this is probably wrong, but there is a 54mph wind barb from the WNW just at the coast of Africa related to the TW that is emerging from the continent.



the black man's way of trolling the West
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Quoting TropicalAnalystwx13:
SAB was revised. Here are the new numbers. T3.0 support an initial intensity of 50 mph, although recon shows winds near 45 mph.

22/2345 UTC 15.6N 62.0W T3.0/3.0 ISAAC -- Atlantic
45kts from TAFB as well. Irrelevant with Recon in there though.

AL, 09, 201208222345, 10, DVTS, CI, , 1560N, 6210W, , 3, 45, 2, 1000, 2, DVRK, , , , , , , , , , , , , , L, TAFB, GR, I, 5, 3030 /////, , , GOES13, CSC, T,
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Bolaven...

What a bola de viento....

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Quoting washingtonian115:
Yes this "list" is known to accommodate inactive hurricane seasons with El nino usually.Might be it's year...
yeah like the one use in 2009 they are usually the 2 inactive list of the Atlantic. 2003 was the active year for that list and maybe 2012 will be the active for this one.
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Quoting Thing342:
Am I the only one who thinks it is rather ironic that when Isaac first formed, it was well organized but lacked convection, but now it has very strong convection and is a hot mess of a storm?


maybe alanis morisette also thinks its ironic... dont you think?
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1585. LargoFl
Quoting trey33:
Largo:

Not sure if you are here tonight, but do you have any updates on tonight's rain? Found a roof leak ... and wonder if we are done for awhile (days?). Thx.

Would love a day of sunshine.....
supposedly tomorrow is supposed to be drier, about 40% chance of rain if i remember right
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Looking from a distance.

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How long till the next plane takes off.
Mission 3 is heading back to St Croix and Mission 4 is still investigating. Mission 5 is when?
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1582. JLPR2
I know this is probably wrong, but there is a 54mph wind barb from the WNW just at the coast of Africa related to the TW that is emerging from the continent.

Member Since: September 4, 2007 Posts: 7 Comments: 8747
1581. pottery
Quoting presslord:


Are you well stocked with rum?!

Oh, Surely !

Have to go to the airport in a little while, so my stock has not DMIN'd today.
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1580. scott39
Quoting congaline:
http://mediamatters.org/video/2012/08/22/limbaugh -o bama-is-hopeful-hurricane-will-hit-ta/189503

Please check out this link. Rush Limbaugh says that Hurricane Isaac hitting Tampa is a plot by the President and the NHC. Would be funny if it weren't so pathetic.
That has to be a joke ...right??
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Quoting Gearsts:


So far no power outage here in Santurce with those bands moving thru.
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Quoting KEEPEROFTHEGATE:
T.C.F.W.
09L/TS/I/CX
MARK
15.49N/63.45W


you see, i just don't know keep... seems to me like it's more to the NE
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1577. SLU
Quoting bajelayman2:


The only other TS this large I can remember entering the Caribbean was TS Jose in 1999 which eventually tightened up as it became a hurricane.

But Isaac is really amazing to maintain this large size almost all the way across the Atlantic in the face of dry air at times.
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Am I the only one who thinks it is rather ironic that when Isaac first formed, it was well organized but lacked convection, but now it has very strong convection and is a hot mess of a storm?
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Quoting LargoFl:
...does anybody think the impossible..that this does NOT pull together and somewhere down the line..falls apart?


And in response to scott as well.

No, this system is not going anywhere. It will be around in some way shape or form. The real question is whether it will ever be able to work out the massive amount of decoupling. It very well could remain a hot wet mess its entire life.

So we all agree that we have a very, very decoupled system and it will have to work that problem out before any sort of significant intensification could even be a possibility. That was sort of my point in going through the open wave debate.
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1574. trey33
Quoting icmoore:


Would not be good for the barrier islands here like the one I live on.


I think most of downtown (read convention area) is at or near sea level. I used to live on Tampa Bay and we were at 8ft above, and were higher than all the neighbors!
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Quoting pottery:

Looks like the worst is past you, except for an area just to your south.

Hi Pottery, from the visible yes, but right now we are getting lathered, moreso than today,

Maybe some stray bands, but boy...some strong showers.
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Quoting washingtonian115:
Be funny if this season also had 19 named storms XD.
Unlikely but at the pace we are going there is a possibility.
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Quoting KoritheMan:


And when you consider it's "that list".
Yes this "list" is known to accommodate inactive hurricane seasons with El nino usually.Might be it's year...
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1570. Chiggy
Quoting SLU:


Yes. Isaac is highly decoupled. That mid-level circulation passed between St. Lucia and Martinique around midday while the LLC just past Guadeloupe.

CIMSS vorticity maps clearly shows the lack of alignment at 850, 700 and 500 my levels
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1569. pottery
Quoting scott39:
Isaac is in its toddler phase. Thia isnt a system that is going to go away any time soon!

Agree.
It's still a HUGE system, which some here seem to have forgotten, from looking at it too long.

Looking at the cloud-top temps is pretty alarming too.
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1568. Gearsts
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1566. scott39
Quoting KoritheMan:


Who's being arrogant?
You are! :) J/K Some of these strong opinionated comments can seem that way sometimes. Hell I am sometimes. Lol Thats why I said thier is a fine line.
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1565. Grothar
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Quoting Seflhurricane:
drak does it look like the center is starting to consolodate, someone in the last post mentions that the midlevel center and the llc are not aligned because of the trade winds ???


wind shear is light
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1562. yoboi
Quoting poikoo:
really! all you people are for the most part trolls! who cares what you have to say! i miss stormw! and drakoen, and all the rest:(



and
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1561. LargoFl
ok this is looking real Bad again for us
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Further south?
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Who thinks Issac will break the trough over Florida and head into the eastern an Central Gulf, any ta
kers?
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Quoting CHANGESALOT:
could daytona beach have 20mph or more sustained winds from isaac or just some rain>


Didn't you say earlier that it wouldn't affect Florida at all and that it will continue to shift west?
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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


I think there are two factors that will come in to play that will determine if Issac is going to move north on the eastern edge of the NHC cone.

1) how far east will the high over Texas move and will the reinforcing trough dig far enough south ?
and
2) how close will TD 10 (Joyce) get to Isaac's east when Isaac begins crossing the islands ?
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People need to learn how to read NHC materials.

1, The forecast cone is based on "present knowledge" and is not set in stone.

2, The meaning of the forecast cone is the cone which is 2/3 probability of the storm being "somewhere" in that cone at the specified times.

This means that on average for any given storm, there is about 1/6th chance of the storm being outside to the left and a 1/6th chance of the storm being outside to the right, since the center line should be the area with greatest probability.


So in reality, there is about a 16% chance of Isaac passing directly over or west of Grand Cayman, based on the forecast cone's real meaning.
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1554. trey33
Largo:

Not sure if you are here tonight, but do you have any updates on tonight's rain? Found a roof leak ... and wonder if we are done for awhile (days?). Thx.

Would love a day of sunshine.....
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1553. zoutil
Hère on Guadeloupe : no Rain no wind on isaac
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Quoting sunlinepr:
Looks a bit like ying and yang
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Quoting stormpetrol:


Look , I know the NHC are the experts, I give them heads up , they get it right the vast majority of the time and I'm just a layman"not even that" but if I had followed the track back in 2004 when Ivan devastated us, I'd been in deep trouble,even more than I've suffered! " Experience is the greatest of all schools , so that even the fools will learn", I will not turn my back on Isaac until he is well North of us(Cayman Islands)either way we will experience probably TS conditions from him, even if not a direct hit!!!


storm's been right a couple of times
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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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