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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2501. angiest
WFO Houston had this to say earlier today:

THE EXTENDED FORECAST IS STILL RATHER MUDDLED. OVERALL MODELS
SHOW UPPER LEVEL RIDGE BUILDING OVER TX SUN/MON. THERE STILL MAY
BE ENOUGH MOISTURE TO SUPPORT AN ISO STORM OR TWO WITH THE SEA
BREEZE SO WILL AT LEAST KEEP THE TREND OF 20 POPS IN THE FORECAST.
MODELS ARE HAVING A TOUGH TIME WITH THE EVOLUTION OF THE RIDGE
OVER THE ROCKIES AND SUB-TROPICAL RIDGE IN THE ATLANTIC WHICH
WILL PRIMARILY PROVIDE STEERING FOR TS ISAAC. GFS LOOKS WEAKER
WITH BOTH RIDGES WHILE THE ECMWF IS STRONGER WITH EACH RIDGE THUS
HAVING A MORE WESTWARD TRACK FOR ISAAC. STILL WAY TOO FAR OUT IN
FORECAST TIME TO DETERMINE THE IMPACTS FROM ISAAC IF ANY BUT
CERTAINLY SOMETHING TO KEEP TRACK OF IN THE COMING DAYS. THINK
THAT BY THE WEEKEND MODELS WILL HAVE A BETTER HANDLE ON THE
EVOLUTION OF EACH RIDGE AND TRACK OF ISAAC.
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2500. GetReal
Quoting Levi32:
See this. The 700mb high is centered over Houston, and the winds are NNE across the entire north gulf coast. How is a storm on the coast of Cuba supposed to move northwest into that lol. Movement should be more NNW or north here.



Stronger Atl ridge than on that forecast map...
Member Since: July 4, 2005 Posts: 204 Comments: 8898
2497. emcf30
.....
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Quoting winter123:
Tilt your head to the left. Looks a lot like a certain symbol! Also, can someone please post the European model? It seems to be doing better with Isaac thus far, keeping it due west and weaker.

It looks like a bug if you turn your head left to me.
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2495. Levi32
See this. The 700mb high is centered over Houston, and the winds are NNE across the entire north gulf coast. How is a storm on the coast of Cuba supposed to move northwest into that lol. Movement should be more NNW or north here.

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From the NHC - 'An air force hurricane hunter aircraft investigated Isaac several hours ago and found that the center was still not well-defined and the intensity estimate of 40 kt was probably a little on the high side. However...Dropsonde data from a NOAA aircraft currently investigating the storm suggest that the center has probably become a little better defined and that the minimum pressure has dropped a bit. There has also been an increase in cold-topped convection near and over the estimated center so the current intensity is held at 40 kt. Isaac has developed an impressive upper-level outflow pattern and dynamical guidance shows that the vertical shear should remain weak during the next several days. Therefore the primary impediment to strengthening should be the interaction with the mountainous terrain of Hispaniola and eastern Cuba. The most recent runs of the HWRF and GFDL show similar amounts of land interaction but are not in good agreement on the intensity forecast as the former model shows much less strengthening than the latter. The official wind speed forecast is in reasonable agreement with the latest statistical-dynamical guidance.

After a slight southward relocation of the center...The initial motion continues basically westward or 270/17. Isaac should continue to move westward to west-northwestward for the next couple of days along the southern periphery of the subtropical ridge. The global models continue to depict a break in the ridge in the vicinity of florida in several days...Which should induce a more northward component of motion. The ECMWF model shows less of a break in the ridge than the GFS solution. Consequently...The ECMWF track is somewhat to the southwest of most of the other track models which would lessen the threat to Florida. The official track forecast for this advisory is quite close to the previous one and also very close to the dynamical model consensus.'
Member Since: September 18, 2007 Posts: 0 Comments: 1074
Gotta give some credit here to WKCM. Still default to the next plane but all available info is pointing to a center that is SSW.

850 Vort


500 Vort


Lower Convergence


Isaac
Member Since: August 19, 2005 Posts: 5 Comments: 5458
2491. emcf30
Quoting Michfan:


Thats an old run.


Crap, my bad. Sorry.
Nevermind
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2490. KEEPEROFTHEGATE (Mod)
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Quoting 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. If this run is already off with the position of Isaac, how far off is it further west into this hypothetical run?


