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 Levi32:
Hey though, pressure was rising for a long while on approach to the wind center. That means the pressure center is farther north and east, so this "center" is not a well-defined anchor for the system yet. Look for fishy stuff northeast of that position.

I'm out for a while, possibly for the night. Later.

Have a good night Levi, thanks for staying up with us.
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Quoting Levi32:
Ah. Bingo. 14.75N.


Very much displaced from the convection. Still a long way to go with this one.

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Pressures are higher in that area lowest I've seen so far is a 1005.6 mb.
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Center: 14.7 N 63.5 W

Should change the models a bit.
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2946. Levi32
Hey though, pressure was rising for a long while on approach to the wind center. That means the pressure center is farther north and east, so this "center" is not a well-defined anchor for the system yet. Look for fishy stuff northeast of that position.

I'm out for a while, possibly for the night. Later.

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ha ha NHC should have waited looks like RECON has found the LLCOC location
14.7167N 63.4833W
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2944. KEEPEROFTHEGATE (Mod)
before i start getting all weird
iam off to bed
see how it looks at 8 am
later all
goodnight


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I guess now we find out how many other swirls there are in there.
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2942. HrDelta
Quoting Levi32:
Ah. Bingo. 14.75N.



I thought it might be south, but not THAT far south.
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2941. 7544
Quoting AllStar17:
Center is about 80 miles S of where the NHC had it at 11 pm.


so now for gom
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2am plot is a geusstimate because it is surely not at 15.5N heck it not even at 15N I guess NHC just put that there untill RECON has got the fix
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Quoting louisianaboy444:


+1 for Trevor ;)


I'm not afraid to admit it.
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RAINFALL...TOTAL RAIN ACCUMULATIONS OF 4 TO 8 INCHES ARE POSSIBLE
OVER THE NORTHERN WINDWARD ISLANDS AND THE LEEWARD ISLANDS. TOTAL
RAIN ACCUMULATIONS OF 1 TO 3 INCHES WITH MAXIMUM AMOUNTS OF 6
INCHES ARE POSSIBLE OVER PUERTO RICO AND THE VIRGIN ISLANDS. TOTAL
RAINFALL ACCUMULATIONS OF 8 TO 12 INCHES...WITH MAXIMUM AMOUNTS OF
20 INCHES...ARE POSSIBLE OVER HISPANIOLA. THESE RAINS COULD CAUSE
LIFE-THREATENING FLASH FLOODS AND MUD SLIDES.

STORM SURGE...A STORM SURGE WILL RAISE WATER LEVELS BY AS MUCH AS
3 TO 5 FEET ABOVE NORMAL TIDE LEVELS IN AREAS OF ONSHORE WINDS ALONG
THE COAST OF HISPANIOLA. A STORM SURGE WILL RAISE WATER LEVELS BY
AS MUCH AS 1 TO 3 FEET ABOVE NORMAL TIDE LEVELS IN THE NORTHERN
LEEWARD ISLANDS...PUERTO RICO...AND THE U.S. AND BRITISH VIRGIN
ISLANDS. NEAR THE COAST...THE SURGE WILL BE ACCOMPANIED BY
DANGEROUS WAVES.

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Quoting RussianWinter:


This confirms my suspicions.

Look to the south of croix. This is vortex one. Look to the south of croix a bit further. This is vortex two.


Yup, I noticed that.......the plot thickens.......
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Center is about 80 miles S of where the NHC had it at 11 pm and 50 miles S of where they had it at 2 am.
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Isaac looks a lot more symmetrical now as opposed to just 8 hrs. ago. Nice breathing (both venting and intake) is starting to take hold in upper quads.....
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"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."


I did not know that.

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Yeah its farther south than 15.5
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Still 10% with the wave coming off Africa.
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2931. Levi32
Ah. Bingo. 14.75N.

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Quoting KoritheMan:
...RECONNAISSANCE AIRCRAFT FINDS THE CENTER OF ISAAC IS REFORMING
FARTHER SOUTH...

And there you have it.


+1 for Trevor ;)
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2929. KEEPEROFTHEGATE (Mod)
Quoting nrtiwInvragn:
Ummmm... are we tracking an open wave?
something is going on its weird
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Quoting Surferdude:

SUMMARY OF 200 AM AST...0600 UTC...INFORMATION
----------------------------------------------
LOCATION...15.5N 63.5W
ABOUT 265 MI...425 KM SE OF SAN JUAN PUERTO RICO
MAXIMUM SUSTAINED WINDS...45 MPH...75 KM/H
PRESENT MOVEMENT...W OR 260 DEGREES AT 20 MPH...32 KM/H
MINIMUM CENTRAL PRESSURE...1003 MB...29.62 INCHES


I know. The center isn't there. Recon proves that. NHC just needed to pick somewhere for now.
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Quoting AllStar17:
I can tell you for sure that the 2 am advisory position is simply just a plug until the recon finds an actual center...it certainly isn't at 15.5 N!


