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 stormpetrol:


Recon fixes equal a WSW movement for now, argue with Recon and WKC, not me :)) J/K.

yeah lol

Quoting superpete:


....and Grand Cayman on the way...

oh boy

Quoting PalmBeachWeather:
stormpetrol.... The saying is "Don't shoot the messenger"


lol

Quoting MAweatherboy1:
The NOAA plane is finding 50mph winds...


HH RECON was finding some near the area too
Member Since: June 13, 2009 Posts: 2 Comments: 12716
Quoting Levi32:
12z UKMET ensembles, mostly gulf:

Quite a shift Levi
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Quoting clwstmchasr:
I'm wondering if this could pull an Ivan on us (not the strength but direction). Everything pointed to Ivan running up the spine of Florida or the West Coast. Subtle changes happened and it shifted significantly West and hit Pensacola.

been wondering that myself
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Quoting washingtonian115:
At least we here in D.C get needed rain in wake of the suffrage of others :D.


Wash..cool avatar!!
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through 360..Im assuming that is Joyce over there in the east with all that rain..

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A mess, but still firing a fair amount of deep convection
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Quoting GTcooliebai:
264 hrs. and future Kirk looks like he wants to pay the East Coast a visit.



No thank you Kirk. Your mom Irene and your great great great great great great great grandmother Izzy was enough for me.

And yes I counted each year as a step up in generation.
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Isaac will decide where his center is between 12AM and 3AM I think. He certainly put in some work at that time last night and early morning.
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12z UKMET ensembles, mostly gulf:

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Quoting ncstorm:
Oh boy!!! 240 hours with the 18z GFS..the entire SE

At least we here in D.C get needed rain in wake of the suffrage of others :D.
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Pressures appear to be down to 1003mb in Isaac now as we approach DMIN. Two centers continue to be competing against each other. Dry air is indeed penetrating the NE side as dew points are fairly low by there, but this will probably become less of an issue as it enters into the Caribbean.

Isaac is really starting to become a no-win scenario. You have two options.
A) Isaac steers into Haiti and the Dominican Republic as a strengthening Tropical Storm, causing landslides to an already hammered region from the last two years. This would be supported by all of the major dynamical models and most of the global models. Hurricane Noel as a tropical storm for example stalled out as it approached Haiti and killed 222 people.


Due to Isaac's broad size and lack of a tight LLC, this would likely result in not as much decrease in intensity as one would expect. Larger systems weaken less over the mountains, especially as tropical storms. After that, Isaac would be approaching Florida where conditions in the Bahamas are very warm and would probably become a minimal hurricane as it approaches Florida. There you have two options, Isaac steers into Florida causing flooding but not strengthening.. or Isaac entering the Gulf of Mexico, only briefly making landfall in Florida and then doing who knows what.


B) Isaac remains weak, unable to fend off the notoriously hostile Eastern Caribbean. This would cause it to miss Hispaniola by a matter of 50 miles or so, which would limit strengthening and cause flooding but no where near as bad as a possible direct hit. From there on forth, Isaac would enter into the Western Caribbean and really begin to take off. It would unlikely hit the Yucatan, though that is possible just very unlikely, it would probably though end up scraping the tip of Cuba and intensifying into a very, very powerful hurricane as it approaches the United States. This scenario is of course as we all know, supported by the reliable ECMWF model and has been very consistent with this, for two days now.


The BEST case scenario would be if Isaac ran through Cuba and Haiti and died over the mountains, but this scenario just doesn't seem as likely and would result in numerous casualties. Isaac is producing massive precip and the threat for massive landslides exists. Storms in the Caribbean in August are a no-win scenario regardless when they are as large as Isaac is and have such strong model support.

The point is, people we're in for a ride regardless of what happens. I shouldn't have to log on and see the amount of bickering we have on now between two of our best bloggers about who's track record is better or not. None of us are trained, 100% certified mets at the National Hurricane Center. The best we can see is some data provided to us while we sit on our armchairs.
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Quoting StormJunkie:


And THAT is the severe threat with what the GFS has been showing fairly consistently.



That'll definitely knock out the drought we have in GA.
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Quoting StormJunkie:


And THAT is the severe threat with what the GFS has been showing fairly consistently.



