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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2373. MBSCEOCHam 3:47 AM GMT on August 23, 2012 +0

Quoting StormTracker2K:
Well I see Levi is still stick to his track toward the Carolina's. He still has a lot to learn when it comes to forecasting it seems. If he needs help I'm here. 😱😱

I can't explain it, but I have a bad feeling on this one for the East Coast. Historically, it seems to me that as time progresses, the recurves tend to move further east/north.;;


MY Thoughts exactly. On track with Levi's solution as well. Not etched in stone, but just the general nature of storms in the past, when the curve happens. Seems to always trend to the right rather than to the west. JMO
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Levi32 - David 1979 tracked up the east coast not northern florida. Made landfall near west palm I think and tracked right up the intercoastal turning out to sea just north of the cape.
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Quoting GetReal:
Please note the location of the 12 hour GFS forecat position.







Isaac better slam on the brakes because he is already approaching where he is forecated to be 12 hours from now!

Just thinking the same thing. Expect a lot of hedging from local mets all over the GOM tomorrow AM.
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2448. KEEPEROFTHEGATE (Mod)
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When is the next recon flight?
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2446. KEEPEROFTHEGATE (Mod)
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Quoting weatherman12345:
nope it hits cuba
Dammit, MH09 can't get a forecast to verify LOL.
Member Since: September 2, 2009 Posts: 130 Comments: 21194
2444. scott39
Quoting Levi32:


It's not like I pinned it down with an elephant at each end. 3-5 day forecasts are always going to fluctuate. My stubbornness in this instance is remaining a bit east of the main model consensus.
Can you give me a short synopsis why? I missed it earlier.
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2443. KEEPEROFTHEGATE (Mod)
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Quoting chrisdscane:
http://mag.ncep.noaa.gov/NCOMAGWEB/appcontroller? pr evPage=Model&MainPage=index&image=&pag e=Param&cycl e=08%2F23%2F2012+00UTC&rname=PRECIP+PARMS& pname=pr ecip_p60&pdesc=&model=GFS&area=NAMER&a mp;cat=MODEL+GUI DANCE&fcast=075&areaDesc=North+America+-+U S+Canada +and+northern+Mexico&prevArea=NAMER&currKe y=model& returnToModel=&imageSize=M

75 hrs



sry to quote myself how do I post this as an image plzz help
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Quoting MiamiHurricanes09:
I guess the fact that the 00z run has more data fed into it helps construct a more accurate forecast, and apparently the more accurate forecast is the slightly more northerly one.

LOL, the only reason I said that is because I compared the 00z position to the 18z position at 48 hours and noticed that the 00z run was already farther north, so I figured this would be a more poleward run.


Yeah which makes no sense since the Center has relocated further south and is probably even more south than what it is initialized lol
Member Since: August 29, 2006 Posts: 22 Comments: 1352
2440. will40
Quoting StormJunkie:


As soon as we start thinking that it throws a curve at us and puts it right on the Eastern tip of Cuba...lol




uh hu i was too quick
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Issac is looking more north this run
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Quoting StormTracker2K:
One could argue Issac is almost an open wave


We actually did debate that earlier. Came to conclusion that west winds were seen on the surface, and by HH with dropsondes at lower levels. That said, we also decided that the MLC and LLC are not getting along well at all and could be misaligned by as much as a hundred miles.
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2436. Levi32
Quoting hunkerdown:

It has to do with opers...the final note of an opera are usually sung by heavier female singers. This is due to the fact that they tend to have a louder, broader range when they sing and it is the perfect way to end the show.


Hmm, interesting. Thanks lol.
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Quoting Levi32:


My track from this morning did not yet go beyond the Bahamas and does not actually show where a potential landfall would be. It could well be northern Florida or Georgia on that kind of a track, similar to David 1979.

Also, it's not over until the lady sings (don't even know what that reference is from lol).


It's an opera reference, hun. :-)
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2432. angiest
Quoting OracleDeAtlantis:
Speaking of "size .... "

This one may be the Special K of hurricanes.







It is Kirock!
Member Since: August 26, 2006 Posts: 16 Comments: 4766
2431. Levi32
Quoting scott39:
So you could move it more either way as time moves on?


It's not like I pinned it down with an elephant at each end. 3-5 day forecasts are always going to fluctuate. My stubbornness in this instance is a theme of remaining a bit east of the main model consensus.
Member Since: November 24, 2005 Posts: 635 Comments: 26700
Quoting Joshsen18:
my wife and I are traveling to cancun Mexico area for our honeymoon......are we safe? She is soooo worried figured maybe someone can give us advice? Really don't want to cancell but I want to be safe?....?
I have advice to your wife-to-be after seeing this post 100 times. Call it off and run!!!
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Beware of the Grey King!
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Quoting Levi32:


My track does not go beyond the Bahamas and does not actually show where a potential landfall would be. It could well be northern Florida or Georgia on that kind of a track, similar to David 1979.

