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 dfwstormwatch:
Link

My forecast for Issac


Seaside, FL....the rich folks'll luv ta for that!!!
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RECON flew bcak NE then W then SE seems like it just took a quick jog WNW-NW how ever I think this is just a jog should continue it past movemenst
Member Since: June 13, 2009 Posts: 2 Comments: 12334
I wouldn't expect a significant shift from the NHC in either direction, although they will probably be a little more confident with the shift in the Euro.
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Large Spread

Image and video hosting by TinyPic
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Link

My forecast for Issac
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Quoting cntrclckwiseSpenn:


it's been on a quick pace for awhile now. most storms intensifying storms slow down in order to intensify or re-intensify. the trade winds also start slowing down, the water gets a lot warmer so this system will balloon should it graze the islands.


I know that mane..
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The 00Z ECMWF has come further east with a forecast landfall shifted from the New Orleans area in the last 12 Z run to Mobile Bay in this latest run.

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Quoting wunderkidcayman:


ok BOC yeah and guys on here say that I'm wishcasting

now this is really wishcasting you must be joking really BOC


Yes I am joking.......I'm also on my 3rd (4th?) glass of Wild Turkey 101 :-D :-D :-D
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Quoting FLPandhandleJG:


Actually I think Isaac is refiring those T'storms and should start get more explosion.. Then eventually in the next 12hrs should def start slowing down a bit.. to get better organized and hopefully we will get a better idea of where Mr. Isaac wants to go visit..


it's been on a quick pace for awhile now. most storms intensifying storms slow down in order to intensify or re-intensify. the trade winds also start slowing down, the water gets a lot warmer so this system will balloon should it graze the islands.
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I c a lil green on here.. starting to fire up maybe.. we will c!
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Quoting wunderkidcayman:
RECON think they missed it now heading N current location SW of last vort fix


Temporary wobble/reorganization to the south west. Many thought it was on a north west trend a day or two ago, but it's not the overall movement component.
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Quoting Kowaliga:
OK---FOR REAL: who wants to prog this onto the BOC 144HRS???



ok BOC yeah and guys on here say that I'm wishcasting

now this is really wishcasting you must be joking really BOC
Member Since: June 13, 2009 Posts: 2 Comments: 12334
Convention blowing up once again west of the center.

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Isaac is playing that game of u see me now and now u don't game.. Go Figure!!!
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3087. Dunkman
Quoting wunderkidcayman:
RECON think they missed it now heading N current location SW of last vort fix


I imagine those guys are pretty annoyed at Isaac right now. The first hint of a wind shift was at 14.3N.
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OK---FOR REAL: who wants to prog this onto the BOC 144HRS???

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Quoting STSUCKS:
My thinking is with the storm not strengthening at a rapid pace and the center reforming to the south we will have a weak to moderate tropical storm moving thru the Caribbean south of PR, the DR and Haiti..it could begin to strengthen to a strong tropical storm as it skirts the north coast of Jamaica and starts a more WNW movement and moves thru the Cayman Is. it starts to intensify into a hurricane and makes landfall in western Cuba..starts to turn NW and intensify rapidly as in traverses the central Gulf of mexico to a cat 3 and starts to turn due north and could make landfall as a cat 4 with 130-140 mph winds along the northern gulf coast..between the mouth of the MS River (Venice, Boothville LA.) and Pensacola, FL.

hmm seem reasionable maybe lets see
Member Since: June 13, 2009 Posts: 2 Comments: 12334
3084. emguy
Quoting GTcooliebai:
So you're not buying Pensacola and points west, you see more of a profound weakness or a turn earlier from the weaker subtropical ridge?


That's right. We may see a track that looks different that what we thought, but it is still a gradual sweeping turn into the weakness by a large storm. In fact, I'd go as far as to say Apalachicola would be the westernmost extent for a landfall point for Isaac, but any ladnfall should occur on the west coast of Florida.
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RECON think they missed it now heading N current location SW of last vort fix
Member Since: June 13, 2009 Posts: 2 Comments: 12334
3082. STSUCKS
My thinking is with the storm not strengthening at a rapid pace and the center reforming to the south we will have a weak to moderate tropical storm moving thru the Caribbean south of PR, the DR and Haiti..it could begin to strengthen to a strong tropical storm as it skirts the north coast of Jamaica and starts a more WNW movement and moves thru the Cayman Is. it starts to intensify into a hurricane and makes landfall in western Cuba..starts to turn NW and intensify rapidly as in traverses the central Gulf of mexico to a cat 3 and starts to turn due north and could make landfall as a cat 4 with 130-140 mph winds along the northern gulf coast..between the mouth of the MS River (Venice, Boothville LA.) and Pensacola, FL.
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According to all the Hurricane Hunter data I've seen, this doesn't deserve to be classified as a tropical storm anymore. Haven't seen any surface winds above 30 mph. Will be interesting to see if the Hurricane Hunters find any changes in their ongoing next pass. If not, then it will be interesting to see how the NHC handles the hurricane hunter data, none of which thus far has supported more than maybe a tropical depression at the moment.
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ok recon has passed over the vortex message plot and it not there looks like it is moving SW via RECON's winds
Member Since: June 13, 2009 Posts: 2 Comments: 12334
Quoting Tribucanes:
Calling Isaac a tropical storm is not generous. 45mph sustained winds easily now. Isaac is no TD. Isaac was a mess, but has clean up nicely. He's now going to assert himself into a cane tomorrow. I'm thinking there is a 75% chance of hurricane Isaac tomorrow. He's behaved just as I thought he would. I hope I'm wrong, but Isaac may be a real killer for Haiti.


