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 1900hurricane:
Looks like Taiwan might be in for a double-dose of fun...



That would be something!
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Quoting chrisdscane:



he prolly does


"wunderkidmobile"
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Quoting angiest:
FWIW, hurricanealley.net has been forecasting west the last several advisories.




To me that seems to be the deadliest track. Hopefully we won't see that
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I know this is splitting hairs but thought I saw speed at 18mph on the homepage when logging in? Initial thought...."oh now its slowing to organize."
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I like the wunderkidcayman quite a bit, and his forecasting doesn't bother me in the least. That being said, Kori your dead on there in 1901. In two/four hours I think we'll be looking at Isaac as one giant ball of convection with a very clear center. It's all coming together now.
Member Since: April 18, 2012 Posts: 0 Comments: 2437
Quoting HrDelta:


I would say Ike did explode....in size. And that size in turn was able to make the storm surge larger.

I really do think that the Saffir-Simpson Scale needs to be retired. It does not take into consideration size. There should be another system, which would take into account size.


Ike didn't even have an eyewall the final 36 hrs prior to landfall.

A little too late for the collapse though.It was such a large storm, the surge energy was already in motion.
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Quoting ironbark:
i guess the texas coast is safe


The "Texas Death Ridge" is protecting us. Lol. Evening everyone!
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1943. A4Guy
A lot can change tomorrow when we have the surrounding atmosphere samples from the Gulfstream IV jet. Models will have a lot more information to use...and more likeley, a defined CoC.
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Cantore just speculated "Florida Panhandle" landfall..."not my call...that's the NHC's"

He actually did a halfway decent little bit, IMO, though...
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1941. ncstorm
I see a lot of people are quoting Jim Cantore with the west movement but this is the same man that was in Miami waiting on a "monster Nicole" to take out the city in 2010 where they maybe had one heavy downpour..
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Quoting clwstmchasr:


He's be saying South and West for five days. We can't take him seriously because not once has he suggested a path that would take it anywhere other than the Caymans. If this system tracked the way he called it, it would have died over South America two days ago.


And yet he conveniently left out that part.

Badabing, badaboom!
Member Since: March 7, 2007 Posts: 575 Comments: 20571
Quoting StormJunkie:


Mind if I ask if your "humble opinion" happens to be because you live in Mobile?



he prolly does
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1938. scott39
Quoting will40:


you stating that just because of one model?
Doesnt the GFS show a Major too?
Member Since: June 13, 2009 Posts: 0 Comments: 6774
1937. WxLogic
Quoting MiamiHurricanes09:
Look on satellite imagery, you can see the poleward outflow channels expanding a little bit as well.


Correct and a slow down should follow with a WNW displacement taking shape soon.
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1936. JLPR2
>:\ ASCAT missed, only a little bit of the eastern half. It does look like it could be a rather large circulation.

Member Since: September 4, 2007 Posts: 7 Comments: 8690
Advisory on 10L is out...now we wait on Isaac.
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will the NOAA jet info change the track on the models?
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Quoting MiamiHurricanes09:
Look on satellite imagery, you can see the poleward outflow channels expanding a little bit as well.
Hey Miami what are your thoughts about that ULL over Cuba could that have some influence on the short term steering and would that be more north or west?
Member Since: August 31, 2009 Posts: 0 Comments: 5628
1930. scott39
Quoting chrisdscane:



models shifted east look at post 1870
They are on Fl. or the W side...this means a trend to the W,compared to where they were 24 hours ago.
Member Since: June 13, 2009 Posts: 0 Comments: 6774
Quoting PRweatherWatcher10:
Well my house is without power or water already!
Where in PR are you?
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Quoting wunderkidcayman:
I think Isaac will end up tracking like what the CLIP CLP5 and the LBAR is showing

Isaac continues to look good and further S and W
last I heard even Jim Cantore was was just talking about this

but I certainly say its near 15.2N 63.3W

with possible relocation to where Jim was saying I think it was somewhere near 14N

I don't know but I will stick to 15.2N 63.3W


Stop it man, just knock the darn wish casting off.
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1927. will40
Quoting scott39:
Are you telling me that the EURO doesnt have a major hurricane in the GOM?


you stating that just because of one model?
Member Since: September 19, 2005 Posts: 2 Comments: 4227
Quoting chevycanes:

shift west?

latest runs at 00z.



