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 newt3d:
Does not compute ... SHIPS is an intensity model, only.


The white line on the computer model traversing Cuba and headed for Central Gulf is the UKMET. Thanks!
Right. Ships does intensity only.
Member Since: July 11, 2006 Posts: 14 Comments: 11328
3750. Grothar
Member Since: July 17, 2009 Posts: 71 Comments: 26151
3749. LargoFl
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Good morning, everyone!

I see the most pronounced change in Isaac this morning is the slowing down of the forward speed of the storm, now at nearly half the speed if was chugging westward yesterday. With this Isaac should finally start getting its act together later this morning and this afternoon. It will be much easier now for the moist air encompassing the southern and eastern quadrants of the storm to wrap into the center and mix out the dry air hindering it.
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Quoting Drakoen:
What we are seeing is the models converging. A track through the straits or southern FL then into the GOM heading northwards towards the panhandle.


Not what I need that's for sure!
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Err...
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3745. ncstorm
Good Morning Everyone..06z GFS precip map

through 120 hours


the entire SE-240 hours


360 hours


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Quoting VR46L:


I believe TD 10 is going to run into major problems shortly.... see the Dry air..




Not TD 10 it's now the J storm
Member Since: May 21, 2006 Posts: 5091 Comments: 115125
Quoting Waltanater:
Are you really seeing his back?! LOL
HaHa :)
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Quoting Tazmanian:



The atcf site all ready did update too ts

Hence the "TS" 10.
Look again, although it is a TS, it is not listed as "Joyce" in the ATCF yet.
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Craig Fugate ‏@CraigatFEMA
Getting Ready for #Isaac? Go Tapeless this Hurricane Season. Why?
http://www.greathurricaneblowout.org/protect_tape less.php
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3739. OneDrop
Quoting Brock31:


I too was hoping to see Isaac make his way north of PR and send some much needed swell our way.
Here Here Brock, I was hoping for the same thing. We need some swell bad here in NE Fla. Summer started out decent and then went into a lull for the last few weeks. Looks like Isaac is giving us some windchop this weekend and then hopefully the offshores kick in just in time to smooth out Joyces swell and then hoping the storm behind Joyce is our Labor Day weekend wave maker. What are your thoughts?
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Quoting weatherh98:


THERE YA GO


technically that is the panhandle of FL/South Alabama...FL takes up much of that coast so its still not much of a shift
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3737. VR46L
Quoting AussieStorm:

TD10 is looking good


I believe TD 10 is going to run into major problems shortly.... see the Dry air..

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3736. GetReal
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AS some on here were saying all summer long that FL is in a bad set up right now. Could have multiple stikes at FL over the coming weeks.

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It also appears they are now tilted with height SW to NE, not just misaligned as in days past. "Tiled With Height" is indicative of a dominant center.
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Quoting Bobbyweather:

What do you mean?

Anyways, current ATCF for TS 10 (Not yet named Joyce yet by ATCF... that's to be updated soon)
...INITIAL CONDITIONS...
LATCUR = 14.9N LONCUR = 41.6W DIRCUR = 295DEG SPDCUR = 15KT
LATM12 = 13.6N LONM12 = 38.8W DIRM12 = 297DEG SPDM12 = 15KT
LATM24 = 12.2N LONM24 = 35.7W
WNDCUR = 35KT RMAXWD = 40NM WNDM12 = 30KT
CENPRS = 1006MB OUTPRS = 1013MB OUTRAD = 225NM SDEPTH = M
RD34NE = 40NM RD34SE = 30NM RD34SW = 0NM RD34NW = 40NM




The atcf site all ready did update too ts
Member Since: May 21, 2006 Posts: 5091 Comments: 115125
3732. Drakoen
Quoting Chicklit:

I thought SHIPS has track further west this morning.


Slight farther west than the 00z runs.
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3731. LargoFl
...wow back on page 8 lol..well here is the GDFL..straight up florida..just like the GFS had a few days ago..GDFL hasnt budged from this track..notice the intensity
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I just put a new blog up... this time asking for the WU community's input (no, it isn't about Issac, but it is related to hurricanes in general)

Hurricane-related Injuries
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3729. newt3d
Quoting Chicklit:

I thought SHIPS has track further west this morning.
Does not compute ... SHIPS is an intensity model, only.
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Quoting Joshfsu123:
Amazing how the blog goes to c*** as soon as some people who are pushing an agenda wake up.

What do you mean?

Anyways, current ATCF for TS 10 (Not yet named Joyce yet by ATCF... that's to be updated soon)
...INITIAL CONDITIONS...
LATCUR = 14.9N LONCUR = 41.6W DIRCUR = 295DEG SPDCUR = 15KT
LATM12 = 13.6N LONM12 = 38.8W DIRM12 = 297DEG SPDM12 = 15KT
LATM24 = 12.2N LONM24 = 35.7W
WNDCUR = 35KT RMAXWD = 40NM WNDM12 = 30KT
CENPRS = 1006MB OUTPRS = 1013MB OUTRAD = 225NM SDEPTH = M
RD34NE = 40NM RD34SE = 30NM RD34SW = 0NM RD34NW = 40NM

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3727. sar2401
Quoting Maineweatherguy20023:

I had this weird dream that isaac opened into a wave then hit Tampa later as a cat 5...
My ACTUAL prediction: Becomes hurricane south of Hispaniola, then skirts just over the southern tip of Haiti and crossing E Cuba before moving through the Keys as an intensifying CAT1 before moving offshore of Tampa as a strong and still intensifying CAT 2. It will then become a CAT3 before hitting the panhandle/peninsula junction moving NE then it will move NNE and affect the outer banks before moving into the atlantic.