There will be flooding yes, but it's not the worst storm Haiti will have. It will be a mild storm with severe effects as usual.
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Quoting Grothar:


16N 62.5W per that satellite view? no?
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Quoting GTcooliebai:
Oh I totally agree with this, especially if it takes the southern route. Anyone know if HH confirmed the center relocation at 14.7 N?

vortex message was sent out now RECON is making another pass as we speek
Member Since: June 13, 2009 Posts: 2 Comments: 12334
Quoting GTcooliebai:
Oh I totally agree with this, especially if it takes the southern route. Anyone know if HH confirmed the center relocation at 14.7 N?


Actually I think Isaac is refiring those T'storms and should start get more explosion.. Then eventually in the next 12hrs should def start slowing down a bit.. to get better organized and hopefully we will get a better idea of where Mr. Isaac wants to go visit..
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hey Dunkman I this it is quite definitive because atleast this time it matches with the mid level spin on sattelite yeah wind don't look all that great at the moment but that should be no more problem cause it can now quickly intensfy more than before
Member Since: June 13, 2009 Posts: 2 Comments: 12334
Don't call my doc gro,I am just behind on the blog,back on page 54. I don't wanna miss a single post. Hope I catch up before ya gotta hit the hay. I will quit posting and try and catch up. I thank everyone of you for all your posts.

:)
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Dully noted Dunkman. Winds may well have not caught up to the organization. This is a big system.
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3072. Dunkman
Quoting Tribucanes:
Calling Isaac a tropical storm is not generous. 45mph sustained winds easily now. Isaac is no TD. Isaac was a mess, but has clean up nicely. He's now going to assert himself into a cane tomorrow. I'm thinking there is a 75% chance of hurricane Isaac tomorrow. He's behaved just as I thought he would. I hope I'm wrong, but Isaac may be a real killer for Haiti.


Well this HH mission has sampled 3 of the 4 quadrant right now and found exactly 0 TS winds. VDM claimed a max of a whole 21 knots.
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Even though it has not been the best model for Tropical Storm Isaac, it's very interesting to note that the 00Z CMC (GGEM) run showed the center moving to the SW and at this time. Well initialized and predicted by the CMC at least in the first few hours of the run.
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Quoting HurricaneHunterJoe:
This is great entertainment for a great price! My DSL is only $14.95 a m0nth.



COOOL!!!!!!!!!
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Good early morning, everyone.

Power was just restored and now I'm trying to catch up with what's been happening the last few hours in here.

Lindy
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look at the huge blow in the last frame here...
he better start this gfs north movement soon or he might shoot the gap between jamaica and yucatanLink
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Grothar is exactly what Grothar means.
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This is great entertainment for a great price! My DSL is only $14.95 a m0nth.
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Quoting HurricaneHunterJoe:
Have the West vs East Casters shown up yet? Or the Texas vs Florida Crowd........No that was Debby,Right? History has a way of repeating itself. I have a feeling this is going to build to a crescendo.


Does it really matter and who cares about the Wish Casters.. U may like playing that game but that game is old.. Just ignore the ones that do play the game.. Thx
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Quoting FLPandhandleJG:


It should rebound and refire more T'storms in Isaac as keeps heading into warmer waters and getting away from those minor setbacks that Issac has/is experiencing off and on..
Oh I totally agree with this, especially if it takes the southern route. Anyone know if HH confirmed the center relocation at 14.7 N?
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Quoting wunderkidcayman:
location on vortex message

14°43'N 63°30'W (14.7167N 63.5W)

by the way which one should we use the one in Barckets or the onee that is not


The first coordinates are the coordinates in degrees and minutes. For plotting purposes, use the second one.
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Quoting cchsweatherman:


I wouldn't be so confident in ruling South Florida out yet. Considering the immense size of the storm, impacts will be felt over a large area around the center, so even if the storm doesn't make landfall in South Florida and goes in the Gulf of Mexico, South Florida could see bands from the backside of the storm.


