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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OK, time for the most-hated quiz of all time:

How strong will Isaac be at peak on the SSHS?

A. TS
B. Cat 1
C. Cat 2
D. Cat 3
E. Cat 4
F. Cat 5

I'm going for C, cat 2 with 90 kt winds, 961 mb about 36.8 nm S of Clearwater, Florida.
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1250. LargoFl

000
WTNT84 KNHC 222335
TCVAT4

ISAAC WATCH/WARNING BREAKPOINTS/INTERMEDIATE ADVISORY NUMBER 7A
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012
800 PM AST WED AUG 22 2012

.TROPICAL STORM ISAAC

PRZ001-002-003-004-005-006-007-008-009-010-011-01 2-013-VIZ001-002-
230300-
/O.CON.KNHC.TR.W.1009.000000T0000Z-000000T0000Z/
/O.CON.KNHC.HU.A.1009.000000T0000Z-000000T0000Z/
800 PM AST WED AUG 22 2012

PUERTO-RICO 18.22N 66.44W
VIEQUES 18.12N 65.43W
CULEBRA 18.32N 65.28W
ST-THOMAS-AND-ST-JOHN 18.33N 64.85W
ST-CROIX 17.74N 64.73W

$$

ATTN...WFO...SJU...
Member Since: August 6, 2011 Posts: 4 Comments: 41000
1248. DDR
Some feeder band activity starting to take shape over Trinidad.
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Quoting Bluestorm5:
Well, one half is saying Isaac is getting strong FAST while the other half is saying it's still struggling... *facedesk*


There is a reason that the recon never found anything lower than 1004mb. Recon was in there not long ago and there is no way it has strengthened by 5mb since that time.
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1246. HrDelta
Quoting Bluestorm5:
Well, one half is saying Isaac is getting strong FAST while the other half is saying it's still struggling... *facedesk*


To be fair, it seems to have been doing that all day.
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Quoting Drakoen:
It is more likely that Issac will impact Florida than it turning east of the state. The subtropical ridge axis extends out towards the Bahamas and that will change little over the next couple of days.


Hey so what do you thing of the latest sat. image over the past hour or so?
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Quoting sunlinepr:
Water Vapor loop

Looks like he is ready to really ramp up here. Amazing the processes it takes for these things to form. Guess that's the attraction...
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So great to see people we haven't seen in a while!
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Are we Ready.... almost here...

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Quoting Levi32:


So far in my "career" it has served me well more times than it has bitten me in the rear.


Quoting hydrus:
Crazy..Busy..trying to keep from falling behind...Which I am barely doin.. Isaac is ready to blossom into a dangerous storm..Naturally I am concerned what this Caribbean Bruiser/Cruiser is gonna do.


Hang in there, brother...this one will become, or it won't.
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Quoting LBAR:
So, Isaac appears to be coalescing towards the more northern blob of convection?

South

Quoting HurricaneSwirl:


Well looks like the two convective blobs are sorta connecting.


well I did say it will come together

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Don't know exactly what is going on with Issac at the moment coc wise but it is currently going through the strongest burst of convection that we have seen all day...
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Quoting sunlinepr:


Which COC is the one to consider?

Which ever becomes more dominant.
Member Since: September 30, 2007 Posts: 9 Comments: 15962
1236. Levi32
Quoting KoritheMan:


Yes, but even taking that into account, a turn east of Florida does not seem feasible if current trends continue. I don't think a sprawling disorganized 45 kt tropical storm is going to have proclivity toward the trough.


"Sprawling" actually helps. The larger it is, the larger of a depression it has in the 500mb height field, which can help it move north, but I can see where you're coming from. Another very interesting storm to track.
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New SHIPS is pretty aggressive, takes Isaac to 107kts.

With land though its much lower.
Member Since: February 11, 2012 Posts: 84 Comments: 7948
Quoting MississippiWx:
Isaac is most likely not sub-1000mb.


Not yet still consolidating however it could be by as soon as 11pm.
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1233. Drakoen
It is more likely that Issac will impact Florida than it turning east of the state. The subtropical ridge axis extends out towards the Bahamas and that will change little over the next couple of days.
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Quoting KoritheMan:


There is a trough, but it might not stick around for long. The weaker Isaac stays, the less pull it gets from the trough also.

No one is out of the woods from Louisiana to the Carolinas.


AHHH change of heart my dear?
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Well, one half is saying Isaac is getting strong FAST while the other half is saying it's still struggling... *facedesk*
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Water Vapor loop

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Quoting Levi32:


I still think it could shift back. Goodness knows we've seen several storms in this position "nailed down" 96 hours in advance only to have the models shift by hundreds of miles during the final 72 hours. If the models over the next 48 hours shift farther westward then I'll probably have to shift, but it's the cat and mouse game every track forecaster plays with the computer.


I am sticking to my reasoning too and I don't see any major reasons to change it at this early stage...my forecast cone would be similar to yours, ending with Isaac situated somewhere near the NW-ern Bahamas by day 5 on a NW-erly heading.
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Quoting zoomiami:


I don't like that at all. Watching channel 10 here in Miami, based on the models/possibility, Miami is going not going to have fun either way.
Hahh, I had it on as well. Max Mayfield speaking that wisdom.
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Quoting MiamiHurricanes09:
Wow thanks a ton!