Also, it's not over until the lady sings (don't even know what that reference is from lol).

It has to do with opera...the final note of an opera are usually sung by heavier female singers. This is due to the fact that they tend to have a louder, broader range when they sing and it is the perfect way to end the show.
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http://mag.ncep.noaa.gov/NCOMAGWEB/appcontroller?pr evPage=Model&MainPage=index&image=&page=Param&cycl e=08%2F23%2F2012+00UTC&rname=PRECIP+PARMS&pname=pr ecip_p60&pdesc=&model=GFS&area=NAMER&cat=MODEL+GUI DANCE&fcast=075&areaDesc=North+America+-+US+Canada +and+northern+Mexico&prevArea=NAMER&currKey=model& returnToModel=&imageSize=M

75 hrs
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Quoting CaneHunter031472:
I don't know about you guys and gals, but to me this look like a significant shift to the west and perhaps the usual shifting to the west trend.

.5 Shift. not very significant shift
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2425. scott39
Quoting will40:
looks like he gonna miss cuba on this run
Is he shooting between the straits?
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2424. Torgen
Quoting Levi32:


My track does not go beyond the Bahamas and does not actually show where a potential landfall would be. It could well be northern Florida or Georgia on that kind of a track, similar to David 1979.

Also, it's not over until the lady sings (don't even know what that reference is from lol).


"It ain't over 'til the fat lady sings" was coined (in the 20s or 30s?) as a reference to opera. Operas back then ended with a big song, usually from a rather large woman with lungs that could inflate the Hindenburg.
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2423. KEEPEROFTHEGATE (Mod)
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Quoting StormJunkie:


It has been for the last 4 nights or so, what should be different about tonight. Wonder why that seems to be a pattern?
I guess the fact that the 00z run has more data fed into it helps construct a more accurate forecast, and apparently the more accurate forecast is the slightly more northerly one.

Quoting opal92nwf:

Don't count your chickens until they hatch ;)
LOL, the only reason I said that is because I compared the 00z position to the 18z position at 48 hours and noticed that the 00z run was already farther north, so I figured this would be a more poleward run.
Member Since: September 2, 2009 Posts: 130 Comments: 21194
Quoting will40:
looks like he gonna miss cuba on this run


As soon as we start thinking that it throws a curve at us and puts it right on the Eastern tip of Cuba...lol

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Quoting trey33:


Surge will be way worse.


Surge....lived through Katrina and worked cleanup and housing the survivors. I also worked Gustav in LA when Ike hit 8 days later. That was truly a sad picture. Our teams ride them out with commodities...I.e...food, water, shelter. I live in MS and soooo do not want it anywhere near here.
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2418. KEEPEROFTHEGATE (Mod)
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2415. will40



60hrs
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Quoting odinslightning:
the sheer size of the broad low pressure this will consolidate together in addition to further signs of southern/western run makes me even more worried.....next 24 hrs this pulls away from the dry air and can build it's own environment.....not to mention the warmer waters from now on.....when i saw the euro run bomb this in combo with the size of the low pressure basin i was worried....i knew the intensity models were junk because it hadn't pulled together enough.....this could be catastrophic especially if Isaac completely avoids DR/Haiti.....
Speaking of "size .... "

This one may be the Special K of hurricanes.





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2413. Grothar
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One could argue Issac is almost an open wave
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Quoting StormJunkie:


Banding is much more impressive now than earlier. Seems that LLC and MLC may have finally worked out some of their differences after lots of yelling, lawyers fees, and maybe a couple of affairs...


I wouldn't get my hopes up, yet. Lol.
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2410. luigi18

http://www.weather.com/news/weather-hurricanes/at lantic-depression-storm-nine-20120817" target="_blank">Link
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Quoting TomballTXPride:

Isaac sure is a pain in the ass.

Not as much as Debby. (or at least yet!)
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2408. KEEPEROFTHEGATE (Mod)
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2407. scott39
Quoting Levi32:


I moved it 50 miles west of yesterday. Adjustments are a 99% probability of being necessary on a 5-day forecast.
So you could move it more either way as time moves on?
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Evening everyone... GFS running yet?
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2405. luigi18
Link
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Quoting Levi32:


My track from this morning did not yet go beyond the Bahamas and does not actually show where a potential landfall would be. It could well be northern Florida or Georgia on that kind of a track, similar to David 1979.

Also, it's not over until the lady sings (don't even know what that reference is from lol).
I got to give it to you as stick to your guns even if I don't agree
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2403. KEEPEROFTHEGATE (Mod)
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2402. Grothar
The GFS might go poleward a little more. Who just wrote that and how did you know? :)
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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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