Well, that covers just about all the known world. :)
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Quoting GTcooliebai:
Here ya go weather98:

00z ECMWF 168 hrs.



THERE YA GO
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Quoting GTcooliebai:
Here ya go weather98:

00z ECMWF 168 hrs.


thanks for the better graphic.

clearly shows panhandle of FL for the 0z Euro model whereas the 12z was LA.
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Quoting Drakoen:
What we are seeing is the models converging. A track through the straits or southern FL then into the GOM heading northwards towards the panhandle.

I thought SHIPS has track further west this morning.
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Quoting TropicalAnalystwx13:
Good...10L took the name Joyce. That leaves the wave behind it to become big, bad Kirk.
Hope he is not a jerk.
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I must stress something HIGHLY important, a more eastern track up the west coast of Florida and or peninsula is based on Isaac gaining at least decent intensity, if Isaac fails to intensify for too long it will most likely go much further west, it's just a matter of physics.

With that said, some of the model shifting to west farther into the central gulf is only because of Isaac's southward relocation and its failure to intensify, if Isaac begins to intensify steadily from here on out however I expect model groups to further tighten into the eastern gulf.

Make sense?
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Quoting TropicalAnalystwx13:
Good...10L took the name Joyce. That leaves the wave behind it to become big, bad Kirk.


Did you see the 0Z Euro?
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According to hurricane hunters who flew into the cyclone overnight, TROPICAL STORM ISAAC is slightly weaker and centered further south than yesterday...which might spare Puerto Rico (radar loop) and Hispanola from a direct hit. As of 6am, ISAAC's moving west at 12 mph with sustained winds at 40 mph. The official track carries the storm across Cuba through Saturday then to near the Florida Keys Sunday night/early Monday morning, and it might be a weak Category 1 hurricane at that point. We'll be watching the west coast of Florida Monday to Tuesday for the most impact.


TROPICAL DEPRESSION #10 may soon become TROPICAL STORM JOYCE but she's not expected to be a menace. The latest round of computer models almost all suggest a turn north into the cooler waters of the open Atlantic.

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3719. GetReal





It looks like Isaac need to start making a right turn about NOW to stay in alignment with a majority of these model runs!!!
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After Issac FL better have all eye's on future Kirk. This is not good almost looks like a Frances set up from 2004. I think FL is in for some serious flooding concerns if this Tropical train sets up and FL is in the cross hairs of these systems.

0Z Euro.
FL say high to Kirk.
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Quoting LargoFl:
..anyone remember charley?..im beginning to feel like that once again..it was the only storm i ever plywooded the windows for..luckily he changed course and came in way south of me That time if i remember the name of the storm right...
Quoting cat6band:


Euro moved east??? I don't think so...Pretty much ALL models have shifted westward..unless I'm missing something..
Wrong, latest gfs 06z and latest Euro moved east, not west. In fact the euro has been shifting west for the past 3 runs. consistently. Check it out, you're missing something.
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Quoting chevycanes:
0z euro



I just showed that model.
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Quoting weatherh98:


I USED WHAT CAME UP ON MY COMPUTER
Here ya go weather98:

00z ECMWF 168 hrs.

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Center is pretty well aligned with the NHC forecast. The area to the SW is the MLC.

850


500mb
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Quoting masonsnana:
Good morning! Good to see your back
Are you really seeing his back?! LOL
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0z euro

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06z run of GFS virtually identical to previous run (00z):

Link
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3708. K8eCane
Quoting gprxomstr:

It's what's underneath that matters, Pete. Common Brother. You should know that.


As a woman myself, that was not very respectful.
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Good...10L took the name Joyce. That leaves the wave behind it to become big, bad Kirk.
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Amazing how the blog goes to c*** as soon as some people who are pushing an agenda wake up.
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You are going to see tracks shift back and forth. What matters is that the entire state of Florida is in the cone so everyone should be prepared to be impacted.
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good morning. i see the euro still shows a Gulf major hurricane. looks like the "center" reformed farther south. interesting
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Quoting chevycanes:

it would help if you posted the latest runs like the person above did.

you can go on believing that the 0z euro didn't shift to the panhandle of FL if you'd like.


I USED WHAT CAME UP ON MY COMPUTER
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Quoting weatherh98:


these are the model run landfalls for the euro



that is not the pan handle


Looks very Katrina-ish/Camille-ish in terms of final outcome.

Hits or grazes lower Plaquemines, with a slight right hook into Biloxi or Mobile Bay.
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Quoting StormTracker2K:


Euro shifted EAST is big time from yesterday's 12Z run.

0Z


12Z


continue the run
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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.