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 Patrap:
The question is where's Isaac?.
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I plan on doing a blog soon about injuries (and how to administer first aid for them) that are common during and after a storm. For those who have been through storms in the last 10 years or so, and also everyone involved with Portlight... what kind of injuries do you see the most with a storm making landfall?

Please send your input via WUmail - I am about to head home from work, and any replies on here will be drowned out quickly by the sheer volume of entries on the blog.
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Quoting TropicalAnalystwx13:

I seventh the poof!


If you guys, especially the younger ones knew the UK definition of POOF, You may not be so keen to do it!

Link

...and it's off topic and most of you SHOULD know better, especially in active times!!!
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Quoting Clearwater1:
Does anyone know how the Weather Underground's model is derived. For example, the GFS runs at 00, 06, 12, 18z. As far as I know there is no run at 5PM, as it states on the model plots, yet there is clearly the gfs plot on this WU chart is significantly different than the 1200 run. I see this also for 8pm 2pm etc. Anyone?


00z 8pm, 6z 2am 12z 2pm 18z 8pm

Some of the models also take longer to initialize as they more complex, that is why you see the GFS and Euro out much later then the statistical models.
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247. SLU
WOW ... so the EURO was right when it showed ISAAC as a large mess with multiple lows.

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Quoting Tazmanian:
ISAAC has a twin

Located fill in the bank
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Sorry guys off topic.

Red, if you're out there, couldn't find out anything on the spin off TX. Looking at water vapor there's dry air to its west and maybe shear above it. So hopefully it won't do anything. But good catch. :)
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Quoting MAweatherboy1:

As odd of an analogy as that is, that's exactly what it looks like, lol.



Looks like red eyes of DOOM :)
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Does anyone know how the Weather Underground's model is derived. For example, the GFS runs at 00, 06, 12, 18z. As far as I know there is no run at 5PM, as it states on the model plots, yet there is clearly the gfs plot on this WU chart is significantly different than the 1200 run. I see this also for 8pm 2pm etc. Anyone?
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Quoting nrtiwlnvragn:
FWIW the HWRF and GFS were initialized at 12Z with significant dry air to the north, GFDL not as much.


So which is more accurate to the actual conditions - lots of dry air to the north or not as much anymore?
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max mayfield on local10 in miami just said a hurricane watch or tropical storm watch appears very likely sometime friday afternoon or evening and that he is very concerned about the florida keys
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Please no more poofs..I really don't like scrolling from peoples post..
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ISAAC has a twin
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Quoting MAweatherboy1:

As odd of an analogy as that is, that's exactly what it looks like, lol.

It's watching the blog!!! No bad words now gang!!!
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Quoting VR46L:
Hmmm seems rather big right now ...



But how will he cope with the dry air ahead



Should cope fine thanks to spiral banding well to it's west. This was one of Ernesto's biggest flaws, dry air from the Caribbean got into Ernesto's circulation. This is only getting dry air from the Atlantic on its NE side.
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How accurately can these models predict the result of interaction with Hispaniola?? The Euro keeps Isaac stuck going west along Cuba, The GFS has Isaac hitting E. Cuba but then pulling north away from it.
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Quoting MAweatherboy1:

As odd of an analogy as that is, that's exactly what it looks like, lol.


We call that 'splitting the mitochondrial sheets' here in TX.
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Quoting MAweatherboy1:
Isaac is still a very big storm. It could end up being kind of like Irene in the sense that its pressure may come way down but the winds never really catch up.

A better example would be Alex.



Pressure of 948MB but winds of just 85KT.
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I poof all the poofers to infinity (and beyond!)

Seriously though... Everyone between TX and NC need to be checking through their hurricane prep plans, and filling in any "holes" of their plans right now.
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Quoting guygee:
Latest GFS wind fields...trending towards a little lower intensity after passing near southern Hispaniola.
Of course, global model intensity forecasts not as reliable as track forecasts.


Beware the JOYCE
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What I find almost as interesting as Issac is the persistent line of thunderstorms over the stalled out front in the Gulf. I don't believe I have seen it persist this long. Shows the tremendous heat out there I suppose.
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Quoting VR46L:
Hmmm seems rather big right now ...



But how will he cope with the dry air ahead

This is a large system, And as it intensifies it may appear to shrink a bit, but this is a big storm that will get bigger unless it gets ripped apart by mountains, and even the it will probably be large.
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Quoting Drakoen:


Indeed. It is obvious that dry air has been affecting the NE corner. All one has to do is turn on the WV imagery.


....at least someone can find the NE quadrant. I'm seeing a mess right now
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Quoting seminolesfan:
I guess co-moving frameworks are the science of which you allude. :)
Lagrangian
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Quoting VR46L:
Hmmm seems rather big right now ...



But how will he cope with the dry air ahead

It'll protect itself from dry air using the western bands
Member Since: August 1, 2011 Posts: 28 Comments: 7906
Evacuation Considerations for the Elderly, Disabled and Special Medical Care Issues


Acknowledgements

The authors would like to thank the Federal Transit Administration for making this study possible. We particularly want to acknowledge Monica McCallum, David Schneider and all others that have worked on this specific grant. We are grateful to the other members of our research team for theirinput,including Jacky Grimshaw, Pam Jenkins, Shirley Laska, and Brian
Wolshon. Graduate assistants Clare Cahalan, Carrie Makarewicz, Robert Peterson, Jason Sappington and Patrick Wontor provided valuable research and assistance with early drafts of this literature review.

