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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I watch all the time but don’t get concerned until landfall is three days out. Models at seven days and NHC tracks at 5 days are never right. By three days they usually have it pretty well nailed down.

Here in NE FL we don’t need any more rain.

I will be leaving soon to take one of my daughters to go see Winter the dolphin. If you know who that is.
Member Since: Posts: Comments:
they all point out to that WNW trajectory which hasnt happened.

Member Since: August 22, 2006 Posts: 2 Comments: 1983
Isaac is weaker, but one thing is that Isaac has certainly slowed down. It went from 20mph to 12mph, that will help it in the long run if that isn't because of the CoC relocation. If not, intensification will begin today.
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Quoting LargoFl:


You do not even need Isaac to make landfall on the West Coast of Florida to cause problems even if Isaac is 25-30 miles off the Coast Florida will still see the worse of it. Most of Debby's wind and rain came she was just offshore.
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3247. LargoFl
Quoting clwstmchasr:


I agree, it is not time to give the all clear. However, we have seen this same type of scenario over and over again. Latest GFS has shifted East a little bit.
boy i hope that proves true, we surely dont need this storm..even just for the rains..the ground is soaking wet
Member Since: August 6, 2011 Posts: 4 Comments: 38541
20 >>> 12mph is interesting.

A slowdown is often a change in direction so...The WNW component could be coming in soon.

Then again, unless Isaac takes this opportunity to get organized (ie..strengthen), the turn may not happen anytime soon.

Personally, until we get some vertical alignment and one sure COC, I'm going to expect it to stay on a western course.

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Quoting dfwstormwatch:
the Hurricane hunters are finding gusts to 70 MPH in the rainbands, and 45 to 50 out in the normal


i dont know where thats coming from. either theres another plane i dont know about, all i see is SURFACE WIND with is obviously less than flight level and that's what counts.

is it surface winds (SFC)?

all im seeing is 40mph max.
Member Since: August 22, 2006 Posts: 2 Comments: 1983
3244. GetReal
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Quoting Autistic2:


Do you remember 05?


Why even quote him? An obvious troll and just trying to stir up trouble.
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Quoting BOGUSMODELS:
typical tracks for florida and it deflector shield one storm goes way east of florida as in td10 andd the other storm goes way west of florida as in issac florida is always lucky


Do you remember 05?
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the Hurricane hunters are finding gusts to 70 MPH in the rainbands, and 45 to 50 out in the normal
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3240. WxLogic
Quoting MahFL:
Is the center reforming closer to 15 N ?


Looks like it... based on the latest HH run, but we'll see if that's the case after they attempt another stab at locating the center.
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3238. shfr173
Do not think Tampa out of woods yet!!
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Quoting shfr173:
ref 3222

My point exactly!!! What did models pick up on??? not just center relocation....

Maybe fluctuations in the High Pressure above Isaac..hard to say though , these storms tend to look for the path of least resistance and follow those.
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There has to be stronger wind in the storm than the previous advisory, because PR radar shows a cell moving 43knots, so that's 50mph.
Member Since: January 25, 2012 Posts: 33 Comments: 1520
3234. WxLogic
Good Morning... I see ECMWF has shifted E when compared to the last runs. Hopefully we'll get more consensus than what we have now after the G-IV run later today.
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3233. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 38541
3231. MahFL
Is the center reforming closer to 15 N ?
Member Since: June 9, 2004 Posts: 0 Comments: 3494
3230. icmoore
Good morning everybody. I just turned on my computer going to grab my coffee now but was checking local NWS and saw this change from yesterday oh and I see currently a more west...

Monday Scattered showers and thunderstorms. Partly sunny, with a high near 88. Windy, with a northeast wind 24 to 29 mph increasing to 38 to 43 mph in the afternoon. Winds could gust as high as 60 mph. Chance of precipitation is 50%.
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3229. LargoFl
Quoting clwstmchasr:


This is typical for Tampa Bay. Big threat then the subtle moves shift it away. We see it all the time. My guess it will be 150 miles offshore heading for Pensacola.
it would be wonderful IF that track came true..at least for our area, but..until maybe saturday, i am not taking my eyes off this one...GDFL has this still..going up into florida..including tampa bay..hasnt changed that track in days..so im watching..and waiting
Member Since: August 6, 2011 Posts: 4 Comments: 38541
3228. shfr173
ref 3222

My point exactly!!! What did models pick up on??? not just center relocation....
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Quoting clwstmchasr:


This is typical for Tampa Bay. Big threat then the subtle moves shift it away. We see it all the time. My guess it will be 150 miles offshore heading for Pensacola.

And the 6z GFS is back towards Tampa..
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Quoting BOGUSMODELS:
whoes that troll thats been saying everyday for 4 days now that the track of issac will keep shifting west and eventually bypass most of florida? i think he was on to something ( not bad forcasting for a amateur ) so far hes been right on

so very sad
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Quoting LargoFl:
yes no one has a good handle on this storm, tomorow morning it could be headed for tampa, could be miami, could be NOLA..then again it could be panama or the yucatan...no one knows yet..im hoping it dies off going thru the islands..its not a strong storm, barely holding on

