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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1351. hydrus
Quoting AussieStorm:
Soon Isaac will have plenty of energy to feed on.



and this is what's still to come. A pass just south of Haiti could be really bad for down the road with so much OHC.

Yes..If this storm continued west unimpeded by the mountainous terrain into that super heated water, then took the turn north into the gulf, it would be almost a mathematical certainty that a cat-4, and possibly a cat-5 would present itself...and I am not doom-casting by any means.
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This is much farther south of Puerto Rico than was anticipated seems like


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The parochial prognostications on this forum are starting to rival Storm2K.
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1348. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 39162
1347. snotly
-90C for cloud tops?

Quoting hurricanehunter27:
These cloud tops are crazy. Coldest of the year in ATL.

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Trough appears somewhat weaker today, especially the southern extent:









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Quoting RTSplayer:


That's colder than -80C and very close to -90C.


Really that cold hmmm...
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Quoting tropicfreak:


Can you say... Dennis?



Look at how fast Dennis intensified! Wow!
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no TS yes from TD 10


AL, 10, 2012082300, , BEST, 0, 135N, 389W, 30, 1007, TD,



grrrrr
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1342. snotly
Z. A Cat 200, winds create space-time wormhole singularity that destroys the universe

Quoting AstroHurricane001:
OK, time for the most-hated quiz of all time:

How strong will Isaac be at peak on the SSHS?

A. TS
B. Cat 1
C. Cat 2
D. Cat 3
E. Cat 4
F. Cat 5

I'm going for C, cat 2 with 90 kt winds, 961 mb about 36.8 nm S of Clearwater, Florida.
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1341. pottery
Quoting Floodman:


And here I was talking about reprobates a minute ago...Pottery, how have you been keeping? This one has to be causing you some concern

That's some kind of legal document, right ? :)::

"Concerned" is the word.
2 deaths last w/e from flooding, and this one looks like it could be bad again.

Have to leave here in an hour to get to the airport for 11:00.
Grrrr....
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1340. DDR
Quoting pottery:

Cant access the local radar.
Hearing some thunder to the south.
Hope it does not get too heavy.
Seems like a lot more to come though, looking at the loops.

DMAX and all......

We'll probably get the usual,a few gusty winds,couple inches of rain,if you ask me.I'm using the Barbados radar,our radar has been offline for over a year.
Member Since: April 27, 2007 Posts: 14 Comments: 1701
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UW - CIMSS
ADVANCED DVORAK TECHNIQUE
ADT-Version 8.1.3
Tropical Cyclone Intensity Algorithm

----- Current Analysis -----
Date : 22 AUG 2012 Time : 234500 UTC
Lat : 16:05:31 N Lon : 61:56:03 W


CI# /Pressure/ Vmax
3.5 / 994.9mb/ 55.0kt


Final T# Adj T# Raw T#
3.0 3.3 3.3

Center Temp : -76.5C Cloud Region Temp : -64.2C

Scene Type : UNIFORM CDO CLOUD REGION

Positioning Method : FORECAST INTERPOLATION

Ocean Basin : ATLANTIC
Dvorak CI > MSLP Conversion Used : ATLANTIC

Tno/CI Rules : Constraint Limits : NO LIMIT
Weakening Flag : ON
Rapid Dissipation Flag : FLAG

C/K/Z MSLP Estimate Inputs :
- Average 34 knot radii : 105km
- Environmental MSLP : 1012mb

Satellite Viewing Angle : 24.1 degrees
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Recon found many, many readings below 1000 millibars...why did the NHC go with 1004 millibars?
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Quoting Chiggy:
Look how far south the 500mb vorticity is from the LLC



Thats should refocus further north.
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1334. HrDelta
Quoting MahFL:
I have a feeling Isaac will just plow over the islands and hardly notice them.


With it's huge size, I wonder if it might do that.
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Definitely some intense convection:

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Quoting tropicfreak:


Can you say... Dennis?


That is comparable, especially the angle it goes after going be Jamaica. If it bombed out as a cat 4 back in July, think what it might be able to do in August now.
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hope.no.boats.are.under.that.deep.convection
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1330. Chiggy
Quoting wunderkidcayman:
interesting the Climo model CLP5 00Z interesting track I think this kinda track may play out I don't buying it hitting Hispaniola and cuba the way the others are

if this current southerly movement continues it will be interesting to see what happens

by the way lets find out when the next recon flys



I am saying passes just south of Jamaica... Anybody agree on this!?
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1329. LargoFl
Quoting MAweatherboy1:
The 0z dynamic model suite was very similar to 18z...
18z:



..see there are several in that run coming into florida..im not taking my eyes off this storm til its way past florida
Member Since: August 6, 2011 Posts: 4 Comments: 39162
1328. HrDelta
Quoting AstroHurricane001:
OK, time for the most-hated quiz of all time:

How strong will Isaac be at peak on the SSHS?

A. TS
B. Cat 1
C. Cat 2
D. Cat 3
E. Cat 4
F. Cat 5

I'm going for C, cat 2 with 90 kt winds, 961 mb about 36.8 nm S of Clearwater, Florida.


