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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Isaac looks like he has 2 heads
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No one knows how strong or weak the system will be. Isaac could very easily huge the west coast and quicky ramp up.
Charley jumped from a cat 2 to cat 4 in a matter of hours off the west coast.
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Quoting sporteguy03:


Being a rainmaker would be bad enough with the flooding and saturated ground already in Florida.


Absolutely, and it looks to be a fairly slow mover in the long term. Not down playing it at all and its potential impacts on the SE; just saying I am finding it hard to come up with ways in which it could be even a Cat 1 when it approaches the CONUS. Also pointing out that the 18z GFS does not show a "strong" tropical system. That said, things change quickly in the tropics as we all know. To top that off, I've been wrong before and will do it again.
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Quoting mcluvincane:


I know, stormjunkie use to have some good views, no its just a bunch of rambling and agreeing with trolls
What ever happens to stormjunkie??,I have no seen him posting in years!!!,maybe he died?.
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For twits, HRD is tweeting from the plane.


Link
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And here comes the 20+ sympathy comments regarding Levi....Levi handled the situation well.It wasn't like he was in any danger or something.No one is accurately right with tropical weather.
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Quoting TropicsGirl:
Levi - please don't take this barking as a personal attack. You are one of the best on this blog. NOBODY knows for sure what this storm is really do until it gets closer. I value and respect your opinions and forecasts - keep up the good work!!
Levi.It seems the majority of us here are onj your side.... This is not an exact science, but you do one hell of a great job. (Uh OH.... I didn't say Brownie)
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Quoting KEEPEROFTHEGATE:
T.C.F.W.
09L/TS/I/CX
MARK
15.35N/61.13W

Well, it's moved a little since I called 15N 61W :)
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Quoting Stormchaser121:

Most of em went west. Im seeing the track of Issac moving ever so slightly to the west. Will it do the same??
it's not going to just stay west it will pull north at some point
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Quoting StormJunkie:


I'm guessing we don't get access to those images? Darn! ;)


Not in real time, but it is available at some point in the future.


Link
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541. TXCWC
I am NOT saying is going here but just noticed a couple of Ensembles incrediblly going to TEXAS now. More important perhaps though the TVCN track is saying Florida Panhandle.

Link
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Quoting NICycloneChaser:


Actually, no. It shows a potent (and very large) Cat 1/2 hurricane taking it's sweet time moving along the Florida coast. More than a rain maker in my eyes.


I know, stormjunkie use to have some good views, no its just a bunch of rambling and agreeing with trolls
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Quoting FOREX:


It's all about ratings. TWC, like many channels would rather have 15 million idiots watching reality shows, than 5 million watching the Tropical updates. Just my opinion.
Central florida has channel 13 news. Tropicals updat every 21 and 51 minutes of the hour and local weather on the 1's .... good stuff
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when you have a real job??? Really Drakoen? A debate by definition is a respectful back and forth. When one gets to being condescending and nasty the debate has been lost whether or not they were right. Let me say I've never seen Drakoen like this before and I think he and Levi are two of the very best. There was just absolutely no need at all for that debate of ideas to have taken a dive off the bridge. Been amazingly polite and educational in here all day. Certainly many have differing ideas and were respectful of other's thoughts today. If others disagree with me that he was being mean and condescending; I respect that opinion. :)
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Quoting StormJunkie:


All in all though, it shows exactly what thewindman was referring to. As much of a mess as this system is, and with the mountains coming up, and then fairly close proximity to Fl. Odds say that this will be a rain maker and that is about it.


Being a rainmaker would be bad enough with the flooding and saturated ground already in Florida.
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Levi - please don't take this barking as a personal attack. You are one of the best on this blog. NOBODY knows for sure what this storm is really do until it gets closer. I value and respect your opinions and forecasts - keep up the good work!!
Member Since: August 22, 2011 Posts: 0 Comments: 32
Barbados radar...


Link
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Quoting StormJunkie:


See post 505. I disagree with your statement based on that. What it does show is a large low pressure area with a relatively small and fairly weak 850mb vorticity field.


In this scenario, winds wouldn't be the big concern, so the vorticity isn't really an issue. But rainfall from this sort of 'large low pressure area' could well be in excess of 10 inches across much of Florida, which makes it more than a rainmaker.
Member Since: August 10, 2010 Posts: 2 Comments: 1971
T.C.F.W.
10L/TD/XX/CX
MARK
12.25N/37.69W
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KFDM 6 Greg Bostwick pointed out Issac, and when he was asked if we here in TX should be concerned about it, he said "We will talk about it, dont be overly concerned yet but we will watch it." It was like he was hinting at the possibility of a more west track...
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T.C.F.W.
09L/TS/I/CX
MARK
15.35N/61.13W
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Quoting GTcooliebai:
For Tampa Bay, notice the drier weather before the storm.



thank goodness........
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Just curious, though I'm almost afraid to ask. If Issac continues to maintain TS status and continues to drift west at the speed it is going now, is that what the ECMWF is seeing as a more westerly track rather than turning north?
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Quoting nrtiwlnvragn:


Kermit's mission is not center fixes, so they would not necessarly be trying to find one. They are doing Tail Doppler Radar, and I think they are on leg 3 to 4.




