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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2001. HrDelta
Quoting trey33:


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.


Accuweather failed both ways in 2005. They made predictions that Katrina would go into Florida, while bashing the NHC.

FEMA used the NHC, but the Department of Homeland Security used Accuweather. As a result, at least 30 people died in the catastrophic week after Katrina.

Then they hyped up Rita hitting Houston, causing a hysteria induced evacuation. Killing somewhere between 90-117 people.

There are many companies I get ticked at for various reasons. The Super-Banks, Big Oil, and that company that sucked up Union Carbide (Bhopal). But no company on the planet makes me more angry than Accuweather. For the inaccuracy of reports, then lobbying to sabotage the NWS. For Katrina and Rita, I wish Accuweather could be sued.

I f---ing hate Accuweather. With the fury of a thousand suns.
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2000. wpb
Quoting caneswatch:
The only TWC hurricane forecaster I ever trusted was John Hope.
the best
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1999. Dakster
We should be able to break a record for the number of times the advisory is posted on the blog...
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Quoting scott39:
No I dont mind and yes I live in Mobile.... as you can tell Im a little anxious. My mind lights up when I see models trend westward and the EURO bombing in the GOM. I went through Major hurricane Fredric when I was 10 years old. That kind of trauma doesnt wear off. So if I phych myself up to a possible something... I will be ready.....if not then thank God I missed another one. Thats how I deal with it.


Yet you were just saying the other day that you wanted to get hit. Sorry, but I don't buy this sort of facade you're pulling now.
Member Since: March 7, 2007 Posts: 553 Comments: 19890
Quoting mynameispaul:
Does NHC have any predictions on how large Isaac could get?


Look under "Forecast Advisory". They give wind field estimates near the bottom of the page.
Member Since: March 7, 2007 Posts: 553 Comments: 19890
Quoting tornadolarkin:
Jim Cantore and Greg Forbes are the only ones from TWC I actually respect.


Norcross is excellent. Cantore is good but overdramatic... I have all the respect in the world for Dr. Greg Forbes.
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Quoting wunderkidcayman:


-9999999999999
I am not wish casting at all


Don't deny it. I don't post much on here, I mostly lurk. And if there is one thing that I have noticed, it is that you are a wishcaster. I am done arguing.
Member Since: June 18, 2010 Posts: 0 Comments: 528
Ugh, Ike, followed twelve days later by Gustav. I don't ever want two hurricanes hitting me less than two weeks apart again.
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looks to me that Isaac has slowed down in the last few hours.
Member Since: August 6, 2007 Posts: 0 Comments: 693
1991. scott39
Quoting StormJunkie:


Mind if I ask if your "humble opinion" happens to be because you live in Mobile?
No I dont mind and yes I live in Mobile.... as you can tell Im a little anxious. My mind lights up when I see models trend westward and the EURO bombing in the GOM. I went through Major hurricane Fredric when I was 10 years old. That kind of trauma doesnt wear off. So if I phych myself up to a possible something... I will be ready.....if not then thank God I missed another one. Thats how I deal with it.
Member Since: June 13, 2009 Posts: 0 Comments: 6730
2 planes reporting now. NOAA & Air Force.
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Jim Cantore and Greg Forbes are the only ones from TWC I actually respect.
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1987. Dakster
To me it is amazing how many times Bermuda is the cone for a storm...
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Rapid scan satellite just started.
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Quoting galvestonhurricane:


+10000000000 wunderkidcayman is absolutely notorious for wishcasting every storm in his own direction. Ridiculous.


-9999999999999
I am not wish casting at all
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Quoting ecupirate:


Stop it man, just knock the darn wish casting off.
The lowest pressure is around 16N/62W...
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Does NHC have any predictions on how large Isaac could get?
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Quoting sporteguy03:

A smidge maybe West but not much though. Maybe 25-50 miles.


I agree. Either no change or very slightly west.
Member Since: June 18, 2010 Posts: 0 Comments: 528
Quoting KoritheMan:


I'm seriously considering it. And I generally have a lot of patience, and even attempt to defend some bloggers that get trounced on.

I gave up on that this week. When the Army of Trolls rolled through... which Doubled the size of my Ignore List.
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Bryan Norcross' Analysis

Status Update
By Bryan Norcross
Isaac is still disorganized tonight, but has a big circulation in the lower levels of the atmosphere. The large circulation makes it harder for a tight inner core to develop, and in addition, dry air has been pushed into the system which his always a negative factor. An inner core is required for significant strengthening. Tonight, however, a cluster of thunderstorms has developed near the center, which may be the beginning of the intensification process. The expectation continues to be that the upper winds around the storm will evolve in a way that the core IS able to develop, so Isaac is still expected to be near hurricane strength before it interacts with Hispaniola. Then the big unknowns begin. The more the core of the storm interacts with the mountains of the Dominican Republic, Haiti, and Cuba, the more disrupted it will be. (We're fearful for a terrible time for our friends on Hispaniola with an extended period of heavy rain.) The large circulation, however, will likely survive it's encounter with the high terrain, however much it is degraded. The strength of the storm at that point will play into how quickly it turns north. The latest American computer forecast model (GFS) is determined to move the a strengthening storm quickly out of the islands toward Florida. The European model (ECMWF) more slowly swings a significantly strengthening storm north of Cuba into the Gulf and toward the northern Gulf coast. Traditionally the most accurate forecast is a compromise between these two and other models, the consensus track, which would mean a strengthening storm bringing bad weather to all of Florida, and high water and the worst weather to the west coast. It's just too early to pin it down. In any case, Thursday looks like decision day on taking action in the Florida Keys with watches posted on Friday if there are no big changes. We should know more tomorrow (Thursday) and even more Friday when the NHC throws their whole arsenal at Isaac to gather data all around the storm, which gives the computer models a better starting point for their forecasts.
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1979. GetReal
Quoting GTcooliebai:
Will Isaac start pulling north because of that ULL over Cuba?