D

Cat 3, 115-120mph, 950s for pressure, scrapes along Florida West Coast, making landfall in the Southern Big Bend area, or just to the South. Like Dixie or Levy Counties.
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Quoting hurricanehunter27:
These cloud tops are crazy. Coldest of the year in ATL.



guess rapid intensification flag will be ON. where's KEEPEROFTHEGATE?
Member Since: August 22, 2006 Posts: 2 Comments: 1983
1326. Grothar
Quoting icmoore:


Ugh Gro don't you have anything better to show me than that, oops that could be a loaded question :) Shift? Which way and how much? :)


I haven't heard that in a long time.
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Quoting MahFL:
I have a feeling Isaac will just plow over the islands and hardly notice them.


Maybe. I mean how much can mountains really rip a system apart that isn't stacked and has zero structure to its core? Matter of fact, I honestly question if it may be an open wave at this point.
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1324. pottery
Quoting DDR:

Hi pottery
The radar is most disturbing,heavy rain is currently falling in south-south east and north-eastern areas.

Cant access the local radar.
Hearing some thunder to the south.
Hope it does not get too heavy.
Seems like a lot more to come though, looking at the loops.

DMAX and all......
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Quoting hurricanehunter27:
This cloud tops are crazy. Coldest of the year in ATL.



That's colder than -80C and very close to -90C.
Member Since: January 25, 2012 Posts: 33 Comments: 1520
slight wind shift found way down where the noaa plane is now
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interesting the Climo model CLP5 00Z interesting track I think this kinda track may play out I don't buying it hitting Hispaniola and cuba the way the others are

if this current southerly movement continues it will be interesting to see what happens

by the way lets find out when the next recon flys

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Quoting tropicfreak:


Can you say... Dennis?


Doubt it...
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Quoting RitaEvac:
Showing E Gulf now



Euro and many ensembles of other models showing central to Eastern gulf anyhwere from Louisiana to Florida...long way from over for anyone I dont care if many of my forecasting buddies think I fell off the rocker :)...I guess this weary soul has just seen one too many major track shifts in the 4 to 5 day out period
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1317. icmoore
Quoting Grothar:
Look what just came out. You think maybe Florida will be out of the cone? By the way, expect a model shift at 11:00.




Ugh Gro don't you have anything better to show me than that, oops that could be a loaded question :) Shift? Which way and how much? :)
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Quoting AstroHurricane001:
OK, time for the most-hated quiz of all time:

How strong will Isaac be at peak on the SSHS?

A. TS
B. Cat 1
C. Cat 2
D. Cat 3
E. Cat 4
F. Cat 5

I'm going for C, cat 2 with 90 kt winds, 961 mb about 36.8 nm S of Clearwater, Florida.

D
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1315. LargoFl
Quoting hydrus:
If the LBAR path came to fruition, would be almost a worse case scenario for S.W.Florida.. Here is the NAM 84 hours..
..yes this really needs to be watched very carefully, suppose both the GFS and the other one going west..are BOTH wrong?..we could be in big trouble huh
Member Since: August 6, 2011 Posts: 4 Comments: 39162
Quoting RitaEvac:
Showing E Gulf now



Can you say... Dennis?

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Quoting AstroHurricane001:
OK, time for the most-hated quiz of all time:

How strong will Isaac be at peak on the SSHS?

A. TS
B. Cat 1
C. Cat 2
D. Cat 3
E. Cat 4
F. Cat 5

I'm going for C, cat 2 with 90 kt winds, 961 mb about 36.8 nm S of Clearwater, Florida.


Gatito 4

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Quoting pottery:

I feel so very reassured at this comment. Thanks.

heheheheheh


And here I was talking about reprobates a minute ago...Pottery, how have you been keeping? This one has to be causing you some concern
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The 0z dynamic model suite was very similar to 18z...
18z:



0z:
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1310. DDR
Hopefully it doesn't getting to bad tonight.Yea the area to our SW is packing.
Member Since: April 27, 2007 Posts: 14 Comments: 1701
1309. MahFL
I have a feeling Isaac will just plow over the islands and hardly notice them.
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Looks like Taiwan might be in for a double-dose of fun...

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All the focus has really been on Issac. I imagine it's because of the RNC. I really have never seen such politically charged weather coverage. My question is: where will Issac go after it exits Florida. Does it go up the East coast in some fashion? Also, I see TD10, probably Joyce sooner or later. Does this system have the potential to hit the U.S.? I've seen this idea be given the yadda, yadda, yadda by mets, saying that it will go out to sea. But I've seen the GFS and and European, and it seems that these models don't recurve it. What does anybody think? Thanks!!!
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1305. Chiggy
Look how far south the 500mb vorticity is from the LLC

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1304. LargoFl
Quoting ILikeIke:
Retire the S name so next time it can be Spock lol
LOL I say the same thing
Member Since: August 6, 2011 Posts: 4 Comments: 39162
1303. hydrus
Quoting LargoFl:
..on this run the GDFL has the same track also....we in florida ought to be very careful, this storm Could fool us
If the LBAR path came to fruition, would be almost a worse case scenario for S.W.Florida.. Here is the NAM 84 hours..
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These cloud tops are crazy. Coldest of the year in ATL.

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Quoting hunkerdown:

But as MH09 said, that should occur by Friday night, or at least know as it will be happening at that time...so I do agree with him. Once we see the shape he's in and what is the result from land interaction, we will also know the strength/position of the trough and condition of the ridge which will give a pretty solid track/forecast.
Oh man, look who it is. I haven't seen you around in way too long.
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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.