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 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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2300. TheMom
I think Orlando's Mets are looking in here for what to say since he pretty much summarized what those I trust in here have been saying. Till we know level of pumped up Isaac gets and how that reacts with the weak spot we don't know if it is east or west. I'm hoping for Isaac to pump up tonight and get headed north early. Thanks to all for the info and insight as always WU is the go to for info I can trust and know that if I don't understand something there are knowledgeable people here to explane.
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You still feeling Isaac may not be going as far west as some are thinking now Levi?
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2298. CCkid00
Quoting HimacaneBrees:
Off topic but

"Authorities late Wednesday were evacuating Baton Rouge residents near Interstate 10 where a tanker carrying flammable isobutane crashed early Wednesday as they prepared to burn off the substance and remove the truck." 2theadvocate.com


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!
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Quoting Clearwater1:
I was wondered that too. lol. But don't think so.

However, when the local met states at 10:30 he personally thinks it will be further out in the Gulf, it makes me wonder if they get the NHC report way before the public, if only to make them look good.


yup. makes you go hmmmmmmm...

I kinda would doubt them using 5 hour old info from one of their more reliable models. Thats what kind of made me think this.

Lets see where the 0z GFS goes....
Member Since: September 30, 2008 Posts: 0 Comments: 108
Quoting GTcooliebai:
We have to consider what takes lives more is it the storm surge or the winds? I'm inclined to go with the storm surge. Buildings can be replaced, lives cannot.

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.
Member Since: May 12, 2012 Posts: 15 Comments: 3156
2295. Michfan
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.


Too far apart.
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Quoting weatherman12345:
0z GFS init. yet?


Not yet that I see...
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Quoting Joshsen18:


we leave Friday for cancun riviera for our honeymoon....does anyone have advice are. We pretty safe there from issac? I would hate to cancel....any advice please I trust people who really know how to read storms? Or any ideas



I think you'll be fine. Maybe some wave action, that's about it.
Member Since: August 26, 2011 Posts: 0 Comments: 72
Quoting hunkerdown:

In MH09's scenario, it would lead to a track more to the East, not West.
Exactly; that's why I don't like to use the east/west terms because they can be pretty confusing. Poleward would be the word.
Member Since: September 2, 2009 Posts: 130 Comments: 21194
2290. trey33
Quoting GTcooliebai:
We have to consider what takes lives more is it the storm surge or the winds? I'm inclined to go with the storm surge. Buildings can be replaced, lives cannot.


Surge will be way worse.
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Quoting Chiggy:
Don't freaking believe the center is at 15.8N - I think NHC just sticking to the previous advisory in light of recon's failure to conclude on a LLC...

I know but yeah maybe yo are right

Quoting ProgressivePulse:
Mission #3 is out early.

FLIGHT THREE --TEAL 71--
A. 23/0600Z, 1200Z
B. AFXXX 0509A CYCLONE
C. 23/0400Z
D. 16.0N 63.0W
E. 23/0530Z TO 23/1200Z
F. SFC TO 15,000 FT



Product: Air Force Temp Drop (Dropsonde) Message (UZNT13 KNHC)
Transmitted: 22nd day of the month at 21:56Z
Aircraft: Air Force Aircraft (Last 3 digits of the tail number are 307)
Storm Number: 09
Storm Name: Isaac (flight in the North Atlantic basin)
Mission Number: 3
Observation Number: 11


wait a minute that is from this last flight
last flight was #3
this one will be #4 and it is listed as #3 in the planner




I think the LLCOC is further S then what NHC had out for 11pm advisory we will have to wait for HH to take off in about 2 hour and 1/2 (1am) I think

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18 hrs.

Member Since: August 31, 2009 Posts: 0 Comments: 5628
Hispaniola South mountain range could push the storm left
http://goo.gl/maps/PIMlj
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Lol Levi you throwing models out I never even heard of now..
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Quoting AtHomeInTX:


RED!
SSHUSH!

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.
Member Since: August 4, 2007 Posts: 0 Comments: 3383
Quoting GTcooliebai:
Buildings can be replaced, lives cannot.


Run from the water, hide from the wind.
Member Since: March 7, 2007 Posts: 603 Comments: 21852
2280. trey33
Quoting Chicklit:
oh oops, thanks.
then the latitude was same at 5 p.m. as it is now!
in other news:
Tampa Mayor Bob Buckhorn said his city is prepared.

"We have contingency plan after contingency plan," he said. "We are ready in the event that it happens. I don't think it's going to be a factor in this particular convention. But we are prepared in the event that it is."

really?! I mean really?


He's actually done a really good job so far with this. Everyone in the city is working together, they have a backup plan (just not announcing it), National Guard will be there anyway along with every LEO agency within 100 miles, along with fantastic mets at Ruskin (NOAA & NWS), and the county emergency mgmt.

RNC isn't the issue as much as the locals who every time we are in the cone, it shifts, and no one takes it that seriously.
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Quoting Levi32:
21z SREF mean has Isaac in the northern Bahamas in 87 hours.



