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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3051. 7544
Quoting cchsweatherman:
The latest Vortex data message has confirmed that the center has indeed reformed quite a bit further south than the analyzed center at 2AM EST. The latest Vortex data message shows the center to be at 14.7N and 63.5W. This means a nearly 60 mile southward shift with the center of Tropical Storm Isaac which likely now makes an immense difference in the future track of the storm.


so would that put south fl out of the pic
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Said last night I was with the Euro - and (south shift)developments since then have added weight to the Euro model's track forecast.

Can't see Isaac clipping west Haiti given his forward speed and somewhwt elongated east-west presentation.

Question now is, will he wreck on Cuba...or shoot the Yucatan channel?
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Have the West vs East Casters shown up yet? Or the Texas vs Florida Crowd........No that was Debby,Right? History has a way of repeating itself. I have a feeling this is going to build to a crescendo.
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3048. emguy
Hmmm...so many interesting developments tonight.

1.) EURO has tracked further east in the Gulf once again...for the 3rd run or so in a row. This is trending...expect a track in the eastern Gulf that is even a little further east on the next run.
2.) Probable center relocation south of 15N is very regrettable news. Irregardless of track, the outcome in Hispanola is relatively the same. Heavy rain and bad mudslides...good news for them would be no hurricane. Track wise, the impacts are significant...as this storm would now need to gain significant lattitude in order to get tangled up in the mountains of Hispanola and Eastern Cuba.
3.) Future impacts...well, the debate of mountainous interraction on motion may go out the window if this is relocated near 14.7...as the impacts would angle more towards the Central Cuba area. In result, a much stronger storm.
4.) Otherwise, the stronger storm would still be expansive, so it would be a gradual turner making the North bend into the weakness over central Cuba instead, just stronger. If trends continue, then the final outcome from there...legit concern that a more significant hurricane may be in the makings for the West Coast of the Florida Penninsula and/or the Florida Big Bend.

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The latest Vortex data message has confirmed that the center has indeed reformed quite a bit further south than the analyzed center at 2AM EST. The latest Vortex data message shows the center to be at 14.7N and 63.5W. This means a nearly 60 mile southward shift with the center of Tropical Storm Isaac which likely now makes an immense difference in the future track of the storm.
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Quoting FLPandhandleJG:
the last few frames looks like Isaac is getting cripple up in the northern part of the storm.. losing some steam for now.. I think its just another minor setback..
Now I know why storms don't get it together in the Eastern Caribbean.
Member Since: August 31, 2009 Posts: 0 Comments: 5628
the last few frames looks like Isaac is getting cripple up in the northern part of the storm.. losing some steam for now.. I think its just another minor setback..
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Quoting gustavcane:
yep looks like us on the northern gulf coast better be ready if Isaac's
center is south of 15n. The rest of the models will shift more west in
line with the ECMWF Model on the next runs.


ECMWF has shifted to Pensacola, which isn't that far from the GFS when one does a homogeneous comparison of the two.
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Quoting GTcooliebai:
If the center is at 14.7N then no, in fact none of the reliable models did.


Exactly
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Quoting Kowaliga:


I just wanna retire and sell fish bait in the Everglades.......off the books of course!!!



Now that's living the dream!!!!!!!
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Quoting HimacaneBrees:
Did the Euro Model initialize in the right spot on this latest run?
If the center is at 14.7N then no, in fact none of the reliable models did.
Member Since: August 31, 2009 Posts: 0 Comments: 5628
A bunch of little things could change the final path of thousands of miles of coastlines and strength could be changed by entire categories.

Just saying.
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Did the Euro Model initialize in the right spot on this latest run?
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Quoting HimacaneBrees:


My jobs not bad, I just really don't wanna be here lol. If it wasn't for the money I really don't think I would be here lol.
Quoting HimacaneBrees:


My jobs not bad, I just really don't wanna be here lol. If it wasn't for the money I really don't think I would be here lol.