NHC was not on the left side of the guidance earlier today. Based on the models listed on this map, no, they shifted east.
Member Since: August 19, 2005 Posts: 5 Comments: 5396
Quoting scott39:
The NHC Cone will shift more W and then N. Isaac will then bomb in the GOM...IMHO :)


Mind if I ask if your "humble opinion" happens to be because you live in Mobile?
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Guess I'll wait for the new advisory before writing my blog.
Member Since: March 7, 2007 Posts: 575 Comments: 20571
with most of the models having the storm taking a north turn by now. how far west do you think the models will shift. isaac has def not made any north movement, if anything the center seems to be just south of 15*
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Quoting Levi32:
The ridge to the northwest of Isaac is starting to weaken a bit. 500mb geopotential height is down from 5900m at 12z to 5880m at 00z at Santo Domingo, DR.
Look on satellite imagery, you can see the poleward outflow channels expanding a little bit as well.
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Quoting FLPandhandleJG:



Anticyclone still off to the west... If Isaac, can attempt to slip under it... Hurricane status will be much easier to achieve over the next 72 Hours.
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Quoting clwstmchasr:
I think it is beginning to look like the Tampa shields are at work again. It would not surprise me at all that Isaac misses Tampa by 200-300 miles to the West. The trend has been shifting slowly westward for two days and I don't see any signs of it stopping. I'm certainly not giving the all clear yet but trends are trends and can't be ignored. Jim Cantore is expecting the models to shift further west.
Like I mentioned, if Tampa is in the early crosshairs, it usually dodges the bullet. Irene had dozens of direct FL model hits, and we know what she did.
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Quoting MrstormX:


Exactly what the NOAA flight noted, this is a very shallow and primitive system.
i agree however seems to be ingesting lots of moister. fuel for the future ?
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Hurricane by tomorrow i say
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1917. angiest
FWIW, hurricanealley.net has been forecasting west the last several advisories.

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Caribbean waters... new scenario...



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Quoting Tropicsweatherpr:
Look way down.





40 frame shows it better.
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1914. trey33
Quoting Clearwater1:
Gusty for a Tampa met to say he was "optimistic", unless he has the inside scoop on the 11 o'clock report.


Same station (different met) that said Charley was the "worst possible scenario" for Tampa. Everyone freaked out, evacuated, in panic and then it went south and they were the last station to report the turn. That met retired not long after. The met who said tonight's statement is better, but I think the station needs to have a training seminar on the open mouth, insert foot theory. And yes, Charley was supposed to be a direct hit, I'm not arguing with the forecast so much as the adjectives used to hype it up.
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1913. KEEPEROFTHEGATE (Mod)



CI# /Pressure/ Vmax
3.5 / 994.8mb/ 55.0kt


Final T# Adj T# Raw T#
3.2 2.8 2.6

Center Temp : -26.4C Cloud Region Temp : -34.9C

Scene Type : CURVED BAND with 0.38 ARC in MD GRAY
Maximum CURVED BAND with 0.40 ARC in MD GRAY
at Lat: 15:20:24 N Lon: 62:19:48 W

Member Since: July 15, 2006 Posts: 171 Comments: 53815
Quoting moonlightcowboy:
SFC MAP shows the conus/BA highs are bridging with the eConus trof weakening and lifting out, increasing the 1016mb isobar over Isaac and westwards into the GoM all the way to the Yucatan at 20n.

Isaac's current disorganization and fast forward speed westwards at 20 mph, along with still mixing some dry air, will like continue its present motion more westwards. That will likely continue now for 24-36 hours - just will not organize that fast. All of which supports the drift westwards of many of the models, but even more so of the ECMWF.

It's a much larger system than Ernesto, a very broad circulation, but all the critical similar detrimental effects are the same: fast forward speed, dry air still, proximity to land (not as bad). The key will be forward speed. It cannot get vertical and mature at that clip - just doesn't happen. If it keeps that up, we're looking again at 75-80w before this thing really gets cranking.


Nods head

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Quoting scott39:
The NHC Cone will shift more W and then N. Isaac will then bomb in the GOM...IMHO :)



models shifted east look at post 1870
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Quoting trey33:


he really said tongue wrap?


Ewww...just....Ewww lol
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Quoting Jwd41190:


That looks more east to me than west but maybe I am not seeing everything right...

which is why i posted that. some keep saying the models are trending west, when there not really trending that way.

for the person who said ships model must take it more west, it does not from looking at the model posted above by washingaway.
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It is so going to S FL
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Quoting chevycanes:

shift west?

latest runs at 00z.



Actually the 00z runs that are shown on there shifted slightly E. Namely the BAM models. Very slight shift though.

One way or another, I know I will be glad when either the EMCWF moves over, or every other model goes to meet it. MLC is the only one providing any sort of data and explanation of why he thinks the EMCWF will verify. Personally don't see it, but he is looking at it more from a data perspective where as I am putting faith in model consensus.
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Look way down.

Member Since: April 29, 2009 Posts: 75 Comments: 14253


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Quoting trey33:


ok good i feel better
you wouldn't believe the thoughts that ran thru my brain


you made me chuckle with that keen observation of Jimmy
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Quoting wunderkidcayman:
I think Isaac will end up tracking like what the CLIP CLP5 and the LBAR is showing

Isaac continues to look good and further S and W
last I heard even Jim Cantore was was just talking about this

but I certainly say its near 15.2N 63.3W

with possible relocation to where Jim was saying I think it was somewhere near 14N

I don't know but I will stick to 15.2N 63.3W


You know, throughout your long tenure here, not once have I ever heard you call for a center reformation that didn't put the cyclone closer to you... And you wonder why people don't take you seriously. It's your fault, and no one else's. Sorry if this seems mean, but enough is enough.
Member Since: March 7, 2007 Posts: 575 Comments: 20571

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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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