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 trey33:


Some in south tampa (my old house - westshore fingers) were stuck in their homes and couldn't get out for 3 days because of Debby's flooding. So I can't imagine
how bad if the GFS track is correct.

I work downtown and my building is closed next week - so that is at least a positive. Downtown is partially evacuated of its workforce anyway for a week starting Saturday when the National Guard moves in.
Get out of town if it looks like Isaac is going to hit. As observers have been saying he is already a very wet storm, and is very large. Even a near pass could be disastrous. We'll open shelters here in Highlands County for Tampa peeps who need a safe haven.
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Quoting Gorty:


Also where they are located also helps to keep them warm...

What!!!!!
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1748. HrDelta
Quoting TropicalAnalystwx13:

Don't hype..


Agreed. Unless something special happens, this will not be Katrina-Class in damage, or (god-forbid) death toll.
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1747. TXCWC
Consensus is now saying FL Panhandle for landfall - BUT NOTE THERE ARE NOW MORE TRACKS POINTING EVEN FURTHER WEST - INCLUDING THE HWFI. If I was a betting person I would say another slight track shift West further off FL West Coast is coming and EURO tonight holds to landfall West of Panhandle.

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Quoting TropicalAnalystwx13:

What mess?


nevermind... had issues with the darn graphic.. I gave up
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Quoting wunderkidcayman:
right now I'd say Isaac's LLCOC is near 15.2N 63.3W or somewhere close to that



hmm because that is a slight shift S and W that is


Spot on I'd say..
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1744. GoWVU
Quoting StormJunkie:


Looks to be a big rainmaker for us. keep an eye on it.


Storm I will, heck today I fired up the generator just to make sure all is working well. rain I can deal with just no winds just put a new roof on the house last month, ugh
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1742. WxLogic
Quoting wpb:
did the noaa jet fly today?


No... the G-IV is expected to fly out tomorrow @ 1:30PM ET
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Quoting GoWVU:

Thanks!! I like toooooo hear that!!


Looks to be a big rainmaker for us. keep an eye on it.
Member Since: August 17, 2005 Posts: 26 Comments: 16541
1740. GoWVU
Quoting AussieStorm:


Can't answer that. you may have Isaac on your doorstep then again, you might not.

Thanks Aussie... I dont want that at all
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1738. HrDelta
Quoting congaline:
Thank you for talking about what MUST BE Thank you for letting us know that discussing the most important weather events ever in the history of man is not "off topic"


I think your computer might have been screwing with you there. I can agree with what you say though.
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Quoting scott39:
Isaac has high potiential to go into the GOM and become one for the ages.

Don't hype..
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1736. Gorty
Quoting AussieStorm:
Sydney is having it's warmest day since May. Currently 81.3°F and still rising. It's still meant to be winter down here. Tomorrow(Friday) is meant to be 66°F.


Also where they are located also helps to keep them warm...
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Quoting GoWVU:
Have a question for all of you guys, how safe are we here in South Carolina with Issac?


Can't answer that. you may have Isaac on your doorstep then again, you might not.
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Quoting HoustonTxGal:


What is this mess?




What mess?
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1732. scott39
Isaac has high potiential to go into the GOM and become one for the ages.
Member Since: June 13, 2009 Posts: 0 Comments: 6775
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1730. HrDelta
Quoting TropicalAnalystwx13:
10L not doing well at all.

23/0000 UTC 13.5N 38.4W T1.0/2.0 10L -- Atlantic


Issac is so damn big, it is shearing little TD10 to pieces.
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Quoting drg0dOwnCountry:


How the arctic ice melt affects extreme weather (including Hurricanes)!
Quoting drg0dOwnCountry:


How the arctic ice melt affects extreme weather (including Hurricanes)!
Thank you for letting us know that discussing the most important weather events ever in the history of man is not "off topic"
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From 18Z today


Quoting NYCyclone86:


That is from a night or two ago I believe.
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1727. trey33
Quoting SOHOGator:
As a souh Tampa resident less than 1/4 mile from the RNC I can deal with an offshore cat 1 but anything stronger is going to be bad bad news...


Some in south tampa (my old house - westshore fingers) were stuck in their homes and couldn't get out for 3 days because of Debby's flooding. So I can't imagine
how bad if the GFS track is correct.

I work downtown and my building is closed next week - so that is at least a positive. Downtown is partially evacuated of its workforce anyway for a week starting Saturday when the National Guard moves in.
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1726. Gearsts
Quoting sunlinepr:


Check

Link

16.2n?
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1725. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 171 Comments: 53827
1724. wpb
did the noaa jet fly today?
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1722. GoWVU
Quoting Seflhurricane:
you guys are looking good

Thanks!! I like toooooo hear that!!
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1721. HrDelta
Quoting louisianaboy444:


I thought he might lol


I still remember Hurricane Katrina & Hurricane Rita, and thus despise Accuweather.
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Quoting TropicalAnalystwx13:
10L not doing well at all.