I just wanna retire and sell fish bait in the Everglades.......off the books of course!!!
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yep looks like us on the northern gulf coast better be ready if Isaac's
center is south of 15n. The rest of the models will shift more west in
line with the ECMWF Model on the next runs.
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3036. Dunkman
Quoting Tribucanes:
We're under an hour from knowing exactly where the COC is.


I don't think there is a definitive COC. It's just a big mess...calling it a TS is probably generous.
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Quoting Grothar:


So we weren't seeing things? This could change a lot of the models.


yep looks like us on the northern gulf coast better be ready if Isaac's center is south of 15n. The rest of the models will shift more west in line with the ECMWF Model on the next runs.

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Quoting KoritheMan:


A little, but the convection still lacks banding.


I know Im just saying its looking a little better than it was before .. ya know!
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Quoting KoritheMan:


This guy's off today. :P



Cool. I get off at 6AM.
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Quoting FLPandhandleJG:




Looks like TD 10 trying to organized.. any thoughts??


A little, but the convection still lacks banding.
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Quoting GTcooliebai:
Should become TS Joyce.


Should be sometime later today if it keeps it up..
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3030. 7544
wso no south fl storm with this new center?
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Quoting FLPandhandleJG:




Looks like TD 10 trying to organized.. any thoughts??
Should become TS Joyce.
Member Since: August 31, 2009 Posts: 0 Comments: 5628
ok guys I just did some calculations with the current forecast plots and I brought it down to current location and ploted new forecast plots using same degree of movements and track takes it S of Haiti clips a bit of cuba near 20N 77W then back out on water off of cuba's S coast cross central Cuba pass W of the Keys and looks like LA/AL heading that is with the first center fix waiting on #2
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Quoting stormchaser19:


The Dilemma of Debby? and if i were to bet definitely would bet for GFS
I think it comes down to timing, the trough will be there, where will the storm be at in longitude, that's the question.
Member Since: August 31, 2009 Posts: 0 Comments: 5628




Looks like TD 10 trying to organized.. any thoughts??
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Quoting louisianaboy444:


Lol one day you will realize I actually know what I am talking about


I never said you didn't. :P
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Quoting FLWaterFront:
It is going to really shake things up in the world of hurricane computer forecast modeling if it turns out that the Euro has been right all along with this one.

Earlier this season it was the GFS that was insisting that TS Debby was going to head for the NE GOM and impact Florida while every other major model was forecasting an entirely different solution. Now it is the ECMWF that has been insisting upon a major striking the Central Gulf Coast while all the other models have been tightly clustered around the Florida peninsula for days.

Too early to say that the Euro is right and everyone else is wrong, and it still may be that the Euro is a bogus outlier after all. But somehow, one gets this uneasy feeling that this may not be the case. We'll see.
The thing is though it has been shifting east for the past 3 runs while the GFS has been shifting west for the past 3 runs so somewhere in the middle they're meeting up. First is first we need a well defined center to track and all these shifts north and south are probably throwing these computer models off and they fail to initialize at the right spot. After that will it impact Haiti or stay far enough away to not have any problems with the mountainous terrain. We still have plenty of time to watch this if we are in FL. or along the Gulf Coast and expect models to continue to shift one way or another.
Member Since: August 31, 2009 Posts: 0 Comments: 5628
Quoting Kowaliga:


Amen to that!....that's why I took the day off!


My jobs not bad, I just really don't wanna be here lol. If it wasn't for the money I really don't think I would be here lol.
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3022. Bitmap7
Fresh 850mb vort map:




Some white finally appearing in there. Depicted center seems almost in line with RECON.
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Quoting HimacaneBrees:
work sucks. just sayin'


This guy's off today. :P
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Quoting KoritheMan:


If it stays weak, then that's very possible. weatherguy03 even mentioned that earlier, and he's a well-respected meteorologist amongst this community.