In reference to that ULL currently near W. Cuba, IMO that will have no effect on the track of Isaac. However, it is in the PERFECT location to help ventilate Isaac. The ULL is being pushed west by the anti-cyclone that is Isaac. Watch the WV over the far East Bahmas as it is being bent towards Florida the last few frames.
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Quoting MiamiHurricanes09:
I don't believe that ULL should have any major impact on Isaac's track nor intensity.
Ok thanks, must be too far away.
Member Since: August 31, 2009 Posts: 0 Comments: 5628
1977. trey33
Quoting muddertracker:


Let's do the tounge wrap agaaaaain...It's just a lick to the right....





Ok that was good!
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Quoting Jwd41190:
Less than 20 minutes or so for the next update? Anyone think the track will shift any??

A smidge maybe West but not much though. Maybe 25-50 miles.
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no shift
Quoting Jwd41190:
Less than 20 minutes or so for the next update? Anyone think the track will shift any??
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Quoting HurricaneDean07:

I ignored him a couple of days ago... Enough is ENOUGH.


I'm seriously considering it. And I generally have a lot of patience, and even attempt to defend some bloggers that get trounced on.
Member Since: March 7, 2007 Posts: 553 Comments: 19890
Quoting HoustonTxGal:
Someone mentioned Ike.. thought I would post this



September 12, 2008, a day which will live in infamy...
Member Since: June 18, 2010 Posts: 0 Comments: 528
Quoting galvestonhurricane:


I think the biggest threat area at this point is from New Orleans to Tampa Bay. But that is just my opinion - I think the models will continue to shift away from S. Florida.
Yes, probably further south
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This is for TD 10.

Member Since: August 31, 2009 Posts: 0 Comments: 5628
Quoting caneswatch:
The only TWC hurricane forecaster I ever trusted was John Hope.


Amen to that and I miss him and his bowtie.
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Quoting KoritheMan:


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.

I ignored him a couple of days ago... Enough is ENOUGH.
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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.


Not only that but basing his reasoning off Climatology models makes no sense when you look at the features surrounding Isaac.
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Quoting GTcooliebai:
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?


I was thinking about Hugo when you said that. Hugo got accelerated to the NW from ULL to it s/sw if I am remembering it right.

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Someone mentioned Ike.. thought I would post this

Member Since: September 18, 2007 Posts: 0 Comments: 1073
Less than 20 minutes or so for the next update? Anyone think the track will shift any??
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Quoting stormhank:
anyone feel a dennis track from 2005 could verify??or a fla west coast hugger up to the big bend?? thanks for any input..as possibly a cat. 1 or 2?


I think the biggest threat area at this point is from New Orleans to Tampa Bay. But that is just my opinion - I think the models will continue to shift away from S. Florida.
Member Since: June 18, 2010 Posts: 0 Comments: 528
Quoting GTcooliebai:
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?
I don't believe that ULL should have any major impact on Isaac's track nor intensity.
Member Since: September 2, 2009 Posts: 130 Comments: 21091
Totally unscientific input: A woman in my office, who is always wrong on her predictions, has declared that the storm is not going to hit south Florida at all. Based on that, I'm figuring we're all doomed.

(just kidding on the doom)

But I do feel that the models will shift eastward by Friday and south Florida is going to get quite a soaking.

As always - thank you to those who are voicing their scientific/learned impressions out here during these storms.
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The only TWC hurricane forecaster I ever trusted was John Hope.
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Yeah, there is a lot of TWC quoting this year. Things were just fine without that.
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The NHC track in red is going over the 90 degree water straits

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1957. trey33
Quoting FSUCOOPman:
Cantore just speculated "Florida Panhandle" landfall..."not my call...that's the NHC's"

He actually did a halfway decent little bit, IMO, though...


Good for Jim

(There is some sayin' about a stopped clock but I can't remember the rest....)
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1956. JLPR2
Quoting sunlinepr:


Mid level circulation is very easy to spot there, due south of the big convection.
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Quoting ecupirate:


Stop it man, just knock the darn wish casting off.


+10000000000 wunderkidcayman is absolutely notorious for wishcasting every storm in his own direction. Ridiculous.
Member Since: June 18, 2010 Posts: 0 Comments: 528
anyone feel a dennis track from 2005 could verify??or a fla west coast hugger up to the big bend?? thanks for any input..as possibly a cat. 1 or 2?
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Quoting chrisdscane:



he prolly does


He does, I was just wondering if that is what was influencing his insistence on the Euro.
Member Since: August 17, 2005 Posts: 26 Comments: 15644
Quoting MiamiHurricanes09:
Advisory on 10L is out...now we wait on Isaac.

Intensity has decreased considerably with their Forecast...
Showing 60 Mph peak at 5 days out.
Instead of 70 Mph.
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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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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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