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 MAweatherboy1:
New forecast cone didn't change much. Either they're not putting much stock in the center reformation or they're waiting for a little more data.


Or a center reformation (in this particular case) is not as relevant to long term track.
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WV Loop dee Loop

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looking at visible images the center of isaac looks to be just east of dominica
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Quoting MAweatherboy1:
New forecast cone didn't change much. Either they're not putting much stock in the center reformation or they're waiting for a little more data.


I think they are waiting for more data.
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Quoting Relix:
So, guys with more knowledge, is is safe to say that PR won't be impacted by the strongest winds anymore right?


You know better. The North will be less vulnerable than the South.
Member Since: August 22, 2006 Posts: 2 Comments: 1975
I wonder how much this will change tonight?

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Almost a 100 comments in 20 minutes...
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As some already pointed out yesterday, Isaac's development cycle is very similar to monsoon cyclones in the western Pacific, right down even to the lack of convection on the north side, which, by the way, is still not because of dry air as keeps being said.
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New forecast cone didn't change much. Either they're not putting much stock in the center reformation or they're waiting for a little more data.
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Quoting SLU:
000
WTNT44 KNHC 222039
TCDAT4

TROPICAL STORM ISAAC DISCUSSION NUMBER 7
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012
500 PM AST WED AUG 22 2012

SATELLITE...AIRCRAFT...RADAR...AND SURFACE OBSERVATIONS INDICATE
THAT ISAAC HAS A BROAD CENTRAL AREA OF LIGHT WINDS WITH MULTIPLE
VORTICITY MAXIMA EMBEDDED. THE LOWEST PRESSURES ARE IN AN AREA
NEAR GUADELOUPE...WHICH IS USED AS THE CURRENT CENTER SINCE IT HAS
SOME CONTINUITY WITH THE CENTER PREVIOUSLY BEING TRACKED.
HOWEVER...THE MEAN CENTER IS LIKELY TO THE SOUTHEAST OF THE
ADVISORY POSITION. SOME ADJUSTMENT OF THE CENTER POSITION MAY BE
NECESSARY WHEN THE CENTRAL CORE AGAIN CONSOLIDATES.
THE INITIAL
INTENSITY IS HELD AT 40 KT...ALTHOUGH THE HURRICANE HUNTER AIRCRAFT
DATA SUGGESTS THIS COULD BE GENEROUS.

Euro may have been on to something all along...Well good afternoon folks.I hope everyone has their plans/kits ready in case you know who tries to make a visit.
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Quoting CybrTeddy:


No, they where recorded at 10,000 feet.

Also, as pointed out on the other blog.. the ECMWF is developing another storm at by 144 hours, here is it by 240

you forgot to refresh. that is an old run.
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Dust is almost gone around Isaac.
Member Since: September 27, 2007 Posts: 1 Comments: 20528
Cheaper?

I can assure you no plans for RECON or other is EVER based on funds.

The NHC has announced ALL available Tools, personnel and resources are now called up for this event.

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Quoting ITSNOTGONNAHAPPEN:
ABSOLUTLY YES YES YES IVE SAID FLORIDA WONT GET ANYTHING FOR THE PAST 4 DAYS NOW


Jfv?
Member Since: August 22, 2006 Posts: 2 Comments: 1975
So, guys with more knowledge, is is safe to say that PR won't be impacted by the strongest winds anymore right?
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Quoting wunderkidcayman:

the plane has not been used for a while I don't think they are and plane is is riding at 10,000 ft too



I dissagree with at location for Isaac

I geuss we will have to wait for HH RECON and NOAA RECON to find a good center 8pm advisory should have the proper LLCOC fix if they could find it

right now Isaac kinda reminds me of the big monssonal systems with multiple LLCOC

Did you read the full advisory, if so then it is explained.
HOWEVER...THE MEAN CENTER IS LIKELY TO THE SOUTHEAST OF THE
ADVISORY POSITION. SOME ADJUSTMENT OF THE CENTER POSITION MAY BE
NECESSARY WHEN THE CENTRAL CORE AGAIN CONSOLIDATES.
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It would help if Antilles Radar site would actually show about 2 hours worth of frames, and on 5 minute intervals, instead of 15 minute intervals.

