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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2701. bappit
Quoting yqt1001:
Tembin still strengthening at a good clip, Taiwan might be in for quite a bit.



Looking very impressive now.

A beast. Hey, little piggy ...
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2700. Grothar
Quoting 1900hurricane:
Looks like the blog may be lagging by about five minutes. It did this last night too.



They are probably laughing at us so hard, they are forgetting to reset the server.
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Quoting scott39:
What I am referring to is that the cone bends and shrinks over time as the track progresses.


A faster storm will have a smaller cone not because the track is more certain (although usually, a quickly moving storm is easier to predict, Wilma for example), but simply because the size of the error circles surrounding each forecast point need to stay the same regardless of storm's speed.
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2698. scott39
Everyone better be hoping that Isaac does not get much room in the GOM!
Member Since: June 13, 2009 Posts: 0 Comments: 6871
2697. Grothar
Quoting bdm2225:
Bonjou from Port-au-Prince. How bad is it for us??


Excuse us for a moment blog.

Bonjou zanmi mwen an, Ki jan ou ye. Pale angle? M’ pale yon kreyol ti kras. Ou pral resevwa pi lapli. Enfomasyon sou van yo se pa definitif, men li pa gade bon.
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Looks like the blog may be lagging by about five minutes. It did this last night too.
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2695. scott39
Quoting weatherboykris:


I think it's probably an optical illusion(maybe I'm misusing that phrase) based on the coastline of florida being closer the the eastern edge of the cone, idk though. But trust me, I know for a fact, the cone is not subjective, nor will one side be wider than the other. My source is Bryan Norcross' Hurricane Almanac, where he gives a very detailed explanation of how the cone is made. I am sure this info can also be found on the NHC website.
A cone will bend and shrink over time. There is more of the cone in the GOM than the E side of Fl, in the Atlantic.
Member Since: June 13, 2009 Posts: 0 Comments: 6871
Testing...
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2693. bappit
Quoting hydrus:
I still cannot figure out why the blog has almost miniturized itself on Firefox, while on Google everything is normal..GFS 120 hours out..

Right click and select view image. Your miniature images will be de-miniaturized.
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2692. yqt1001
Tembin still strengthening at a good clip, Taiwan might be in for quite a bit.



Looking very impressive now.
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Hey guys ...and the incredibly smart ladies....what are the chances of it hitting between Mobile and New Orleans? I keep seeing it moving west. Shouldn't it be headed more WNW and turning to keep in line with the forecasts?
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2690. JLPR2
Yeah...

I belive this is the first time I have seen decent circular convergence with Isaac.
Member Since: September 4, 2007 Posts: 7 Comments: 8735
Quoting weatherboykris:
With how good the outflow and overall structure of the storm is, as soon as the inner core gets set up it's gonna start intensifying real fast.

I would love a good microwave pass about now. It would give us a nice look to see how things are organizing at the moment.
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2688. HrDelta
Quoting scott39:
Isaac is ugly under those pretty cloud tops right now. Continue to see the models shift W and N. The structure will get it together...just farther W like the EURO has trended.


Not that far west. I still believe it will end up being the West Coast Grazer.
Member Since: October 4, 2007 Posts: 0 Comments: 451
I have a hard time seeing Isaac avoiding the impact of that trough and making it anywhere west of Mobile in the GOM. Much more likely to me to ride up Florida's spine or the West Coast. To be a threat to SC or NC, it would basically need to bomb out pretty quick.
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Quoting uncwhurricane85:


which mighteven be mainland south florida


Exactly.
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2685. scott39
Quoting weatherboykris:


I think it's probably an optical illusion(maybe I'm misusing that phrase) based on the coastline of florida being closer the the eastern edge of the cone, idk though. But trust me, I know fober a fact, the cone is not subjective, nor will one side be wider than the other. My source is Bryan Norcross' Hurricane Almanac, where he gives a very detailed explanation of how the cone is made. I am sure this info can also be found on the NHC website.
What I am referring to is that the cone bends and shrinks over time as the track progresses.
Member Since: June 13, 2009 Posts: 0 Comments: 6871
I've been good. Occupied with plenty of things. Garden has been keeping me very busy but I'm starting to find extra time now so I should be here on a more regular basis. Especially so with the tropics now firing up.
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He is starting to build, this will likely mean a quicker turn to the WNW.
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Link


Very cold tops
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Just looked at the latest GFS run. Right up the coast and into the Panhandle. Looks like the Keys are going to get hit first. Is Issac ever going to actually wrap up into something?
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2680. hydrus
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2679. KEEPEROFTHEGATE (Mod)
hello sully stranger in our mist
Member Since: July 15, 2006 Posts: 174 Comments: 54616
I need to mention the ants. Maybe it is the approaching fall season (yes Miami has one) or maybe the less than stellar housecleaning on my part, but several of the window sills have been overrun with ants this past week. Just saying...
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Is Joyce not going to be a factor for the CONUS? Does one hurricane tend to follow another or does Joyce answer to a different steering pattern?
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2676. bappit
Quoting scott39:
Go look at the current cone at the NHC site....maybe I need some new glasses??

Nah, he's right. I looked it up not so long ago. The info is on the NHC site somewhere.

"the size of each circle is set so that it encloses 67% of the previous five years official forecast errors. The cone is then formed by smoothly connecting the area swept out by the set of circles."
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Howdy All!



Been busy with work lately so I have not been on hardly at all.


Interesting to see that after Ernesto, we have seen so many systems so quickly, and now two brand new cyclones.