I agree SJ..rain is going to be bad..i posted yesterday that NC will catch it bad due to all the rain we have already been having and to have a TD track over it..will not be good at all and the bad part is that we werent even asking for any rain..LOL
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Quoting Stormchaser121:
KFDM 6 Greg Bostwick pointed out Issac, and when he was asked if we here in TX should be concerned about it, he said "We will talk about it, dont be overly concerned yet but we will watch it." It was like he was hinting at the possibility of a more west track...


Those of us familiar with Greg "Aggie Boy" Bostwick know to listen to him concerning SE Texas and Golden Triangle weather. I don't get real concerned until he does. No hype or ratings grab for him, just clear analysis of the weather that affects his viewing area.
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Quoting Patrap:
While I'm packing my Scuba gear for Cancun, er that 21/18Z Graph in #553 is yesterdays 18Z Run.

Checks Passport, wu ID

Pat... Was out last weekend... Seems we are being inidated with Lion Fish here in northern Palm Beach county... Saw a few also on the bay side a month ago in Islamorada and Key Largo area... Very scary.
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Quoting Tribucanes:
Cantore was just on the nightly news. He said until a center can be determined initialization of a true path will be difficult. He also said this will be a large storm and a different beast in the coming days. He didn't push the panic or doom button but he did take a step towards it. He said there are three possible centers. NHC going off the one in the middle of the two blobs. He did not use the word blob. :)

Well did he say where he'd be going next?
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Quoting ncstorm:
Oh boy!!! 240 hours with the 18z GFS..the entire SE



And THAT is the severe threat with what the GFS has been showing fairly consistently.

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Quoting serialteg:
Drama on the blog?


always..and it aint the women
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Quoting coffeecrusader:
Isaac looks like he has 2 heads



Looks like it! We will have to see later what the imagery produces. Looks like a center that has regenerated further south and just passed the islands.
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Oh boy!!! 240 hours with the 18z GFS..the entire SE

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Quoting StormJunkie:


And at this point, I see that as being the biggest danger with Isaac. Unless either the EMCWF verifies, it manages to miss the mountains (highly unlikely), or it gets off the E coast of Fl and tracks towards NC.


Indeed. Center is on the west coast for the ride but it looks right loaded. Winds still wouldn't be too bad on the east coast but 30 hours of constant rain.
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Quoting MAweatherboy1:
Did anyone else find it interesting that in the model skill graph Doc posted the long range skill of the BAMM, at least in 2011, was as good as that of the GFS and TVCA and not far off of the OFCL and ECMWF? I usually disregard the BAM suite as trash but I thought that was pretty interesting.


The BAMs are very simple models that can run in 30 minutes, but occasionally a season will come around where the baroclinic flow on average is relatively "simple", and these models can do a quite good job forecasting storm tracks at times.
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by the looks of it NOAA's P-3 is diving into the center of Isaac look like it about to hit HH RECON newest center fix
Member Since: June 13, 2009 Posts: 2 Comments: 12716
Drama on the blog?
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The NOAA plane is finding 50mph winds...
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Quoting Drakoen:
Some day you will learn when you have a real job...there is a right and a wrong. The engineer cannot miss the minus sign or the bridge collapses. I'm gonna take a little break now.
The environment is different. Everything has an influence on everything. You can not discount anything. This isn't a bridge where we are analyzing different loads. It is obvious that the low to mid levels isn't perfectly moist just as it is obvious outflow and divergence is limited to the north and northeast as seen on wv. Don't be afraid of the water vapor loop.

Anyway, later drak, kinda disappointed to see that immature side come out again. Saw it a lot reading back on blogs in 08 and 09, was hoping that was over.
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Quoting Patrap:
While I'm packing my Scuba gear for Cancun, er that 21/18Z Graph in #553 is yesterdays 18Z Run.

Checks Passport, wu ID



have a good trip stay safe
Member Since: July 15, 2006 Posts: 178 Comments: 55983

KIRK @ 324 hrs
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Quoting aislinnpaps:


I've heard that the sharkburger in Trinidad is fantastic. One of my son's has that on his bucket list.

Shark&Bake.....

Still no rain here to talk about.
I have to go to the airport to meet a 11:10pm flight.
It's bound to be a bad night, even if it stays dry........
Member Since: October 24, 2005 Posts: 0 Comments: 24873
Quoting lottotexas:


KIRK @ 252 hrs
Lol.Leslie in the making the wave train is in full action.Accuweather's forecast is about to get blown.
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Quoting stormpetrol:


Recon fixes equal a WSW movement for now, argue with Recon and WKC, not me :)) J/K.
stormpetrol.... The saying is "Don't shoot the messenger"
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Quoting clwstmchasr:
I'm wondering if this could pull an Ivan on us (not the strength but direction). Everything pointed to Ivan running up the spine of Florida or the West Coast. Subtle changes happened and it shifted significantly West and hit Pensacola.


