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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00z GFS will be more poleward than the 18z run.
Member Since: September 2, 2009 Posts: 130 Comments: 21194
keeping linking the new gfs my other internet server is down
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Doesn't look like we're gong to get a good look from here....too far S....

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Quoting redwagon:

Seriously. Have you seen what that little strip of dry air just did? Practically evaporated all the GOM moisture.

I have perfect confidence our atmosphere would actually *run at* any system 'threatening' us.


Lol. Maybe. But we don't challenge Mother Nature here. :)
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Quoting scott39:
I see alot of bias toward the GFS over the EURO. What happened to the EURO being the king? Lol


It lost when the GFS nailed Debby and Ernesto.
Member Since: March 7, 2007 Posts: 582 Comments: 20768
2346. angiest
Quoting bappit:

We haven't had a death ridge all summer in SE Texas. Been a cool summer.


Central and North Texas have not been as fortunate. And it did spend a little of July and August over us (Houston), likely keeping Ernesto away. Probably a similar setup exists today to what gave us the original forecasts for Debby, supporting a storm coming in from the East as opposed to the south as is more normal for us.
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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. 😱😱
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Quoting AllyBama:
Isaac doesn't need to go to the big bend area of FL. There are still roads impassable from the flooding 2 months ago. With all of the rain that has fell over the past few weeks, the water levels are not receding very quickly. More rain will only aggrevate the situation and bring flood levels back to or higher than they were.



Down here in Central Florida we don't need tropical storm/hurricane rains either, Pinellas County utilities is having a water backup issues and I've seen plumbers being called all over my neighborhood.


I've seen nearly 8 inches of rain just since this last weekend, we had 11 inches with Debby and, over 12 inches for the month of July from intense rainy season pattern rains, and now approaching 10 inches for August, yeah, we've had A LOT of rain...


Don't get me wrong, the last few years we were pretty much below average rainfall all the time, rainy season or not. I was worried we were starting to depart from the reliable soaking summer rains. Well now they are back this year, and with a vengeance lol. With that said the rainfall is very beneficial, but a tropical storm on top of this would become destructive just from rainfall alone.
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Quoting JLPR2:
Is it me or is the GFS slower, still south of me at 21hrs.



slower or maybe center relocation further west...lol
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Quoting opal92nwf:

In my comment I meant to imply that the winds would accompany what you were talking about previously (storm surge). The 21' hurricane had a bad storm surge.
Oh ok I understand you now, lol you agreed with me every time, I'm going to need to get some sleep after tonight before I get grouchy. I was just wondering if the person that made that comment realized that a skirt can be just as bad as a direct hit, as well as considering any population increases.
Member Since: August 31, 2009 Posts: 0 Comments: 5628
2339. KEEPEROFTHEGATE (Mod)
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2338. JLPR2

Looks rather decent. :\

42hrs.
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When Debby came through, my area (SW Fl) had two days of tornado touchdowns so that's another thing to keep in mind as this storm passes.
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Quoting Gearsts:
And closer or is just me?


Yes,is closer.
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2334. Grothar
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2333. bappit
Quoting angiest:


The very active sea breeze indicates we are not totally protected.

We haven't had a death ridge all summer in SE Texas. Been a cool summer.
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Quoting galvestonhurricane:


Not good for anyone really, especially the US. With more time in the GOM, Isaac could easily strengthen into a Cat. 2 storm.


Even if Isaac's not a Cat 2 storm. Due to his size (if the current NHC track validates) Tampa will potentially see Cat 2 or higher equivalent surge w/ heavy rainfall trying to drain into it.

I know wind velocity gets the most attention, but ain't everything. Serious situation for the entire west coast of Fla here..
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Quoting Grothar:





Thanks!
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2330. Michfan
Quoting Felix2007:


Jeez, Ivan's even worse than Ernesto at persisting!


At least no one mentioned Karen.
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2329. JLPR2
Quoting Gearsts:
And closer or is just me?


Yep, but just slightly.
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2328. scott39
Quoting KoritheMan:


I'd say more like tomorrow or Friday. Don't forget about that beautiful G-IV mission scheduled for tomorrow.
Oh yea.
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2327. KEEPEROFTHEGATE (Mod)
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2325. emcf30
LOL.

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2324. Grothar
Quoting aasmith26:


Not yet that I see...


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Quoting drg0dOwnCountry:

Irene was costly not for wind damage but the size and rain. Just Ivan's size and rain will overwhelm the saturated Florida soils. He does not even have to interact with land mass to cause damage.


Jeez, Ivan's even worse than Ernesto at persisting!
Member Since: July 12, 2008 Posts: 7 Comments: 383
Quoting angiest:


Ivan?? I know he came back but are his remains still circulating all these years later?
I meant Irene 2011.
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2321. scott39
I see alot of bias toward the GFS over the EURO. What happened to the EURO being the king? Lol
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0z NAM...=\ Not falling for it yet.
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Quoting popartpete:
Who thinks that Issac and TD10 are starting to have a Fujiwara, where close together storms begin to cyclonic rotate around each other? If so, would 10 be slingshoted into the U.S. East Coast, ahead of the remnants of Issac? Could be an interesting scenario.