With those steering patterns and constantly relocating circs, it may never get around to curving until it can miss the GOM altogether.
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Anyone read this?:
THE 00Z
GFS...ECMWF...AND UKMET MODELS DID AN OUTSTANDING JOB PREDICTING
THE RECENT SOUTHWESTWARD JOG OR REFORMATION OF THE CENTER OF ISAAC.
THOSE SAME MODELS ARE ALSO FORECASTING ISAAC TO MAKE A SHARP JOG TO
THE NORTHWEST OVER THE NEXT 12-24 HOURS...AND THEN STEADY OFF ON A
GENERAL WEST-NORTHWESTWARD MOTION THROUGH 48 HOURS.
AFTER THAT...
THE GLOBAL AND REGIONAL MODELS HAVE COME INTO BETTER AGREEMENT ON
ISAAC MOVING NORTHWESTWARD TOWARD A BREAK IN THE SUBTROPICAL RIDGE
THAT IS FORECAST TO DEVELOP AFTER 72 HOURS ACROSS FLORIDA AND THE
EASTERN GULF OF MEXICO. THE ECMWF MODEL HAS CONTINUED ITS EASTWARD
SHIFT OVER THE PAST 36 HOURS...AND EVEN THE LATEST NOGAPS AND
CANADIAN MODEL RUNS HAVE SHIFTED WELL TO THE WEST ACROSS THE
FLORIDA PENINSULA ARE NOW NO LONGER CONSIDERED TO BE OUTLIERS. DUE
TO LESS SPREAD IN THE LATEST NHC MODEL GUIDANCE...AND THE FACT THAT
ECMWF HAS SHIFTED CLOSER TO THE PREVIOUS FORECAST AT DAYS 4 AND
5...THE OFFICIAL FORECAST TRACK IS JUST AN UPDATE OF THE PREVIOUS
ADVISORY TRACK...

I mean the models are indicating this even the GFS and Euro so it was not unexpected. If anything the Euro and GFS are coming into better agreement and that is a good thing. It is a weak system so lets see what happens today.
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3221. ackee
Quoting RTSplayer:
Honestly, this is starting to look more and more like the Euro or just a mid to western caribbean storm, with a re-curve into the Gulf.

I'm thinking Jamaica will see more wind than DR.
agree
Member Since: July 15, 2008 Posts: 0 Comments: 1373
3219. LargoFl
there are still models putting this storm into florida,so we must watch this one, tracks change day to day
Member Since: August 6, 2011 Posts: 4 Comments: 38541
Honestly, this is starting to look more and more like the Euro or just a mid to western caribbean storm, with a re-curve into the Gulf.

I'm thinking Jamaica will see more wind than DR.
Member Since: January 25, 2012 Posts: 33 Comments: 1520
3217. keisa90
NHC can't change the track until the GFS makes a move further west, even though recon is finding the center further south. GFS has performed well this year, and I think they are hesitant to stray too far from its forecast at this point. Especially with the center continually trying to reform.
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Quoting VR46L:
I dont think TD 10 is going to make it ...





but the 10% a different story....





TD10 totally looking poorly! But, so many have have real poor spells this year, so maybe can pull off Joyce still?!

@negriltracy - probably won't know til later today just how much at risk Jamaica is...everything is depending on this N/W pull many are expecting. If it really starts that way, Jamaica should be in the clear, but th elonger it takes, the more likely Jamaica will be in a bad spot. I'm afraid if it does head that way, you're not going to know for sure until not soon beforehand to be honest...IMO...but is how it's looking. I'd certainly be as prepared as I can be so that if worse came to worst, it wouldn't be too big an upheaval. But so much is inthe air right now until it/if it makes the turn. All the best

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3215. LargoFl
..................unless something changes, like it explodes into a strong hurricane..i dont know if im even going to plywood the windows..by saturday we should know more
Member Since: August 6, 2011 Posts: 4 Comments: 38541
Morning everyone. I just got up so my brain's not working but recon looks to have found 45mph winds. I don't know where they are though.
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3211. mfcmom
Quoting panamasteve:
Darn. Looks like it's time to refill the gas cans for the generator. Nobody yet paying too much attention to Isaac here in Panama City Beach.


We are.
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3209. LargoFl
Quoting redwagon:

Wonder whose storm this will be when I get off work in 9 hours.... I'm sure NOLA will be in an uproar by noon.
yes no one has a good handle on this storm, tomorow morning it could be headed for tampa, could be miami, could be NOLA..then again it could be panama or the yucatan...no one knows yet..im hoping it dies off going thru the islands..its not a strong storm, barely holding on
Member Since: August 6, 2011 Posts: 4 Comments: 38541
Quoting redwagon:

Wonder whose storm this will be when I get off work in 9 hours.... I'm sure NOLA will be in an uproar by noon.


Most of the heavy convection does seem to be driven south.

Peculiar, but viewing the loop, looks almost as if that convection is turning back to the islands....

Until one looks at general wide motion and it is going west. Agree that the official forecast track is looking a bit suspect right now.

But NHC has a good record with official tracks, we will see. Only the odd storm throws them off.
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3205. ackee
Quoting yonzabam:
The NHC forecast track takes the center of Isaac through southwest Dominican Republic, then through the middle of Haiti. It will have to start moving a bit N of WNW from now on for this to happen. Can't see it, myself.


me either THE NHC wont change there track until model shift more west and south
Member Since: July 15, 2008 Posts: 0 Comments: 1373
Quoting LargoFl:

Wonder whose storm this will be when I get off work in 9 hours.... I'm sure NOLA will be in an uproar by noon.
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3203. LargoFl
.......................Until these models change we must watch this storm very closely
Member Since: August 6, 2011 Posts: 4 Comments: 38541
The NHC forecast track takes the center of Isaac through southwest Dominican Republic, then through the middle of Haiti. It will have to start moving a bit N of WNW from now on for this to happen. Can't see it, myself.


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3201. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 38541

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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.