....and Grand Cayman on the way...
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Kirk?? I am going to have to quit teaching or retire soon as I have no idea we even had Joyce or where she went!
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Easy there Kirk.

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Quoting aislinnpaps:


I've heard that the sharkburger in Trinidad is fantastic. One of my son's has that on his bucket list.
Fried shark is the best if you ever get a chance to try it.
Member Since: August 31, 2009 Posts: 0 Comments: 5628


Recon fixes equal a WSW movement for now, argue with Recon and WKC, not me :)) J/K.
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Quoting NYCyclone86:
IF Isaac were to clip south west Florida, those in Central and East Florida would be getting the worst of whatever is left of him... And I say left of him due to the land interaction. Again, this storm can still go out to see, friction with Haiti with a weird jog or two can start a new ball game. Wait and see, and lets hope he continues to struggle.
Be careful. There is still the real possibility that Isaac may not have that much land interaction at all before reaching the gulf, and with a sudden change in your direction. And more than enough heat to crankit up before hitting you.
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Did anyone else find it interesting that in the model skill graph Doc posted the long range skill of the BAMM, at least in 2011, was as good as that of the GFS and TVCA and not far off of the OFCL and ECMWF? I usually disregard the BAM suite as trash but I thought that was pretty interesting.
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Quoting ProgressivePulse


UGG, looked like about 30hrs in my location on the last GFS.


And at this point, I see that as being the biggest danger with Isaac. Unless either the EMCWF verifies, it manages to miss the mountains (highly unlikely), or it gets off the E coast of Fl and tracks towards NC.
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Quoting TXCWC:
I am NOT saying is going here but just noticed a couple of Ensembles incrediblly going to TEXAS now. More important perhaps though the TVCN track is saying Florida Panhandle.

Link


Yeah one of my local guys said the high over us may move eastward. Maybe that's what the EURO difference is? Anyway in his update he gave the official track with this caveat

Keep in mind, a very reliable model takes the system much further west than this, and has Isaac as a powerful hurricane striking the New Orleans area by next Wednesday.
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KIRK @ 252 hrs
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Quoting StormJunkie:
Well thanks for the complement on my "previous" self mcluvin...lol

Again, those mountains, poor current structure, proximity to Fl will all have an impact on this system and what it is capable of becoming. Right now the GFS concurs that this will be a weak wind field as it approaches Fl.


Seriously doubt it will even track over the mountains at this point
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If the storm breaks up a Fujiwhara will form, maybe with similar effects:

Typhoon No.17 in 2009 (T0917) caused severe damage to various parts of the Indochinese Peninsula, especially in the Philippines because it remained stagnant around Luzon for a very long time with complex movement due to interaction with Typhoon No. 18 in 2009 (T0918). The interaction between two typhoons is called the Fujiwhara effect. We clarified the process of the interaction between T0917 and T0918 by using satellite images and typhoon tracks. The southward movement of T0917, which is a typical consequence of the Fujiwhara effect, was observed. Link
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Quoting Michfan:
Drak gets like this every once in a while during a heated debate. Some people have to be right all the time and/or can't admit when they are wrong. When you grow up you will learn humility eventually. Even the weather experts get it wrong once in a while. We are all here to learn and experience the science of hurricanes. Enjoy it.

On a happier note it will be interesting to see which center takes control and could effect the long term track a bit. Issac is going to end up a doozy.
Meet the new boss, Same as the old boss
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Quoting Patrap:
er, while I'm packing my Scuba gear for Cancun, er that 21/18Z Graph in #553 is yesterdays 18Z Run.


Checks Passport, wu ID


I've heard that the sharkburger in Trinidad is fantastic. One of my son's has that on his bucket list.
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Quoting TropicalAnalystwx13:
Another depiction of the 12z ECMWF.



That looks disgusting...been there done that...I sure hope he stays an outlier.....
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264 hrs. and future Kirk looks like he wants to pay the East Coast a visit.

Member Since: August 31, 2009 Posts: 0 Comments: 5628

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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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