Isaac appears to be intensifying at this hour and as it poses a threat to the GOM I will be watching it very carefully.
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Scott39, per the NHC's website:


" To form the cone, a set of imaginary circles are placed along the forecast track at the 12, 24, 36, 48, 72, 96, and 120 h positions, where the size of each circle is set so that it encloses 67% of the previous five years official forecast errors. The cone is then formed by smoothly connecting the area swept out by the set of circles."
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04:32:00Z 17.700N 64.817W

Taking off now.
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2672. hydrus
I still cannot figure out why the blog has almost miniturized itself on Firefox, while on Google everything is normal..GFS 120 hours out..
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Quoting sullivanweather:
Hey, Kori, 1900.

How's it going?

Can't speak for Kori, but I'm doing fairly well. Can't complain (well, more than normal at least :P).
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Quoting WCSCTVCharleston:
Levi at least Joe Bastardi agrees with you via his twitter "GFS may be too far west given west coast trough offshore and ridge over colorado. Much more like psns for east coast, not gulf storm."


And what's the unofficial take around the news room?
Member Since: August 17, 2005 Posts: 26 Comments: 16874
2669. Levi32
Quoting Kowaliga:


Yeah, and the NHC has eluded to the GOM having a thinner layer of TCHP (save for the loop current)which depletes faster, right?


Yes, but the gulf is abnormally hot this year, and TCHP would only be an issue if the storm slowed down significantly.
Member Since: November 24, 2005 Posts: 635 Comments: 26661
Quoting 1900hurricane:

Very good post.
Yes! At this point one needs to go with the center of the overall cloud mass represented on satellite.
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2667. Grothar
Quoting 954FtLCane:
Thanks Grothar for the tip. Kinda sorta worked but not for all hours. Huffman's 850/300mb shear maps 850 vort and 500 mb pmsl almost all say file not found. Especially the higher hour maps.

No white fly yet btw! hope yours have dissapeared.


Getting worse. I was in Oakland Park on 33rd last week and the neighbor is loaded. Killing all of the Gumbo Limbo and bamboo and most of the palms.
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Quoting GetReal:


The only flea in the ointment that I can see if the ULL near E Cuba. It is currently located in an ideal place and is ventilating Isaac. IMO it will back off towards the west, being pushed by the anti-cyclone over Isaac.

I simply cannot believe the strength and animus of that dry air pushing back all the GOM moisture.
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Quoting 1900hurricane:
Wow, both Sully and weatherboykris have shown themselves tonight, talk about a blast from the past. How are y'all!



Haha I'm doing great....much older than when I signed up a long 5 years ago haha. It's funny to go back through some of my old blog posts and stuff....how are you??
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2664. scott39
Isaac is ugly under those pretty cloud tops right now. Continue to see the models shift W and N. The structure will get it together...just farther W like the EURO has trended.
Member Since: June 13, 2009 Posts: 0 Comments: 6871
Quoting ClevelandBob:
Just a little eerie that Isaac may make landfall on August 29
was just sayin that to my daughter......making plans already
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Quoting Levi32:


Storms running right up the west coast of Florida do tend to struggle. The 1935 Labor Day hurricane is a great example of this. The key is how much separation there is. If the storm moved up 100 miles west of the peninsula and made landfall in the panhandle instead of getting sucked right into the bight of Florida, then significant strengthening would be likely. It just can't get too close.


Yeah, and the NHC has eluded to the GOM having a thinner layer of TCHP (save for the loop current)which depletes faster, right?
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Quoting scott39:
Go look at the current cone at the NHC site....maybe I need some new glasses??


I think it's probably an optical illusion(maybe I'm misusing that phrase) based on the coastline of florida being closer the the eastern edge of the cone, idk though. But trust me, I know for a fact, the cone is not subjective, nor will one side be wider than the other. My source is Bryan Norcross' Hurricane Almanac, where he gives a very detailed explanation of how the cone is made. I am sure this info can also be found on the NHC website.
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2660. HrDelta
Quoting scott39:
Wasnt thier a hurricane that was moving over 20mph, that hit the East Coast years ago?


I know Wilma did that.

In more personal news, my Depression is acting up again.
Member Since: October 4, 2007 Posts: 0 Comments: 451
Hey Cowboy!

I missed that avatar! lol
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2658. ThePass
I hate this time of the year.
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Wow, both Sully and weatherboykris have shown themselves tonight, talk about a blast from the past. How are y'all!
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Hey, Kori, 1900.

How's it going?
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Quoting sullivanweather:


The overall circulation is very broad and lacks a true inner core. Most of the deep blowups of convection have been the result of convergence along the differing airmasses being entrained into the storm (dry air to the north meeting up with moist air streaming into the storm from the south). Each time one of these large bursts of convection takes place smaller vorticies at the surface develop and spiral into, or away from, the overall broad and elongated center of the storm. This is what's causing all these 'jumps' of the center as each individual vorticy becomes dominant over another. It is better to pay little attention to these little vorticies and instead track the broad center unti a true inner core develops. Until that occurs Isaac will likely struggle to intensify.


Sully!! Good to see you, bud. ;) Nice post. Thanks.
Member Since: July 9, 2006 Posts: 184 Comments: 29610
2654. 7544
Quoting AllStar17:
Mission 5 is in the air.


good thanks looks like they will be there during dmax and issac starts to blow up
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Greetings, folks. Looks like we're in for it on the west coast of Florida, huh? We're already so saturated. Crap.
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With how good the outflow and overall structure of the storm is, as soon as the inner core gets set up it's gonna start intensifying real fast.
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Quoting gordydunnot:
Man that GFS run is brutal, can the state of Texas take everyone from FL. if we bring rain.
Oh, if you're bringing rain then don't stop in Texas. Come right on in to New Mexico.
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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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