For some areas, you can't even figure out what the relative motion is on that crappy internet site.
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No shift west...
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Yeah you better not get married if your worried about the furniture. It's that dam honeydew list.
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Quoting chevycanes:
SATELLITE...AIRCRAFT...RADAR...AND SURFACE OBSERVATIONS INDICATE
THAT ISAAC HAS A BROAD CENTRAL AREA OF LIGHT WINDS WITH MULTIPLE
VORTICITY MAXIMA EMBEDDED. THE LOWEST PRESSURES ARE IN AN AREA
NEAR GUADELOUPE...WHICH IS USED AS THE CURRENT CENTER SINCE IT HAS
SOME CONTINUITY WITH THE CENTER PREVIOUSLY BEING TRACKED.
HOWEVER...THE MEAN CENTER IS LIKELY TO THE SOUTHEAST OF THE
ADVISORY POSITION. SOME ADJUSTMENT OF THE CENTER POSITION MAY BE
NECESSARY WHEN THE CENTRAL CORE AGAIN CONSOLIDATES. THE INITIAL
INTENSITY IS HELD AT 40 KT...ALTHOUGH THE HURRICANE HUNTER AIRCRAFT
DATA SUGGESTS THIS COULD BE GENEROUS.


So 45mph is being too Generous by the NHC.
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Quoting Levi32:


It's like a new husband and wife scuffling over how to arrange the furniture in their new house.
levi looking at the computer models they are in an agremment in TS isaac affecting southern florida either as a direct hit or a close call, but the euro remain to the west and has been consistent but the GFS has also been rather consistent also. do you believe its very very likely that Southern florida is going to be visited by Isaac , looking at all the info i have avaliable i think its going to be a very very close call
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Quoting floridaboy14:
do you guys think at the 8pm advisory the center will be moved further south?

No. 11pm advisory is more likely. Will take a bit for the centers to sort themselves out.
Member Since: August 3, 2008 Posts: 16 Comments: 5875
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Quoting moonlightcowboy:


You've never been married have you, Levi? That's a no-brainer, the wife wins! ;)

I don't know why he thinks it stops at some point either....

Still, Hot, Humid, and Unpleasant here at 11n 61w.
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73. SLU
000
WTNT44 KNHC 222039
TCDAT4

TROPICAL STORM ISAAC DISCUSSION NUMBER 7
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012
500 PM AST WED AUG 22 2012

SATELLITE...AIRCRAFT...RADAR...AND SURFACE OBSERVATIONS INDICATE
THAT ISAAC HAS A BROAD CENTRAL AREA OF LIGHT WINDS WITH MULTIPLE
VORTICITY MAXIMA EMBEDDED. THE LOWEST PRESSURES ARE IN AN AREA
NEAR GUADELOUPE...WHICH IS USED AS THE CURRENT CENTER SINCE IT HAS
SOME CONTINUITY WITH THE CENTER PREVIOUSLY BEING TRACKED.
HOWEVER...THE MEAN CENTER IS LIKELY TO THE SOUTHEAST OF THE
ADVISORY POSITION. SOME ADJUSTMENT OF THE CENTER POSITION MAY BE
NECESSARY WHEN THE CENTRAL CORE AGAIN CONSOLIDATES.
THE INITIAL
INTENSITY IS HELD AT 40 KT...ALTHOUGH THE HURRICANE HUNTER AIRCRAFT
DATA SUGGESTS THIS COULD BE GENEROUS.

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Quoting VAbeachhurricanes:
Are those Kermit reading legit??? 999.8??

the plane has not been used for a while I don't think they are and plane is is riding at 10,000 ft too

Quoting wxchaser97:
...CENTER OF ISAAC MOVING THROUGH THE LEEWARD ISLANDS...NEW WARNINGS AND WATCHES ISSUED...
------------------------------------------------- -------------------------------

5:00 PM AST Wed Aug 22
Location: 16.0°N 61.2°W
Moving: W at 22 mph
Min pressure: 1004 mb
Max sustained: 45 mp


...TROPICAL DEPRESSION OVER THE EASTERN TROPICAL ATLANTIC HAS CHANGED LITTLE IN STRENGTH...
------------------------------------------------- -------------------------------

5:00 PM AST Wed Aug 22
Location: 13.1°N 37.4°W
Moving: WNW at 16 mph
Min pressure: 1007 mb
Max sustained: 35 mph



I dissagree with at location for Isaac

I geuss we will have to wait for HH RECON and NOAA RECON to find a good center 8pm advisory should have the proper LLCOC fix if they could find it

right now Isaac kinda reminds me of the big monssonal systems with multiple LLCOC
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do you guys think at the 8pm advisory the center will be moved further south?
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Quoting jeffs713:

So based on analogs, Issac will hit somewhere between Honduras and Nova Scotia.