We would also like to thank Carol Short, Paulette Simon and Robert Peterson for editing this document.

Disclaimer

This report has been prepared under a Federal Transit Administration grant # LA-26-8001,
however, the content of this report, including any errors or omissions are solely the responsibility
of the authors.
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Latest GFS wind fields...trending towards a little lower intensity after passing near southern Hispaniola.
Of course, global model intensity forecasts not as reliable as track forecasts.



PS Also more land interaction with FL peninsula. Wet and sloppy?
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Quoting schistkicker:
Heh. Looking at the satellite imagery, it almost looks like a reproducing amoeba in the middle of its mitosis.

I wonder if any tropical entity has ever split into two distinct storms before?

As odd of an analogy as that is, that's exactly what it looks like, lol.

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Hurricane Preparation 2012


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217. wpb
noaa or air forcerecons need to collaborate there instraments reading differ big time
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Quoting mfcmom:
Word up guys I am pretty much a lurker, but live in Panama City, please keep us posted as to probabilities, I need an extra day or two to prep, have seven adopted kids five are handicapped. I am really depending on you all and thank you so much.

I am a Disaster Recovery Center Manager for FEMA. Please contact your local EM and let them know your situation and where you are. If the storm moves your way, I'm sure there will be an evacuation of special needs folks long before a storm gets there. However, if they don't know about you, they won't be able to help. Giving your local Emergency Manager a heads up before anything develops will certainly make things a lot easier. Perhaps you have gone through this before and all of this is redundant but it certainly doesn't hurt to remind all folks with special needs and critical care patients to get their information to the local authorities ahead of time.
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Quoting Seflhurricane:
drak do you think this will be a south florida event


Right now I think the GFS and UKMET have the best tracks for this system. It may be a South Florida or even more so a Florida straits event. But it is still too early to say.
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Heh. Looking at the satellite imagery, it almost looks like a reproducing amoeba in the middle of its mitosis.

I wonder if any tropical entity has ever split into two distinct storms before?
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FWIW the HWRF and GFS were initialized at 12Z with significant dry air to the north, GFDL not as much.







Link
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Quoting catastropheadjuster:
Just a question you all. Do you think the folks in Mobile,Al should be worried about this, or is it most likely going to do what the GFS shows? I'm probably like alot of other folks just don't have the money to go out and buy a bunch of stuff, seeing know that all my kids and there wife and grandchildren all live with us again. It's hard these days to make it, and a lot of you all know what I am talking about, I have a extra 8 mouths to feed not counting hubby and I. I have water and batteries.

Sheri
Are you kidding.?/ If your in Mobile, AL I would start making preparations now for a significant hurricane. Do not go crazy., just pace yourself and go down the hurricane prep list. If you dont have one, Patrap and KEEPEROFTHEGATE have a complete list on there blog. Starting now saves you from rushing and making mistakes later.
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Quoting Patrap:



west they go....
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Quoting MississippiWx:


...Or science, but you know. To each his own, I guess.
I guess co-moving frameworks are the science of which you allude. :)
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Quoting Patrap:


Er, dat is the COC in between and as I mentioned earlier, the "Bi-lobing" has balanced the Stack some, and the mean is rotating around that center.


look at radar...
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Quoting Drakoen:


Indeed. It is obvious that dry air has been affecting the NE corner. All one has to do is turn on the WV imagery.
drak do you think this will be a south florida event
Member Since: July 14, 2009 Posts: 0 Comments: 2990
double storm reminding me of Jeanne:(
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Looks like Joyce will hopefully be reJOYCing out to sea.Get it?.You know with Joy being in the storms name?.Please don't throw tomatoes...
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205. VR46L
Hmmm seems rather big right now ...



But how will he cope with the dry air ahead

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My thinking is with the storm not strengthening at a rapid pace and having trouble getting vertically stacked and the center possibly reforming to the south we will have a weak to moderate tropical storm moving thru the Caribbean south of PR, the DR and Haiti..it could begin to strengthen to a strong tropical storm as it skirts the south coast of Jamaica and starts a more WNW movement and moves just south of the Cayman Is. it starts to intensify into a hurricane just prior to the Yucatan channel and the center passes of that thin spit of land in extreme western Cuba..starts to turn NW and intensify rapidly as in traverses the central Gulf of mexico to a cat 3 and starts to turn due north and could make landfall as a cat 4 with 130-140 mph winds along the northern gulf coast..between the mouth of the MS River (Venice, Boothville LA.) and Pensacola, FL.
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Quoting WxGeekVA:


I fourth the POOF.



I FIFTH the POOF! :)
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Quoting 69Viking:


So were to believe you're right while Dr. M and all he experts at the NHC are wrong? Normally I have no problem taking your word for it but too many experts are talking about the dry air in the NE corner of Isaac and you're saying they're all wrong. I'm no weather expert but even I can see the upper level low driving dry air into the NE corner of Isaac, pretty obvious on the water vapor loops.



Indeed. It is obvious that dry air has been affecting the NE corner. All one has to do is turn on the WV imagery.
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IMO... The Islands are looking as a strong TS and the Sothern tip of Fl.... TS or Cat 1 hurricane at best. I am mot trying to down play this at all, because I know what damage flooding and wind can do. Although if Isaac winds up in the GOM for very long, as the EURO portrays.... it would be even worse on the Gulf Coast.
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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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