Bands are a far cry from a landfall, though.
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Quoting 7544:


so would that put south fl out of the pic


yes and the keys as well I did some calculations with current forecast plots and move them to current fix and at the end it is W of Fl and the keys maybe just skirting the exterme W key
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Quoting FLWaterFront:
It is going to really shake things up in the world of hurricane computer forecast modeling if it turns out that the Euro has been right all along with this one.

Earlier this season it was the GFS that was insisting that TS Debby was going to head for the NE GOM and impact Florida while every other major model was forecasting an entirely different solution. Now it is the ECMWF that has been insisting upon a major striking the Central Gulf Coast while all the other models have been tightly clustered around the Florida peninsula for days.

Too early to say that the Euro is right and everyone else is wrong, and it still may be that the Euro is a bogus outlier after all. But somehow, one gets this uneasy feeling that this may not be the case. We'll see.


I was never on the east coast recurve bandwagon. I thought and still think this is a GOM storm. The Euro may win out in the end, but while the high is strong now, and the storm is weak moving it more west (I believe it will graze the islands, and have stated that a couple days now), the high may weaken bending it back towards the west coast of Florida in the long run, which is my biggest fear with the warm water (we saw how Charley blew up in a short time period).
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Quoting 7544:


so would that put south fl out of the pic


I wouldn't be so confident in ruling South Florida out yet. Considering the immense size of the storm, impacts will be felt over a large area around the center, so even if the storm doesn't make landfall in South Florida and goes in the Gulf of Mexico, South Florida could see bands from the backside of the storm.
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Calling Isaac a tropical storm is not generous. 45mph sustained winds easily now. Isaac is no TD. Isaac was a mess, but has clean up nicely. He's now going to assert himself into a cane tomorrow. I'm thinking there is a 75% chance of hurricane Isaac tomorrow. He's behaved just as I thought he would. I hope I'm wrong, but Isaac may be a real killer for Haiti.
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Quoting Dunkman:


I don't think there is a definitive COC. It's just a big mess...calling it a TS is probably generous.



Agree. Seen a lot of these storms with competing centers. Sometimes they never come together. And to hang some models out for not predicting these coc relocations and their implications on future track is really putting too much faith in computed forecasting.
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Quoting GTcooliebai:
Now I know why storms don't get it together in the Eastern Caribbean.


It should rebound and refire more T'storms in Isaac as keeps heading into warmer waters and getting away from those minor setbacks that Issac has/is experiencing off and on..
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I think all of the model tracks except for the Euro will shift a little west on the next runs.
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Hey Gro? Have you been pumping iron again?
Member Since: September 18, 2010 Posts: 0 Comments: 5237
Quoting emguy:
Hmmm...so many interesting developments tonight.

1.) EURO has tracked further east in the Gulf once again...for the 3rd run or so in a row. This is trending...expect a track in the eastern Gulf that is even a little further east on the next run.
2.) Probable center relocation south of 15N is very regrettable news. Irregardless of track, the outcome in Hispanola is relatively the same. Heavy rain and bad mudslides...good news for them would be no hurricane. Track wise, the impacts are significant...as this storm would now need to gain significant lattitude in order to get tangled up in the mountains of Hispanola and Eastern Cuba.
3.) Future impacts...well, the debate of mountainous interraction on motion may go out the window if this is relocated near 14.7...as the impacts would angle more towards the Central Cuba area. In result, a much stronger storm.
4.) Otherwise, the stronger storm would still be expansive, so it would be a gradual turner making the North bend into the weakness over central Cuba instead, just stronger. If trends continue, then the final outcome from there...legit concern that a more significant hurricane may be in the makings for the West Coast of the Florida Penninsula and/or the Florida Big Bend.

So you're not buying Pensacola and points west, you see more of a profound weakness or a turn earlier from the weaker subtropical ridge?
Member Since: August 31, 2009 Posts: 0 Comments: 5628
location on vortex message

14°43'N 63°30'W (14.7167N 63.5W)

by the way which one should we use the one in Barckets or the onee that is not
Member Since: June 13, 2009 Posts: 2 Comments: 12334
3051. 7544
Quoting cchsweatherman:
The latest Vortex data message has confirmed that the center has indeed reformed quite a bit further south than the analyzed center at 2AM EST. The latest Vortex data message shows the center to be at 14.7N and 63.5W. This means a nearly 60 mile southward shift with the center of Tropical Storm Isaac which likely now makes an immense difference in the future track of the storm.


so would that put south fl out of the pic
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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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