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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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
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. :)
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Quoting TropicalAnalystwx13:
Another depiction of the 12z ECMWF.

Pretty colors.
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Quoting nrtiwlnvragn:


Kermit's mission is not center fixes, so they would not necessarly be trying to find one. They are doing Tail Doppler Radar, and I think they are on leg 3 to 4.



really cause so far that is not the flight track

Quoting MAweatherboy1:
The two blobs are pretty much completely separate now:


yeah kinda looks that way hmm
Member Since: June 13, 2009 Posts: 2 Comments: 12566
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.


Good point, maybe the Euro and GFS will compromise towards the panhandle.
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Quoting hydrus:
Talk about looking all too familiar.


No S#$t.
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Quoting ProgressivePulse:
Evening All.


XTRP is spot-on.
Member Since: July 7, 2010 Posts: 0 Comments: 5725
Quoting TropicalAnalystwx13:
Another depiction of the 12z ECMWF.

Talk about looking all too familiar.
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Quoting Sfloridacat5:


Okay, then I don't understand the comment.
He was looking for the yearly comments
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Quoting breald:
More rain for the Southeast. What are the odds of this thing missing FL all together?


As of right now, the odds of Isaac TOTALLY missing Florida and have NO impact at all on the state are slim.

Tomorrow those odds may change.
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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.
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Another depiction of the 12z ECMWF.

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TD10 starting to trend more westerly and not hook out to sea.

Also... Isaac models "feeling" the future Trough in the neighbourhood and starting to trend it more North now. It could possibly make a famous "hook" or "right turn" into Tampa Bay. Gawd forbid, of course. :)
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Quoting StormJunkie:


Absolutely, and it looks to be a fairly slow mover in the long term. Not down playing it at all and its potential impacts on the SE; just saying I am finding it hard to come up with ways in which it could be even a Cat 1 when it approaches the CONUS. Also pointing out that the 18z GFS does not show a "strong" tropical system. That said, things change quickly in the tropics as we all know. To top that off, I've been wrong before and will do it again.


UGG, looked like about 30hrs in my location on the last GFS.
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Quoting MAweatherboy1:

I mean drawing a straight line in the direction a storm is moving! Brilliant!

Quite the computer model! It's incredible, I've never seen anything like it...
Member Since: July 7, 2010 Posts: 0 Comments: 5725
While I'm packing my Scuba gear for Cancun, er that 21/18Z Graph in #553 is yesterdays 18Z Run.

Checks Passport, wu ID

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Quoting breald:
More rain for the Southeast. What are the odds of this thing missing FL all together?

It's big enough that even if it stays well off the FL coast some rain will work in... Very unlikely FL gets away with no impacts at all.
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What I love is when someone says "my forecast" and they are just going right down the middle of the models forecast.
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Quoting hydrus:
I did the math, and my deductions conclude that daisies only make up approximately 2 septillionths of the Earths mass. Hence your 4th statement is more than accurate..:)
Actually I didn't want to get too involved with the flora and Fauna, but here is south Florida I have quite an array of beautiful orchids...(And every kind of bug and critter imagineable)
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Quoting KEEPEROFTHEGATE:
can't see my house lol to much color
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Evening All.

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More rain for the Southeast. What are the odds of this thing missing FL all together?
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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.

SG03, agreed that he (thewindman) did not have the best approach on trying to make his point.
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Quoting mcluvincane:


Don't quit your day job
too dry in late august really ???? lol
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Quoting WeatherNerdPR:

Can't deny the XTRP's infallible knowledge!

I mean drawing a straight line in the direction a storm is moving! Brilliant!
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Quoting StormJunkie:


Absolutely, and it looks to be a fairly slow mover in the long term. Not down playing it at all and its potential impacts on the SE; just saying I am finding it hard to come up with ways in which it could be even a Cat 1 when it approaches the CONUS. Also pointing out that the 18z GFS does not show a "strong" tropical system. That said, things change quickly in the tropics as we all know. To top that off, I've been wrong before and will do it again.