I'm guessing we don't get access to those images? Darn! ;)
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For Tampa Bay, notice the drier weather before the storm.

Member Since: August 31, 2009 Posts: 0 Comments: 5628
I must say...the euro may be on to something...this storm has not gotten it's act together, and that's pretty much why it has it going into the gulf. Wouldn't that just suck if it pans out.....
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Im tasting the first of isaac's rain droplets
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Quoting Levi32:
It forever amazes me the inability of this blog to consider views different from the consensus of experts, models, what have you, just because it's not the generally accepted explanation. Does that make such views automatically wrong? Do I have to answer that?

I love meteorological debate but you guys make it so difficult to have fun tracking the weather.


Being meteorology is a science, no one should ever stop questioning. I find it hard to imagine a day when the experts will really be 100% accurate with weather. Fresh ideas and thoughts should always be welcome if it indeed is a science.

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522. TXCWC
Quoting MississippiWx:
There is something to be said about the GFS/Euro consistency in the long range. Both of them seem to want the system at least in the Eastern Gulf. I'd split the difference of the two for now and say FL Panhandle final landfall.


Seems reasonable to me :)
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Can someone post the other colored version of the model please?..
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Quoting floridaboy14:
can you give me an appoximate location? (coordinates)


000
URNT12 KNHC 222207
VORTEX DATA MESSAGE AL092012
A. 22/21:44:50Z
B. 15 deg 44 min N
061 deg 25 min W

C. 850 mb 1470 m
D. 27 kt
E. 131 deg 90 nm
F. 073 deg 13 kt
G. 278 deg 13 nm
H. 1005 mb
I. 18 C / 1525 m
J. 19 C / 1524 m
K. 18 C / NA
L. NA
M. NA
N. 1345 / 8
O. 0.02 / 0 nm
P. AF307 0309A ISAAC OB 10
MAX FL WIND 40 KT NE QUAD 19:17:30Z
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Quoting NICycloneChaser:


Actually, no. It shows a potent (and very large) Cat 1/2 hurricane taking it's sweet time moving along the Florida coast. More than a rain maker in my eyes.


See post 505. I disagree with your statement based on that. What it does show is a large low pressure area with a relatively small and fairly weak 850mb vorticity field.
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Quoting RTSplayer:


Maybe not, but steering still calls for potentially more southerly jogs and wobbles.

1000mb




990mb




This is why the Euro thinks it's going farther west, not to mention a continental high over the SE U.S. during the final approach.


Too early to know for sure until it organizes a bit more though.


Notice the US high is further SW into Mexico opening up a CLEAR weakness over Florida at 990 mb system should be much lower than 990 mb once it feel starts to feels that weakness.
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NEW VIDEO BLOG UPDATE
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Quoting Levi32:


Oh, meteorology is that clear cut? Gee if I hadda' known that I would have pursued a degree in something more challenging...
Levi, You do a great job.... I respect your forecast.....Please don't let this upset you.... The world isn't all daisies
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Well the latest GFS run is getting closer in agreement with the Euro, moving west ever so slightly to a landfall at Apalachicola.
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Quoting StormJunkie:


All in all though, it shows exactly what thewindman was referring to. As much of a mess as this system is, and with the mountains coming up, and then fairly close proximity to Fl. Odds say that this will be a rain maker and that is about it.


Actually, no. It shows a potent (and very large) Cat 1/2 hurricane taking it's sweet time moving along the Florida coast. More than a rain maker in my eyes.
Member Since: August 10, 2010 Posts: 2 Comments: 1971
HR 144 FINAL

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The two blobs are pretty much completely separate now:

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There is something to be said about the GFS/Euro consistency in the long range. Both of them seem to want the system at least in the Eastern Gulf. I'd split the difference of the two for now and say FL Panhandle final landfall.
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Quoting Drakoen:
Some day you will learn when you have a real job...there is a right and a wrong. The engineer cannot miss the minus sign or the bridge collapses. I'm gonna take a little break now.
Couldnt leave without taking a shot huh. . And to think I used to like that guy!
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Quoting StormJunkie:
Kermit didn't find a wind shift at all?


Kermit's mission is not center fixes, so they would not necessarly be trying to find one. They are doing Tail Doppler Radar, and I think they are on leg 3 to 4.


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Quoting Levi32:
Looks like a center reformation near Dominica.

can you give me an appoximate location? (coordinates)
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Quoting MAweatherboy1:
Pretty strong storm on 18z:



This is not a 850mb vort signature of a "strong" system.

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Center over Guadeloupe now. Link
Little wind. Maybe 20 km/h
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That's a heck of a lot of precip over South Florida in the GFS.
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502. TXCWC
Quoting Levi32:
Looks like a center reformation near Dominica.



If so, could make a big difference with land interaction with Haiti
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Quoting noraneinsite:
I really enjoy reading analysis from some of the more informed members here and wait to see what unfolds down the road to see who was more accurate. So I think I speak for alot of lurkers when I say give it a rest. I have never heard Levi attack anyone for being wrong and I value his opinion as well as yours.

JMO Weather/tropics is never set in stone unless God Himself says it..been on this blog awhile and never seen Him on here. Weather is an imperfect science which makes it so interesting and up for debate. Think thats why we are all here. Again, JMO
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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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