Well, that narrows it down.

big improvement on Panama to Portugal forecast of a few days ago:)
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Thanks, Doc. So the center may be a little Southwest. I thought I was seeing things. Every was saying North.
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So the NHC still thinks TD 10 will become Joyce. Does anyone else agree?
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Quoting tropicalmommie:


I am in Key West and those darn sugar ants have been driving me crazy for a couple days now. They won't go away!

Stop putting sugar on the floor, then.
Member Since: August 3, 2008 Posts: 16 Comments: 5875
Quoting ITSNOTGONNAHAPPEN:
wow isnt that what ive been saying for 4 days now more west west west florida gets nothing




WOW look what i this did POOF you go a long with your other names
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Quoting waterskiman:
Hey what are the bugs doing at your place. In the upper keys and the sugar ants are going nuts like they just did a shot of boochie


I am in Key West and those darn sugar ants have been driving me crazy for a couple days now. They won't go away!
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Quoting moonlightcowboy:


You've never been married have you, Levi? That's a no-brainer, the wife wins! ;)


It boils down to this in marriage...

"Would you rather be right, or be happy?"
Member Since: August 3, 2008 Posts: 16 Comments: 5875
Quoting Thrawst:


That's TD-10 is it not?



no that is not TD 10 the recon cantfly out that far
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SATELLITE...AIRCRAFT...RADAR...AND SURFACE OBSERVATIONS INDICATE
THAT ISAAC HAS A BROAD CENTRAL AREA OF LIGHT WINDS WITH MULTIPLE
VORTICITY MAXIMA EMBEDDED. THE LOWEST PRESSURES ARE IN AN AREA
NEAR GUADELOUPE...WHICH IS USED AS THE CURRENT CENTER SINCE IT HAS
SOME CONTINUITY WITH THE CENTER PREVIOUSLY BEING TRACKED.
HOWEVER...THE MEAN CENTER IS LIKELY TO THE SOUTHEAST OF THE
ADVISORY POSITION. SOME ADJUSTMENT OF THE CENTER POSITION MAY BE
NECESSARY WHEN THE CENTRAL CORE AGAIN CONSOLIDATES. THE INITIAL
INTENSITY IS HELD AT 40 KT...ALTHOUGH THE HURRICANE HUNTER AIRCRAFT
DATA SUGGESTS THIS COULD BE GENEROUS.
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Quoting scott39:
The exact opposite.... the GFS is coming into line with the EURO.


I'm, the last GFS I saw showed Florida, like it always had. Did I miss something?
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Quoting DavidHOUTX:
THE NATIONAL HURRICANE CENTER HAS REQUESTED THE INITIATION OF
SPECIAL 6 HOURLY SOUNDINGS STARTING AT 18Z THU AUG 23 FROM WFO
SAN JUAN AND ALL OFFICES WITHIN THE FOLLOWING STATES:
FLORIDA...GEORGIA...ALABAMA...AND MISSISSIPPI.
HANDEL/SDM/NCO/NCEP

Link

Smart. (and cheaper than the G-IV in the GOM)
Member Since: August 3, 2008 Posts: 16 Comments: 5875
Quoting CybrTeddy:


No, they where recorded at 10,000 feet.

Also, as pointed out on the other blog.. the ECMWF is developing another storm at by 144 hours, here is it by 240


That's TD-10 is it not?
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Tropical Storm ISAAC
...CENTER OF ISAAC MOVING THROUGH THE LEEWARD ISLANDS...NEW WARNINGS AND WATCHES ISSUED...
5:00 PM AST Wed Aug 22
Location: 16.0°N 61.2°W
Moving: W at 22 mph
Min pressure: 1004 mb
Max sustained: 45 mph
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Quoting Levi32:


It's like a new husband and wife tussling over how to arrange the furniture in their new house.


You've never been married have you, Levi? That's a no-brainer, the wife wins! ;)
Member Since: July 9, 2006 Posts: 184 Comments: 29594
Nice work blog. Seems the NHC agrees that the real center is farther SE as we expected.
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THE NATIONAL HURRICANE CENTER HAS REQUESTED THE INITIATION OF
SPECIAL 6 HOURLY SOUNDINGS STARTING AT 18Z THU AUG 23 FROM WFO
SAN JUAN AND ALL OFFICES WITHIN THE FOLLOWING STATES:
FLORIDA...GEORGIA...ALABAMA...AND MISSISSIPPI.
HANDEL/SDM/NCO/NCEP

Link
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This wont be a Major hurricane until it is in the GOM for a little spell.,,,
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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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