I agree SJ completely with you and how you feel the mountains will effect it and being a weaker storm. The poster that you mentioned thewindman seemed to be downplaying it saying people will be happy with wind and rain, right now I doubt Florida wants or needs either. Nothing wrong in being wrong we all are but you do provide good insight always :)
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Quoting Hurricane1956:
I'm sorry I means STORMW


Permabanned...

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Quoting PalmBeachWeather:
He knows that Sfl.....


Okay, then I don't understand the comment.
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I'm not sure if I've ever seen the ECMWF be such a maverick before and consistently predict so far away from consensus. Would be quite a coup for the Euro if it ended up being correct.
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Quoting MAweatherboy1:
The XTRP says Belize... That's pretty much case closed.


Can't deny the XTRP's infallible knowledge!
Member Since: July 7, 2010 Posts: 0 Comments: 5725
In the immortal words of Rodney King.

Can't we all just get along?
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Quoting PalmBeachWeather:
Levi, You do a great job.... I respect your forecast.....Please don't let this upset you.... The world isn't all daisies
I did the math, and my deductions conclude that daisies only make up approximately 2 septillionths of the Earths mass. Hence your 4th statement is more than accurate..:)
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Quoting Sfloridacat5:


Xtrap is the current dirction the storm is moving.
He knows that Sfl.....
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Quoting aislinnpaps:
Just curious, though I'm almost afraid to ask. If Issac continues to maintain TS status and continues to drift west at the speed it is going now, is that what the ECMWF is seeing as a more westerly track rather than turning north?


That and a stronger ridge in place over Florida...
Member Since: July 4, 2005 Posts: 204 Comments: 8898
Quoting PalmBeachWeather:
MA... Are you looking for the annual comments about the XTRAP? LOL

Yes :)
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Quoting MAweatherboy1:
The XTRP says Belize... That's pretty much case closed.



And on that note, I'm off to bed :P

Night all.
Member Since: August 10, 2010 Posts: 2 Comments: 1971
Quoting Levi32:


000
URNT12 KNHC 222207
VORTEX DATA MESSAGE AL092012
A. 22/21:44:50Z
B. 15 deg 44 min N
061 deg 25 min W

C. 850 mb 1470 m
D. 27 kt
E. 131 deg 90 nm
F. 073 deg 13 kt
G. 278 deg 13 nm
H. 1005 mb
I. 18 C / 1525 m
J. 19 C / 1524 m
K. 18 C / NA
L. NA
M. NA
N. 1345 / 8
O. 0.02 / 0 nm
P. AF307 0309A ISAAC OB 10
MAX FL WIND 40 KT NE QUAD 19:17:30Z
thanks levi
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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

I suppose with the convention a transient flap had to be made about FL.....
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Quoting MAweatherboy1:
Don't ignore the consistency of the GFS:

Never do the GFS is always and will always be the model I follow. Just my preference but the GFS is pretty acurate... Look what happened with debby. GFS was right
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Quoting MAweatherboy1:
The XTRP says Belize... That's pretty much case closed.

MA... Are you looking for the annual comments about the XTRAP? LOL
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Quoting MAweatherboy1:
The XTRP says Belize... That's pretty much case closed.



Xtrap is the current dirction the storm is moving.
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Quoting Tazmanian:
the longer TD 10 stays weak the more it gos W and a less of a ch of going out too sea




A few models bring it towards the east coast in fact Taz. Shear from Isaac may keep it in check for a little while.
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we all come here for the same reasons
and if we take all that everyone posts
we can come to a reasonable consenus
of the outcome

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The XTRP says Belize... That's pretty much case closed.

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Quoting Tribucanes:
when you have a real job??? Really Drakoen? A debate by definition is a respectful back and forth. When one gets to being condescending and nasty the debate has been lost whether or not they were right. Let me say I've never seen Drakoen like this before and I think he and Levi are two of the very best. There was just absolutely no need at all for that debate of ideas to have taken a dive off the bridge. Been amazingly polite and educational in here all day. Certainly many have differing ideas and were respectful of other's thoughts today. If others disagree with me that he was being mean and condescending; I respect that opinion. :)
Trib. I agree with....... Totally uncalled comment here
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Isaac looks like he has 2 heads
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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.