I have been waiting or someone to bring that up.....
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2318. Gearsts
Quoting JLPR2:
Is it me or is the GFS slower, still south of me at 21hrs.

And closer or is just me?
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2317. Michfan
The G-1V mission is going to help a ton to lock down a better track.
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anyway I say Isaac is further S and not at 15.8N well will have to wait for recon(21/2 hours from now) to find out in the mean time I am going to hit the shower and top up on the coffee and redbull
Member Since: June 13, 2009 Posts: 2 Comments: 12138
2314. KEEPEROFTHEGATE (Mod)
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2313. angiest
Quoting drg0dOwnCountry:

Irene was costly not for wind damage but the size and rain. Just Ivan's size and rain will overwhelm the saturated Florida soils. He does not even have to interact with land mass to cause damage.


Ivan?? I know he came back but are his remains still circulating all these years later?
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Quoting scott39:
Its really impossible to lock down a possible land fall on the Northern Gulf Coast at this time. IF Isaac makes it that far N.....The NHC should be able to narrow it down more around Sunday or Monday


I'd say more like tomorrow or Friday. Don't forget about that beautiful G-IV mission scheduled for tomorrow.
Member Since: March 7, 2007 Posts: 582 Comments: 20768
Quoting zoomiami:
Did anyone besides me notice that it is almost always the male species of the blog who create the "blow ups"?

I think that answer is to have a few more of the female species on board.



Hahaha, I was reading thru the comments, trying to get caught up, and kept thinking it was getting "catty" in here. :-)
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2309. JLPR2
Is it me or is the GFS slower, still south of me at 21hrs.

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Quoting KoritheMan:

brSome have said that this blog has an elitist mentality. I tend to agree.


Take some of the smartest people from each class in a school district and you wil get a lot of the people who post on this blog. Elitest is far too harsh a word. There is just a lot of bright, opinionated people here. I am not one of the smart ones but have taught many like the bright ones here. Often, all that is needed is a hand on the shoulder just softly suggesting everyone step back. It is so hard for me to not butt in and ask people to please not fight personally. This is especially true when it is good posters who are just obviously having an off night. I am just an old experienced teacher enjoys reading all of your ideas and thoughts.

Kori, I meant to plus you apology but I hit minus because I was in too big a hurry. Sorry, and I look forward to your blog. Oh, and WChaser97 wondering about our thoughts too.

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Quoting moonlightcowboy:



So right. Isaac's forward speed is a big hindrance to its development and maturity. Hard to turn? Exactly, if it stays like it is. The fact that it has a large broad circulation does not make it susceptible to any weak troughing. Only strengthening, heightening storms have better potential to feel weaknesses. The fact that the circulation or wind field is large is a moot point. A storm has to be more organized, more intense, more mature.

As it goes now, Isaac is still really barely just a weak TS. Long ways off from anything substantial yet.

Irene was costly not for wind damage but the size and rain. Just Ivan's size and rain will overwhelm the saturated Florida soils. He does not even have to interact with land mass to cause damage.
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2306. GetReal
12 hour




I have a problem already with this run.... GFS is initializing the forward speed much slower. Hell it is aleady near the 12 hour forecast point in reality.
Member Since: July 4, 2005 Posts: 204 Comments: 8896
2305. scott39
Its really impossible to lock down a possible land fall on the Northern Gulf Coast at this time. IF Isaac makes it that far N.....The NHC should be able to narrow it down more around Sunday or Monday
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Issac still remains fairly weak. Meaning a more western component, in my humble opinion, grazing the islands, and not feeling the weakness as much to the north. I think this bad news, because it will have more warm water to work with regardless of where it heads (Louisiana, Mississippi, Panhandle, West Florida. Tampa Bay seems immune to these storms, but I keep telling anyone who will listen that we're due. Charley was a close call, and this might be another Charley. A major hurricane in the mouth of the Tampa Bay would have the similar bath tub effect to Katrina! Still lots of variables in play, and may just stay off the coast of florida, and fall apart all together, but the fact that its staying weak at this point when it was forecasted to be much more intense and recurve should be a slight worry to all of us along the gulf.
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Quoting TomballTXPride:

He's grasping at straws at this point. So desperate. So sad. ;-)


I'm messing with him. He still has good points.
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Quoting weatherman12345:
0z GFS init. yet?


Yes, but only 12hrs in to the run.

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Quoting KoritheMan:


There is a spin at 14N, but it appears to be mid-level to me, as the convective pattern doesn't really fit with a new center forming in that direction.


Could be but there definitely is something there...could be closer to 15N though 15.8 seems a bit North to me
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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.