23/0000 UTC 13.5N 38.4W T1.0/2.0 10L -- Atlantic



Eeeeeek why you iassce murder TD 10 iassce needs too go to jail for life
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Quoting Michfan:
Isaac will take about 24 hours to consolidate and line up its centers because it is so large. The after effect of that though is that once it does line itself up it will be hard to tear it apart because of its size. Its a catch 22.



Im with you on that. Not much to impede that consolidation right now.
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Quoting vince1:

Partisans will be partisans.

And wow, first thinkprogress and now mediamatters? What's next, MSNBC? ;)
no irene was already!!
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Quoting GoWVU:
Have a question for all of you guys, how safe are we here in South Carolina with Issac?
you guys are looking good
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Sydney is having it's warmest day since May. Currently 81.3°F and still rising. It's still meant to be winter down here. Tomorrow(Friday) is meant to be 66°F.
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What is this mess?



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Let's take a look back at where the list in terms of named storms was at in past years at 0000 UTC August 22:

1992 - TS Andrew (After unnamed STS One)
1993 - TS Cindy (disappated)
1994 - TS Chris (weakening)
1995 - TS Gabrielle (disappated)
1996 - TS Dolly (weakened from Cat 1)
1997 - Hurricane Danny (Cat 1, disappated)
1998 - TS Charley
1999 - Hurricane Cindy (Cat 1)
2000 - Hurricane Debby (Cat 1)
2001 - TS Chantal (post-landfall)
2002 - TS Christobal (disappated)
2003 - Hurricane Erika (Cat 1, disappated)
2004 - TS Earl (disappated)
2005 - Hurricane Irene (Cat 2, disappated)
2006 - TS Chris (disappated, After unnamed STS Two)
2007 - TS Erin (disappated)
2008 - TS Fay (after 1st FL landfall)
2009 - TS Claudette (disappated)
2010 - TS Colin (disappated)
2011 - TS Irene
2012 - TS Isaac

Incidentally, this is right around the time climatologically when Atlantic hurricane activity tends to pick up in earnest.
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1713. GoWVU
Have a question for all of you guys, how safe are we here in South Carolina with Issac?
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Quoting TropicalAnalystwx13:

That was the position of the last VDM. I'll do another image just as soon as the new one comes out.


Check

Link

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Here is the website for Venezuela's Met department.
Could someone skilled in Spanish see if they can find
data from Isla De Aves, on this site?

thanks

http://www.meteorologia.mil.ve/
Member Since: July 12, 2007 Posts: 0 Comments: 6032
10L not doing well at all.

23/0000 UTC 13.5N 38.4W T1.0/2.0 10L -- Atlantic
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1708. scott39
If you look at the Vorticity maps, Isaac does need to work on stacking itself. I do not believe that Isaac is an open wave. There is clearly a battle within itself.... between the NE and SW of the storm. It is a very elongated system....this will take time for the coc to tighten up. Have no doubt that Isaac will be a powerful force of nature.
Member Since: June 13, 2009 Posts: 0 Comments: 6775
Quoting allancalderini:
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.
1988 was too!! in fact that was the only active list 4 except 2000!!! this year probally could have 15 or more storms if this rate continues..the last active elnino was 2004 and 1969 was also active regarless of elnino!!
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18z GFS ENSEMBLE are going for a Hurricane going for S FL
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Best wishes to all the Florida folks on here. Stay safe and hopefully you all get thru the storm ok.
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The increased dvorak numbers are due to the enormous amount of green pixels...when a storm is in the ts stage, the dvorak picks up on, among many things, the intensity of convection in the CDO. Below the funktop image is the dvorak image which I believe is used in the estimates.





Find for yourself 15.7/62.2 which is the current NHC center position. We saw the two competing centers swirl together and collapse, and now they have, in my opinion, reformed in a broader center at 15.3/63.

Bottom line for me is that I honestly, in my greatest logical capacity, expect to see a strong TS tomorrow morning (5:00 update). And it will be strengthening.
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Quoting SFLWeatherman:
From Joe Bastardi GFDL on my path. Funny, how little that impresses me US hurricane models very rarely are in touch if euro not with em


I thought he might lol
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Levi,the trade winds in the Eastern Caribbean are as strong as when Ernesto moved thru?
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As a souh Tampa resident less than 1/4 mile from the RNC I can deal with an offshore cat 1 but anything stronger is going to be bad bad news...
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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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