Anything to support your forecast track, huh? Lol.
Member Since: July 15, 2006 Posts: 17 Comments: 10285
To my very....very untrained eye....Isaac is getting bigger(in size) and the strenght(intensity) is not keeping up with it...this seems like a really bad set-up for someone...I'm sure this is why the euro has been dead set on a central GOM landfall..All I know is that "K" had 2 days in the gulf (I think) this one may have 3-4 days in the Gulf...it's not gonna be pretty for whoever gets this one....
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Quoting louisianaboy444:


I think the center is further south


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.
Member Since: March 7, 2007 Posts: 603 Comments: 21852
Quoting opal92nwf:

Oh sure, if a skirting storm had it's eyewall brush Tampa with major hurricane winds in the city, I would consider that the same magnitude as the 21' hurricane.
We have to consider what takes lives more is it the storm surge or the winds? I'm inclined to go with the storm surge. Buildings can be replaced, lives cannot.
Member Since: August 31, 2009 Posts: 0 Comments: 5628
Quoting opal92nwf:
What the... is that supposed to mean!?!?
img src="Image and video hosting by TinyPic">


Saw that too and LOL'd
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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.
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Quoting Levi32:
21z SREF mean has Isaac in the northern Bahamas in 87 hours.


It does look discombobulated (whatever that word is, lol)
Member Since: May 12, 2012 Posts: 15 Comments: 3156
GFS.


Looks like the trough picks up Isaac near PCB, FL, then ROCKETS off to the ENE....
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Can you feel that? That real strong like magnetic field pulling from the east? That's the Florida casters trying to hold on to that cone of uncertainty that keeps slowly slipping west. LOL just joking folks don't kill me.
Member Since: August 23, 2010 Posts: 0 Comments: 1038
2268. angiest
Quoting galvestonhurricane:


However, steering patterns show that a storm moving that far west would take a track more like Ernesto's (except maybe a little farther north).


Are you referring to the CIMMS steering charts? I think you may be extrapolating too far if you are.

I don't remember who I was discussing it with, but a track for Ernesto similar to Claudette was plausible if the ridge had been centered slightly further north, as it was during early July when we got ~15" of rain in one week.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
Quoting AnthonyJKenn:
It looks more and more like we can split the difference between GFS and Euro and go for Mobile to Destin to Big Bend landfall now.

NOLA-Biloxi is still in play, as is a rake of W Fl coast, but anything further W of Grand Isle or E of the Keys would be the outliers.

I'm calling my shot....let's see if it pans out.





Pretty good call. I'm a bit more inclined toward a Pensacola - Panama city landfall tho. I guess all bets are off.
Member Since: August 1, 2007 Posts: 0 Comments: 206
Quoting MississippiWx:
Interesting tight spin down around 14N. It is bound to be mid-level, but the only reason I brought it up is the fact that it's the only spin I can find. Lol.

i think the nhc track is too far east in the begining. shows it moving wnw with a bend NW over hati/cuba. i see right now a due west track with a GRADUAL bend wnw due to the weakness in the ridge which should move it south of cuba. what do you think?
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Quoting KoritheMan:
Assuming the NHC center coordinates are correct, Isaac should pass near this buoy in the next few hours.


I think the center is further south
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Quoting GTcooliebai:
Well it doesn't need to be a direct hit to be bad here in fact a storm skirting the coastline is the worst case scenario because of the storm surge and the water that would be pushed up onshore.

Oh sure, if a skirting storm had it's eyewall brush Tampa with major hurricane winds in the city, I would consider that the same magnitude as the 21' hurricane.
Member Since: May 12, 2012 Posts: 15 Comments: 3156
This looks like a track that will put a hurting on the east side of the Appalachians. GA mountains all the way up through Richmond, I'd pay attention to Isaac.
Member Since: August 26, 2011 Posts: 0 Comments: 72
2261. scott39
Quoting RitaEvac:
The day 4 and 5 cone is very wide for good reason, but take note that the bulk of the cone is over the Gulf now, which is a tale tell sign that this is becoming more likely of a Gulf system downstream.

Thats been my experience.
Member Since: June 13, 2009 Posts: 0 Comments: 6917
Quoting mynameispaul:


When these kids can't play video games for 3 weeks and have to sleep in the heat they might change their minds.


scott isn't a kid. He just said he was 10 when Frederic hit. Way to assume, though.

I know Isaac is giving everyone stress, but you know what? We can't do anything about it. Watch, sit back, and if need be, prepare.
Member Since: March 7, 2007 Posts: 603 Comments: 21852
2259. hydrus
Quoting angiest:
FWIW, hurricanealley.net has been forecasting west the last several advisories.

Thats a dangerous track..Tell them to change it now....Jk..:)
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check out the pin hole for a slit sec
Link
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2257. Torgen
Quoting Grothar:
Why wait?




KITTY DOES NOT WANT.

http://www.catpictures.co/pictures/Kitty_Does_Not _Want.jpg

Don't think this old roof will take a direct hit.
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2256. Levi32
21z SREF mean has Isaac in the northern Bahamas in 87 hours.

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Quoting RitaEvac:
This storm is getting further and further west by each hour of over 15 mph. The storm is large and weak and will be hard for a storm like this to turn, it's basically top heavy and like a bowling ball once the momentum is one way it usually goes that way



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.
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2254. emcf30
....

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Quoting 954FtLCane:
hmmmm... Im thinking the 0z gfs will be right in line with the center of our new cone of doom.

If so, just one question, are they getting the GFS ahead of time, so it can be plugged into the advisory?

I was wondered that too. lol. But don't think so.

However, when the local met states at 10:30 he personally thinks it will be further out in the Gulf, it makes me wonder if they get the NHC report way before the public, if only to make them look good.
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2251. RitaEvac
3:30 AM GMT on August 23, 2012
The day 4 and 5 cone is very wide for good reason, but take note that the bulk of the cone is over the Gulf now, which is a tale tell sign that this is becoming more likely of a Gulf system downstream.

Member Since: July 14, 2008 Posts: 2 Comments: 9774

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Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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