He hasn't updated it since Arlene from last year. :( Does he post on the blog?


Yes, usually late night.
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Quoting sullivanweather:
Photobucket


It's amazing how large the circulation field of Isaac is. Due to how large it is a good portion of the circulation field covers the dry air regions of the subtropics. This dry air impinging on the inner circulation field of the storm is masking the true center of the storm. Also due to the southwest displacement of the storm center in relation to the overall circulation field it can be deceiving.

Photobucket

A closer inspection, with streamlines, shows the elongation of the center. Also, the apparent southwest movement of the center, which appears to me as a center of vorticity being expelled from the storm as it has become too elongated. I fully expect the dominant center to take over on the north side of the elongated center, closer to the center of the overall circulation of the storm.


preach brother
Member Since: August 22, 2006 Posts: 2 Comments: 1988
Do we have an estimate on the actual size (across) Isaac is? I have not seen that.
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update for 12am is in


AL, 09, 2012082300, , BEST, 0, 158N, 622W, 40, 1004, TS
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1223. icmoore
Quoting Floodman:


Yep, we hope so too...

Just Someone with a camera? LOL...yep, and Picasso was just this guy with a paintbrush. You have a wonderful knack...

As fior Press getting his mind out the gutter? That would require some kind fo surgery...he's a filthy old reprobate and I love him like a brother...LOL


Sorry guys last personal note I know it's busy but I have to say thank you so much, blushing, very nice of you to say! :))
As far as you know who I agree but what kind of surgeon? :)
Member Since: July 18, 2005 Posts: 9 Comments: 4146
Quoting KoritheMan:


No.


The reason they are called Cyclones is because they have one eye. Just like the Cyclopes (one eye) Re: the two eyed hurricane question from a few posts back.
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Quoting SLU:
ISAAC is an extremely decoupled storm. The mid-level center seems over 150 miles to the south of the LLC according to the radar and RECON data.


really?
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The best graphics are the animated ones. Keep em coming folks!

Regarding Models:
I'm not falling for any model run right now...
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1219. MahFL
Isaac is exploding now, must have got organized at last.
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1218. Levi32
Quoting weatherb0y:
Hey Levi, do you think it is Isaac's swift foward speed that is prohibiting rapid development at this point? When or what area is Isaac expected to slow down in?


Movement faster than 20mph certainly isn't helping, and neither are the accelerating trade winds to the west. Some of Isaac's struggles are very similar to Ernesto's when he was in this position.
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Quoting MississippiWx:
Isaac is most likely not sub-1000mb.


Thank you.
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1215. Grothar
Very strong storms and cold cloud tops to the NE of the center look they are beginning to wrap around the center.

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Quoting AtHomeInTX:
MH9 Bobby's blog. :)
Wow thanks a ton!

He hasn't updated it since Arlene from last year. :( Does he post on the blog?
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Quoting KoritheMan:
Sorry, but I don't buy sub-1000 mb pressure readings with a storm as disorganized as Isaac. Likely contaminated.


Not yet however I though the reorganization process would have been longer though. TC is unpredictable so much can happen intensity-wise in mere hours.
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Quoting Levi32:
Still no wind shift SW of the center....this is getting comical.

They are flying 2000 feet higher than less pass. They are flying the same hight that they passed on the first round i wish theyed fly lower
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1211. LargoFl
..on this run the GDFL has the same track also....we in florida ought to be very careful, this storm Could fool us
Member Since: August 6, 2011 Posts: 4 Comments: 41000
1210. 7544
is issac getting stronger at this hour
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1209. divdog
Quoting Hurricanes305:


I HAVE BEEN SAYING THAT SINCE 8pm the sudden drop in pressure is a response to a sudden monster blowup of convection RAPID INTENSIFICATION could be COMMENCING!!!
Funniest one liner of the day
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1208. xcool
hydrus.. ya lol
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1207. HrDelta
Quoting Levi32:
Still no wind shift SW of the center....this is getting comical.



So, uh, where exactly is the center? Are we dealing with a Dolly-like storm again?
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Quoting weatherb0y:
Hey Levi, do you think it is Isaac's swift foward speed that is prohibiting rapid development at this point? When or what area is Isaac expected to slow down in?


dont have to be levi to answer that one
tis true
and when he'll slow down? when he intensifies hehe.
Member Since: August 22, 2006 Posts: 2 Comments: 1988
Quoting Levi32:


The significance of that statement though is that it implies that the perception of many people is that Isaac is struggling more than expected to organize. Relative to the initial NHC intensity forecasts which were overdone, this is true, but then none of the global models have shown any kind of real deepening until Isaac gets west of the lesser Antilles. In that sense, the storm is still right on schedule, taking its time, likely starting to strengthen a bit more just before running into Hispaniola.


Yes, but even taking that into account, a turn east of Florida does not seem feasible if current trends continue. I don't think a sprawling disorganized 45 kt tropical storm is going to have proclivity toward the trough.
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Does it look to you like the center is consolidating now.
Thanks
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Isaac is most likely not sub-1000mb.
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Quoting drg0dOwnCountry:


Which part is it? Quote it.


I'm not gonna argue about it. Let's just drop it.
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Quoting xcool:


HWRF

HEY It's Been a Long Time


Hey xcool! :)
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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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