I was thinking the samething.. I just didnt think that they want to make it a TD or a wave even though they think it would organized again..
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though they may find it in next batch wind have gone more N so instead of ENE then NE its now NNE so their getting close
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Quoting AllStar17:
I can tell you for sure that the 2 am advisory position is simply just a plug until the recon finds an actual center...it certainly isn't at 15.5 N!

SUMMARY OF 200 AM AST...0600 UTC...INFORMATION
----------------------------------------------
LOCATION...15.5N 63.5W
ABOUT 265 MI...425 KM SE OF SAN JUAN PUERTO RICO
MAXIMUM SUSTAINED WINDS...45 MPH...75 KM/H
PRESENT MOVEMENT...W OR 260 DEGREES AT 20 MPH...32 KM/H
MINIMUM CENTRAL PRESSURE...1003 MB...29.62 INCHES
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AT 200 AM AST...0600 UTC...THE CENTER OF TROPICAL STORM ISAAC WAS
LOCATED NEAR LATITUDE 15.5 NORTH...LONGITUDE 63.5 WEST. ISAAC IS
MOVING TOWARD THE WEST NEAR 20 MPH...32 KM/H. SOME ERRATIC MOTION
TOWARD THE WEST-NORTHWEST OR NORTHWEST COULD OCCUR LATER THIS
MORNING...BUT A GRADUAL TURN TOWARD THE WEST-NORTHWEST IS EXPECTED
BY THIS AFTERNOON AND CONTINUE INTO FRIDAY. ON THE FORECAST
TRACK...THE CENTER OF ISAAC SHOULD PASS TO THE SOUTH OF THE VIRGIN
ISLANDS AND PUERTO RICO TODAY...AND APPROACH THE DOMINICAN REPUBLIC
TONIGHT AND FRIDAY.

MAXIMUM SUSTAINED WINDS ARE NEAR 45 MPH...75 KM/H...WITH HIGHER
GUSTS. SOME STRENGTHENING IS FORECAST DURING THE NEXT 48 HOURS...
AND ISAAC COULD BECOME A HURRICANE ON FRIDAY.

TROPICAL-STORM-FORCE WINDS EXTEND OUTWARD UP TO 140 MILES...220 KM
FROM THE CENTER.

THE ESTIMATED MINIMUM CENTRAL PRESSURE IS 1003 MB...29.62 INCHES.
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Oh well.
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Quoting Kowaliga:


Must be one of those loose vorts...ya think?



This confirms my suspicions.

Look to the south of croix. This is vortex one. Look to the south of croix a bit further. This is vortex two.
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...RECONNAISSANCE AIRCRAFT FINDS THE CENTER OF ISAAC IS REFORMING
FARTHER SOUTH...

And there you have it.
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Quoting Levi32:
Wouldn't be too surprised if the plane finds a secondary center or northeastward extension just west of Dominica and Guadeloupe later. The circulation still looks rather binary to me.



Hi Levi:

Been nice to see you so active on the blog today. Always like reading your information --- helps to understand the graphics.

How is college? Is this your 2nd or 3rd year?
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Good image right here..
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2918. 7544
hwrf shift back east to south fl again
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I can tell you for sure that the 2 am advisory position is simply just a plug until the recon finds an actual center...it certainly isn't at 15.5 N!
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Quoting Levi32:
Wouldn't be too surprised if the plane finds a secondary center or northeastward extension just west of Dominica and Guadeloupe later. The circulation still looks rather binary to me.


He's looked kinda like a barred spiral galaxy all day to me.
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HWRF 126 hrs.

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...RECONNAISSANCE AIRCRAFT FINDS THE CENTER OF ISAAC IS REFORMING FARTHER SOUTH...
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i repeat... its doing the same thing its done the past 3 nights...its displacing the blow up of thunderstorm off to the south...new convection will fire under the true center which is still at around 15.5-15.7
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2:01 AM and still no update or TWO on the NHC website.
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Quoting nrtiwInvragn:
Ummmm... are we tracking an open wave?


This would be an impressive wave of a storm..
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Quoting Levi32:
The thing is showers moving from the NNW are visible around 15.5-16.0N on PR radar, implying a center east of there, not necessarily the only center:



Must be one of those loose vorts...ya think?

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Quoting nrtiwInvragn:
Ummmm... are we tracking an open wave?


No. Isaac has a large envelope of lower pressure, which will naturally take awhile to consolidate.
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Quoting nrtiwInvragn:
Ummmm... are we tracking an open wave?


That thought has crossed my mind...
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2906. JLPR2
Well, I'm tired and Isaac seems to be composed of light winds. So I'm off. xD



Good night everyone!
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GFDL 60 hrs.

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2904. Levi32
Quoting Stormchaser121:

Link?


EMC Cyclone Tracking
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Quoting GTcooliebai:
While you guys look for the center, Euro initialized:

Yessiree!
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2902. Levi32
Wouldn't be too surprised if the plane finds a secondary center or northeastward extension just west of Dominica and Guadeloupe later. The circulation still looks rather binary to me.

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Quoting AllStar17:
Any center is probably SOUTH of 15N.


I agree at this point .. as the possible center for this storm.. We will c.. Might have some more tricks up his sleeve.. grrrr
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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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