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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Meanwhile in the North West Pacific basin...

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Not exactly beautiful outflow.
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I guess when we have a US threat. It brings out all the trolls. Not what we need right now. What we need is the correct information, not misleading or incorrect information.
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Quoting AussieStorm:

I always get those two mixed up. Why is English so hard.
Because people are making up new words every second...
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4346. snotly
Na, you'll be fine, I suggest you have the wedding on a large superficially constructed flotilla made out of duct tape, wire ties and bamboo, set sail for the Florida straits where it will be especially romantic this time of year and set anchor for the wedding of a life time... however short that may be...

DISCLAIMER: my lawyer is telling me that I should tell you that you should always consult with a professional before taking my advice, but what does he know.


Quoting Stormcow6:
I am hosting a wedding in my backyard Sunday afternoon in Loxahatchee, Fl. Should I reschedule the tent delivery. I am not sure what to do. Canceling this wedding is not an option but I am nervous that it might rain. Any suggestions....
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If/when Isaac gets truly vertical, convergence looks to be dynamic for strengthening. Not happening in the present though - all over the place.
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11am advisory should be out shortly
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Quoting bappit:

affected

Just a personal affliction. The effect of "effect" misuse affects me deeply.

I always get those two mixed up. Why is English so hard.
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4342. CJ5
Quoting popartpete:
Who else thinks that td10 is becoming aware of Issac, and it is rotating around it? Could this mean that 10 or Joyce would be sling shoted into the U.S East Coast ahead of Issac?


No.
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:)
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Quoting Gearsts:
Look at the low clouds where they are moving and tell me where the center is.Link
indeed looks like a WNW heading but i would like for recon to comfirm but looks WNW
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Quoting Bluestorm5:
Joyce now, Aussie :)

Yes it is.
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4335. bappit
Quoting AussieStorm:

To far away for a sling shot. It could be effected by Isaac's outflow causing shear on TD10.

affected

Just a personal affliction. The effect of "effect" misuse affects me deeply.
Member Since: May 18, 2006 Posts: 10 Comments: 5915
Full House for Isaac? Well if a Cat5 is a royal flush, Cat4 a straight flush, Cat3 four of a kind, Cat2 would be a full house. Isaac right now is like holding J7 offsuit.
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4333. Relix
Quoting TomballTXPride:


There is the gap. Its either moving WNW now or it will soon
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4332. MTWX
Unless Isaac picks up a good deal of strength over the next 12 hours or so, I'm going to have to go with the following odds:

West Coast/ Panhandle of Florida from Cape Coral to Panama City

60%

Panama City to Mobile Bay

35%

West of Mobile Bay

5%
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Quoting TomballTXPride:
Even the NHC has trouble with his name. I knew I wasn't the only one!!

000
WTNT34 KNHC 231252 CCA
TCPAT4

BULLETIN
TROPICAL STORM ISAAC
INTERMEDIATE ADVISORY NUMBER 9A...CORRECTED
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012
800 AM AST THU AUG 23 2012

CORRECTED SPELLING OF ISAAC IN HEADLINE

They spelt Isaac correctly.
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Quoting sunlinepr:

Holy jeebus. Bolaven is HUGE.
Member Since: August 3, 2008 Posts: 16 Comments: 5869
4329. GatorWX
Quoting Saltydogbwi1:


Here's where I see the rough center inside the broader spin


That's exactly my thought at the moment. However, we should all be aware at this point that there has been a mean center and multiple vorticies rotating around it. This latest "possible center" does appear to be becoming dominant and one can observe banding features developing and wrapping into it. I think this system will look a bit better in about six hours, but it is certain he has a long way to go. Outflow is terrific which should help him ramp up quicker than some people think once he's vertically stacked.
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Isaac looking healthy
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Quoting Bluestorm5:
You just joined few days ago... likely to troll on us. I ain't gonna chill just because you said so. What a dumb statement.
Quoting PensacolaDoug:






Yours is a pathetic post.

Please. Stop attacking people. Bluestorm5, would you like it if I said, "I joined 6 years ago, so I don't have to listen to you."?
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4326. Gearsts
Look at the low clouds where they are moving and tell me where the center is.Link
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4325. wpb
nws miami giving information no panic. just watch keep in touch with nhc info.
nhc forecaster in is 6am write wed out of line. go read it
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4324. scott39
The convection is impressive with Isaac, but he has a ways to go before a hurricane happens. He simply is not stacked.
Member Since: June 13, 2009 Posts: 0 Comments: 6728
Quoting AussieStorm:

To far away for a sling shot. It could be effected by Isaac's outflow causing shear on TD10.
Joyce now, Aussie :)
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Hmm... the farther west Isaac goes, the farther west "future" Joyce could go. The ULL by her does not seem to be as pronounced as before too. Don't count Joyce out, she might get close to the east coast.
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Quoting RTSplayer:
Who else has 13 weather related tabs open in multiple browsers?!

LOL.


Haha so true!
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We OFFICIALLY have TS Joyce. 10-3-0 so far for 2012.

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Quoting Elena85Vet:


+1

Totally irresponsible journalism created for nothing but the 'WOW' effect.


They are smoking somethinf there in MSN here in San Juan is sunny.
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max mayfield also say ISAAC is starting to become better organized but ahs work to do and the satelite presentation is its becoming come symetrical
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Quoting popartpete:
Who else thinks that td10 is becoming aware of Issac, and it is rotating around it? Could this mean that 10 or Joyce would be sling shoted into the U.S East Coast ahead of Issac?

To far away for a sling shot. It could be effected by Isaac's outflow causing shear on TD10.
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The heaviest ball of convection is doing a loop-de-loop at the moment in the S-SE quadrant and the coc is probably outrunning the convection somewhere to the NE. The Hunters are going to have a tough time figuring out exactly where the dominant coc is located.
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4313. FOREX
Quoting StormTracker2K:
The blog will go nuts come later today as Issac shows his face and once he does it will be one ugly face!


I'm beginning to think that this will stay a weak Tropical storm and dissipate after crossing Cuba.
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Quoting Bobbyweather:

Don't ignore even the newest users.
I'm not ignoring him, but I ain't listening to him.
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max mayfield just did an update on local10 that officials in the florida keys are likely to begin evacuations of visitors and tourists tomorrow and that a desicion would be made tomorrow morning
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Quoting Relix:


Hahaha wow! There's even sun over my house right now :P


MSN Smoke something, here still sunny in San Juan.
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Quoting Bluestorm5:
*look at join date* And I'm supposed to listen to you? *SMH*

Don't ignore even the newest users.
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Quoting StormTracker2K:
Issac has a Full House right now.

Well, I have 3 A and 2 K which would beat anything he has.
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Quoting AussieStorm:


The only people I am worried about right now is the people of Haiti. Will anyone be game to tell them there is a TS/Possible Hurricane on it's way. I wouldn't, it could start mass panic.


There are some protocols in place to move people to safer ground....they aren't very good and aren't gonna be very effective...but at least there are some efforts being made...
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GOES East RSO visible loop
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4302. yoboi
Quoting Tribucanes:
Haitians already know it's coming. Tent camp leaders have been preparing to take as many people as possible out of the tents into hurricane sustainable buildings. Problem is, there is not enough buildings for the some 400,000.


i think the US military is helping with stronger portable shelters also....
Member Since: August 25, 2010 Posts: 7 Comments: 2325

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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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