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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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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Quoting LSUCaneGirl:


Oh, okay thanks..I'm no expert :)


Me either... always learning here from the masters... and from the available resources...

Link

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The late night show begins for Isaac. Amazing how fast both circulations have blobbed together. NHC did a good job choosing the center one between the two earlier as their center. Amazing organization considering how far apart the two were just a few hours ago.
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00Z

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Quoting StormJunkie:
So no one can provide on piece of evidence that this is not an open wave at the moment?

So, those talking about RI, or even significant strengthening...Open waves of this size do not just go through RI. It's just the way it is. I will gladly listen to any evidence that supports otherwise.

This is a dangerous rain maker for many, but for now, that is all it is.
Looks to my eye like it's more of an open wave, but I imagine the NHC will stick with the long and/or elongated center bit, and or diffuse, or hard to pinpoint, or presently restructering, multiple vortices, etc., ....and keep it as TS Isaac at 11.
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Quoting MiamiHurricanes09:
Actually, based off of surface observations and Recon data it's the closest estimate you're going to get of the surface circulation. ;)

Little?

Uh-oh, I've said too much -- back to hiding.



LOL
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1695. WxLogic
DMAX should assist in deepening the system a bit more.
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Quoting ProgressivePulse:
Evening All.

At least on this map models have trended eastward this evening.

GFS ensembles tightened up alot.
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Quoting HrDelta:


And with his FEMA head, things won't be as bad.

Also, let's not talk about politics if we can avoid it. I will likely go into a flying rage, as the past year has made politics real personal for me.


AGREE... Isaac is the topic, not politics.
Member Since: September 18, 2007 Posts: 0 Comments: 1074
1692. Chiggy
Quoting NYCyclone86:


I will take a physical recon plane over a computer generated graphic any day of the week.

Those maps are pretty accurate - well the recon hasn't found anything conclusive so.....
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This is spot on like Alex of 2010 in my mind. Alex was massive and took forever to organize. But once he organized, he strengthened rapidly. I believe that Isaac will explode once he is organized. Also, another trend to note is that the longer he stays weak, the further west he will go.
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Quoting bajelayman2:
Just a weather alert to you guys up North, we in Barbados are on the very far edges of Isaac, but tonight we are getting a LOT of water, very heavy showers sporadically.

So, this storm a lot of water with it. Clsoer to the centre must be loaded with moisture.


Thanks! We're getting bands of wind and rain now and have been for the last couple of hours.

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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
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Quoting FLPandhandleJG:


Lol its snowing over the Lesser Antilles tonite.
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Quoting sunlinepr:


High cloud tops...


Oh, okay thanks..I'm no expert :)
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1686. pottery
Major Nasty weather coming in fast here from the S/W.
Loads of lightening, and loud grumbles.

Oh well, I'm off to the airport to meet a flight from North, scheduled for 11:10 pm..
Bet it was a bumpy one too !
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Member Since: September 18, 2007 Posts: 0 Comments: 1074
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


Follow recon VDM's...

Not going there, lol. The plots are east of where they were 4 hours ago. The population of east coast FL landfalls has increased.
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1683. WxLogic
Good Evening...
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Quoting WeathermanAG:

Isaac is taking shape.
I'd say 55 mph at 11pm.


Bright red AVN makes anything look good, the NOAA flight just found Isaac to be pretty shallow and neither recon or surface obs support a 55 mph storm IMO. Next advisory 45-50mph
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Quoting RTSplayer:


That's definitely not the center.

Don't know where you got that, but if anything, the center is south of the official fix, unless something was updated in like the past 10 minutes.
Actually, based off of surface observations and Recon data it's the closest estimate you're going to get of the surface circulation. ;)

Quoting MahFL:


Limbaugh is clearly a little insane.
Little?

Uh-oh, I've said too much -- back to hiding.
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Quoting ProgressivePulse:
Evening All.

At least on this map models have trended eastward this evening.




I dont see it. Earlier today, models were east of key west. Not so now.
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Quoting LSUCaneGirl:


What are the white specs?


High cloud tops...
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Let us check in on what Hurricane Irene was doing at this time a year ago.

HURRICANE IRENE SPECIAL DISCUSSION NUMBER 10
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092011
830 PM AST MON AUG 22 2011

THE HURRICANE HUNTER AIRCRAFT FOUND A 850-MB FLIGHT LEVEL WIND
MAXIMUM OF 106 KT ON ITS FIRST PASS THROUGH THE CENTER OF IRENE.
BASED ON THIS...THE INITIAL INTENSITY IS ADJUSTED UPWARD TO 85 KT.
OVER THE PAST COUPLE OF HOURS THE SATELLITE PRESENTATION OF THE
HURRICANE HAS CONTINUED TO IMPROVE...ALTHOUGH NO EYE IS VISIBLE YET
IN GEOSTATIONARY IMAGERY. HOWEVER...AN EYE HAS BECOME APPARENT ON
THE SAN JUAN DOPPLER RADAR AND WAS ALSO SEEN IN A 2230 UTC SSMIS
PASS. THE CENTRAL PRESSURE HAS FALLEN TO 981 MB BASED ON A
DROPSONDE OBSERVATION OF 982 MB WITH 10 KT OF WIND AT THE SURFACE.
GIVEN THAT THE ENVIRONMENT APPEARS CONDUCIVE FOR ADDITIONAL
STRENGTHENING...THE INTENSITY FORECAST HAS BEEN ADJUSTED UPWARD BY
15 KT THROUGH 36 HOURS WITH A SMALLER UPWARD ADJUSTMENT AT 48 AND
72 HOURS. ONLY SLIGHT WEAKENING IS INDICATED AT DAYS 4 AND 5. THE
NEW FORECAST NOW SHOWS IRENE BECOMING A MAJOR HURRICANE ON TUESDAY
AS IT MOVES THROUGH THE SOUTHEASTERN BAHAMAS AND THE TURKS AND
CAICOS ISLANDS. THE WIND RADII HAVE BEEN EXPANDED BASED ON AIRCRAFT
DATA AND THE NEW INTENSITY FORECAST.