Lol one day you will realize I actually know what I am talking about
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Quoting HimacaneBrees:
work sucks. just sayin'Quoting HimacaneBrees:
work sucks. just sayin'


Amen to that!....that's why I took the day off!
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With the way things are going, I have this nagging feeling that Isaac will explode within the next 12-36 hours. This is going to be very bad news for anybody, this guy has the potential to be the next "I" name to be retired.
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We're under an hour from knowing exactly where the COC is.
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Quoting louisianaboy444:


I am starting to think that this may go south of the Greater Antilles if current trends continue
Don't be too surprised if there continues to be wsw travel for another day or two.This storm has been doing that for several days,taking jogs to the wsw but averaging to be west.
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Pulling in dry air from the NE.......I dunno...

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work sucks. just sayin'
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Quoting RussianWinter:
Sigh, I swear, there is MORE THAN ONE CENTER!!!!!

THIS FACT ALONE CHANGES EVERYTHING IF YOU THINK THERE IS THIS ONE CENTER SOUTH OF 15N.


GFS is predicting this exactly just looks the 850 vorticity of FSU page
Link
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Quoting Dunkman:
Euro hits Pensacola next Wednesday. It certainly seems that the Euro and GFS are coming together.
yes the eruo has moved way more than the gfs! also there is more than one center currently.
Member Since: August 4, 2010 Posts: 0 Comments: 572
It is going to really shake things up in the world of hurricane computer forecast modeling if it turns out that the Euro has been right all along with this one.

Earlier this season it was the GFS that was insisting that TS Debby was going to head for the NE GOM and impact Florida while every other major model was forecasting an entirely different solution. Now it is the ECMWF that has been insisting upon a major striking the Central Gulf Coast while all the other models have been tightly clustered around the Florida peninsula for days.

Too early to say that the Euro is right and everyone else is wrong, and it still may be that the Euro is a bogus outlier after all. But somehow, one gets this uneasy feeling that this may not be the case. We'll see.
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Quoting AussieStorm:
How much longer will Isaac continue to travel west??? The more west he goes the more chance he has of shooting the gap between Cuba and Yucatan and into the GOM.


Anything is possible Aussie.. Storms do have a few tricks here and there.. we will c tho in the next few days..
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3009. Dunkman
Euro hits Pensacola at 168h. It certainly seems that the Euro and GFS are coming together.
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Quoting RussianWinter:
Sigh, I swear, there is MORE THAN ONE CENTER!!!!!

THIS FACT ALONE CHANGES EVERYTHING IF YOU THINK THERE IS THIS ONE CENTER SOUTH OF 15N.
Eventually, one predominates and right now it is the second reformation to occur farther south that holds sway.
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Quoting louisianaboy444:


I am starting to think that this may go south of the Greater Antilles if current trends continue


If it stays weak, then that's very possible. weatherguy03 even mentioned that earlier, and he's a well-respected meteorologist amongst this community.
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How much longer will Isaac continue to travel west??? The more west he goes the more chance he has of shooting the gap between Cuba and Yucatan and into the GOM.
Member Since: September 30, 2007 Posts: 9 Comments: 15977
Wrap up is all but complete now with Isaac. What an amazingly impressive last eight hours for this system. If this organization trend continues we'll be looking at a 75-90mph storm by tomorrow afternoon. Friday may may bring a cat2 very close to Haiti. NHC will be upping the mph potential of Isaac by morning.
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Quoting GTcooliebai:
So the question is does it hug the south coast of Cuba, or go over Haiti and hug the north coast of Cuba like the GFS?


The Dilemma of Debby? and if i were to bet definitely would bet for GFS
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Quoting HimacaneBrees:


Yeah but so far with TS Isaac folks have called it the outlier. I think it may be a little closer to what is happening though. I'm no expert and I don't know anything about forecasting. But I think this may be MS,AL,FL panhandle lanfall. Just call it a gut feeling.
Yep, EURO has some good data.
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Sigh, I swear, there is MORE THAN ONE CENTER!!!!!

THIS FACT ALONE CHANGES EVERYTHING IF YOU THINK THERE IS THIS ONE CENTER SOUTH OF 15N.
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Quoting KoritheMan:


The truth lies somewhere in between.

Seriously, I don't know anymore.


I am starting to think that this may go south of the Greater Antilles if current trends continue
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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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