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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2400. angiest
Quoting bappit:

Yep, and of course Oklahoma's summer has been absolutely brutal.


BTW, I'm speaking hypothetically. I'm not expecting Isaac to come this far west (on or about 9/18 is when we will get hit :P ), I'm just saying it's non-zero. This is most decidedly not the summer of 2011 (thanks for nothing, Don!)
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Quoting weatherman12345:
seems like your cuba miss senario may come to reality this run...
Possibly; it's about 2˚ north of the 18z position at 60 hours.
Member Since: September 2, 2009 Posts: 130 Comments: 21194
2398. 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!
Member Since: July 4, 2005 Posts: 204 Comments: 8898
Quoting CCkid00:


the interstate has been closed for 19 hours in both directions, at one location. BR traffic was a nightmare this morning....taking over 4 hours to drive 30 minutes!


It took me three hours to get to work on a usually 15 minute commute! It was a nightmare!
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2396. will40
looks like he gonna miss cuba on this run
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2395. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 186 Comments: 57687
2394. 7544
Quoting weatherman12345:
I would say most likely sofla this run


same here looks like more east run this trip will the nhc follow and shift the cone back east again again the am imo yes
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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...
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Quoting hydrus:
The upper low should fizzle out...Sounds like you knew that..:)
I noticed it was filling in, but I asked because if I remember correctly ULL can pull a storm more towards the North or was it West as it creates its own weakness in a larger pattern.
Member Since: August 31, 2009 Posts: 0 Comments: 5628
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.



To be clear, thanks, I'm acutely aware of damage for TS systems, rain and flooding and even wind. The point here was to confirm Rita's post that this system, though broad/large, is not a strong system and at this point is less susceptible to any weakness north of it.

One injury, one life lost is a tragedy even in a gale or microburst. I won't ever discount that. Please be sure to note the context in which a post is made in reference before suggesting otherwise. Thanks.
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Quoting MiamiHurricanes09:
00z GFS will be more poleward than the 18z run.

Don't count your chickens until they hatch ;)
Member Since: May 12, 2012 Posts: 15 Comments: 3094
Typhoon Bolaven Is like 1600x1500 miles,that is huge.
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2388. Grothar
Quoting hunkerdown:

Gro - do you have the shakes tonight with an itchy posting finger or are my eyes playing with me ;) I keep seeing you post that same map...



Everybody kept asking if the 00z was in yet. I figured they were bound to read one of them.
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2387. Levi32
Quoting TomballTXPride:

So in other words you finally budged and moved it. Good for you!


I moved it 50 miles west of yesterday. Adjustments are a 99% probability of being necessary on a 5-day forecast.
Member Since: November 24, 2005 Posts: 635 Comments: 26731
http://forecast.weather.gov/MapClick.php?w0=t&w1=td &w2=hi&w3=sfcwind&w3u=1&w4=sky&w5=pop&w6=rh&w7=thu nder&w8=rain&w11u=1&w12u=1&pqpfhr=6&AheadHour=48&F cstType=graphical&textField1=25.70212&textField2=- 80.39110&site=all&unit=0&dd=0&bw=0&AheadDay.x=77&A headDay.y=7


hurricane force gusts in forecast for dade county monday morning
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Quoting KEEPEROFTHEGATE:


With the center where it is and the continued Westward motion I still think this is too far north but if the center has relocated it will take until at least the 12Z tomorrow to take it into account....still think it will pass just south of the DR..
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Quoting MississippiWx:


Anything to support your forecast track, huh? Lol.


Like a dog with a bone. ;-)
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Joshsen18 whats the worst that can happen? You can't leave the hotel room.
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2381. bappit
Quoting angiest:


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.

Yep, and of course Oklahoma's summer has been absolutely brutal.
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Quoting MiamiHurricanes09:
00z GFS will be more poleward than the 18z run.


It has been for the last 4 nights or so, what should be different about tonight. Wonder why that seems to be a pattern?
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2379. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 186 Comments: 57687
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2375. Levi32
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. %uD83D%uDE31%uD83D%uDE31


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).
Member Since: November 24, 2005 Posts: 635 Comments: 26731
2374. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 186 Comments: 57687
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.
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2372. trey33
Quoting gordydunnot:
Well it's a short ride from Tampa on I-4 to Disney World. If they had been smart they would have set it up there anyways. That's where all the winners go. No strip clubs though.


excellent point
Member Since: August 16, 2007 Posts: 0 Comments: 499
We are getting smacked again here in Sydney.

90km/h + winds.


loop
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2370. Michfan
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?....?


You should be just fine. Just stay in contact with your travel agent and the hotel just in case.
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2369. scott39
Quoting MiamiHurricanes09:
00z GFS will be more poleward than the 18z run.
Do you mean a quicker turn out of the Caribbean and a farther inland hit into S Fl. or farther N in the GOM?
Member Since: June 13, 2009 Posts: 0 Comments: 6914
850 mb vort at 48 hrs on the north coast of hispanoila.

Member Since: August 6, 2007 Posts: 0 Comments: 693
2367. bappit
Quoting :

Quoting KoritheMan:


I'd say more like tomorrow or Friday. Don't forget about that beautiful G-IV mission scheduled for tomorrow.


Odd how people on here argue about the skinny line while the NHC is busy gathering more data to make a forecast because they know they need more. Of course forecasts on the blog are just a game. Wish casting on here is really not all that bad. Just imagine if the NHC forecasters did that!
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2366. hydrus
Quoting GTcooliebai:
Will Isaac start pulling north because of that ULL over Cuba?

The upper low should fizzle out...Sounds like you knew that..:)
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Well it's a short ride from Tampa on I-4 to Disney World. If they had been smart they would have set it up there anyways. That's where all the winners go. No strip clubs though.
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2363. Gearsts
GFS shows the tail over PR for like 48hours.
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Quoting KEEPEROFTHEGATE:


The models are still initializing this further northward...I do not think they have quite resolved the board nature of the low pressure...it will come down as to where the center eventually takes shape as to what track it takes in the Caribbean..
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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.


It will take awhile to spin up and so far the dry air has promoted a more widespread southerly strong component. The ECMWF track is most likely based on previous forecasts of other hurricanes. New Orleans may have to think about getting ready.
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Quoting Grothar:

Gro - do you have the shakes tonight with an itchy posting finger or are my eyes playing with me ;) I keep seeing you post that same map...
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Here in Seminole County if we get a tropical then flooding will be the biggest concern as Issac is huge and flooding is already becoming a problem
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Quoting GTcooliebai:
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.

Yes, I'm not trying to argue. I was starting out sort of general, but when I saw you questioning, I then wanted to show that I wasn't ignorant. (:
Member Since: May 12, 2012 Posts: 15 Comments: 3094
500 mb vort at 48 hrs. looks to me to be further north than last run.

Member Since: August 6, 2007 Posts: 0 Comments: 693
2354. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 186 Comments: 57687
2353. 7544
well if the nhc is going by the gfs lets see what this run shows and we can tell how the cone will change tomorow at 5 am stay tuned
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Quoting Joshsen18:
We are leaving for our honey moon in the cancun riviera area...... Is it safe from issac? Any advice would be great I would hate to cancell.......


I don't know if you've seen it, but a few of us have answered your question. The general agreement is no. At most you will get some northerly to northwesterly winds off the Gulf.
Member Since: March 7, 2007 Posts: 602 Comments: 21780
00z GFS will be more poleward than the 18z run.
Member Since: September 2, 2009 Posts: 130 Comments: 21194

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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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