NO CHANGES HAVE BEEN MADE TO THE TRACK FORECAST ON THIS SPECIAL
ADVISORY.

FORECAST POSITIONS AND MAX WINDS

INIT 23/0030Z 19.7N 68.7W 85 KT 100 MPH
12H 23/0600Z 20.1N 70.2W 95 KT 110 MPH
24H 23/1800Z 20.8N 72.5W 105 KT 120 MPH
36H 24/0600Z 21.5N 74.0W 110 KT 125 MPH
48H 24/1800Z 23.0N 75.5W 110 KT 125 MPH
72H 25/1800Z 26.0N 78.0W 110 KT 125 MPH
96H 26/1800Z 29.5N 79.0W 105 KT 120 MPH
120H 27/1800Z 34.0N 79.0W 95 KT 110 MPH...INLAND

$$
FORECASTER BRENNAN

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Quoting scooster67:
We got a night storm, meaning he likes the night life....the night time. If he keeps this up...........



He likes to boogie?
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1673. Grothar
Good banding. Tightening up.

Member Since: July 17, 2009 Posts: 71 Comments: 27125
Quoting saltydog1127:
I know it's always a crap shoot guys/gals... but give me ya'll's projects.... I'm in Pascagoula, MS...


10%.
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1670. 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.
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Member Since: August 15, 2008 Posts: 0 Comments: 2133
right now I'd say Isaac's LLCOC is near 15.2N 63.3W or somewhere close to that

Quoting ProgressivePulse:
Evening All.

At least on this map models have trended eastward this evening.



hmm because that is a slight shift S and W that is
Member Since: June 13, 2009 Posts: 2 Comments: 12712
Quoting WeathermanAG:

Isaac is taking shape.
I'd say 55 mph at 11pm.


Nope I believe they stop giving out 55 mph on storm its either 50 or 60, which really dont make sense.
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Yeah, Isaac is getting that "Beastly" look again. What a fickle storm.
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Quoting Levi32:
You know the circulation is a wreck when a radar view of the entire Antilles island chain doesn't help you find the center lol.
I see 4 small swirls rotating around a bigger vortex....lol
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I know it's always a crap shoot guys/gals... but give me ya'll's projects.... I'm in Pascagoula, MS...
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Quoting WeathermanAG:

Isaac is taking shape.
I'd say 55 mph at 11pm.



They don't do 55mph winds any more
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1662. Gearsts
Levi when do you see Isaac getting his candy together and strengthening?
Member Since: August 2, 2010 Posts: 0 Comments: 1993
1661. MahFL
Quoting saltydog1127:
Not sure if the blog has been updated...just jumped on for the first time in years... is it looking like a FL landfall as of now?


Come back in 2 days time, we might have an accurate answer.
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Quoting RTSplayer:


That's definitely not the center.

Don't know where you got that, but if anything, the center is south of the official fix, unless something was updated in like the past 10 minutes.

That was the position of the last VDM. I'll do another image just as soon as the new one comes out.
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Quoting RTSplayer:
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.
There is a product that the NHC puts out with the probabilities of TS and 50+ winds. It's found here:
Link
.
Enclosed within as far as the Caymans go are the following probability. Note it's not 0, and note the cumulative total over the next 5 days(1 in 10) and 50+ winds(1 in 33). So, it can happen, it's just a longshot. And we first of all sort of need a storm to hold together till it gets nearby.
.
.
GRAND CAYMAN 34 X X( X) X( X) X( X) 1( 1) 7( 8) 2(10)
GRAND CAYMAN 50 X X( X) X( X) X( X) X( X) 2( 2) 1( 3)
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Quoting StormJunkie:


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.


Its going to have to build its CDO which is the system is doing. It should have a well define CDO also note convection fired in the NE quad of the CDO which means the new COC is focusing on building the system eastern part of the the storm. Once that is realize the vorticity should move north and realign itself with the strengthening LLCOC. Once thats done which will take tonight and most of the mourning on Thursday it might just explode.
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Quoting sunlinepr:
Bolaven...

What a bola de viento....



What are the white specs?
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1656. Chiggy
This is where the LLC is - south of where NHC has it....
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1655. Grothar
Member Since: July 17, 2009 Posts: 71 Comments: 27125

Isaac is taking shape.
I'd say 55 mph at 11pm.
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Quoting TropicalAnalystwx13:
The box marks the center.



That's definitely not the center.

Don't know where you got that, but if anything, the center is south of the official fix, unless something was updated in like the past 10 minutes.
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I believe Isaac is going to show its true face sometime soon.. and its not going to be pretty.. IMO
Member Since: August 15, 2008 Posts: 0 Comments: 2133

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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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