You will see another shift to the west...anyone on the Gulf Coast better watch very close...
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2488. KEEPEROFTHEGATE (Mod)
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The cone of the unknown.........boy ain't dat the truth!! Intensity and track is a big guess. It is so typical of a large envelope system to take it's time to get out of the dust bowl E Caribbean**** If Sir Isaac gets wound up and can avoid the mouse traps(landmasses) look out. Cantore shut down Norcross on this battle of Westcaster VS Eastcaster!! Geeez the track is hardly ever down the middle on a slow developer/huge envelope system.
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Quoting KEEPEROFTHEGATE:
It is doing the little scrapey scrapey along the Cuban coast I was talking about though. Interested to see what happens beyond the initial Florida impact -- if it does anything in the Gulf.
Member Since: September 2, 2009 Posts: 130 Comments: 21194
Tilt your head to the left. Looks a lot like a certain symbol! Also, can someone please post the European model? It seems to be doing better with Isaac thus far, keeping it due west and weaker.
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I found this curious (even though I understand this plane was flying at a higher altitude):
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2482. KEEPEROFTHEGATE (Mod)
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Quoting galvestonhurricane:


Texas Death Ridge will prevent that from happening.

The Texas death ridge is forecast to move to the northeast which actually may allow Isaac to go into the western gulf. So he may be right.
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GFS 500 mb. height and vorticity loop
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Quoting AllStar17:


It appears they are returning to Barbados now, though. When's the next one?


2 am
Member Since: May 19, 2009 Posts: 0 Comments: 252
2477. Michfan
Quoting emcf30:


Come right through Orlando this run on Monday and Gainesville on Tuesday



Thats an old run.
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Quoting yqt1001:
Oooh! pretty outflow.

Looks annular.
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2475. angiest
Quoting hunkerdown:

It has to do with opers...the final note of an opera are usually sung by heavier female singers. This is due to the fact that they tend to have a louder, broader range when they sing and it is the perfect way to end the show.


The canonical example is the end of Wagner's Goetterdaemerung, where Bruennhilde has a ~15 minute aria to conclude the work (typically 4-4.5 hours), which also marks the conclusion of Der Ring des Nibelungen, a cycle of four operas. Wagner in particular typically requires big singers to project over the orchestra, which is usually louder than in other operas.
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Quoting angiest:


It is Kirock!
More like Price Line ...
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2473. KEEPEROFTHEGATE (Mod)
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2472. KEEPEROFTHEGATE (Mod)
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2471. gator23
Quoting redwagon:

Just thinking the same thing. Expect a lot of hedging from local mets all over the GOM tomorrow AM.

All over the GOM? It isnt forecasted to effect Texas
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2470. 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. If this run is already off with the position of Isaac, how far off is it further west into this hypothetical run?
Member Since: July 4, 2005 Posts: 204 Comments: 8898
Jim Cantore thinks Issac is forming a new center at 14.9 north
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Quoting MississippiWx:


They are in right now.


It appears they are returning to Barbados now, though. When's the next one?
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2467. KEEPEROFTHEGATE (Mod)
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Quoting Michfan:


You should be just fine. Just stay in contact with your travel agent and the hotel just in case.


You just jinxed the path. Just kidding.
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2464. yqt1001
Oooh! pretty outflow.

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Quoting MiamiHurricanes09:
I guess the fact that the 00z run has more data fed into it helps construct a more accurate forecast, and apparently the more accurate forecast is the slightly more northerly one.

LOL, the only reason I said that is because I compared the 00z position to the 18z position at 48 hours and noticed that the 00z run was already farther north, so I figured this would be a more poleward run.

Yes, I just had to use that phrase. lol (:
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2462. Levi32
Quoting marknmelb:
Levi32 - David 1979 tracked up the east coast not northern florida. Made landfall near west palm I think and tracked right up the intercoastal turning out to sea just north of the cape.


Florida scrape and Georgia full landfall?

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Quoting 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!



GFS must be seeing a stall or a wall or there's a kink in the code!
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2460. emcf30
....
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lmao Levi :) That gave me a damn fine laugh...

Would pinning it down with elephants at both ends be sort of like writing in stone?
Member Since: August 17, 2005 Posts: 26 Comments: 16874
Quoting StormJunkie:


We actually did debate that earlier. Came to conclusion that west winds were seen on the surface, and by HH with dropsondes at lower levels. That said, we also decided that the MLC and LLC are not getting along well at all and could be misaligned by as much as a hundred miles.
Sir Issac Newton maybe wanting to go now
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I'm still not convinced that Isaac won't relocate farther south.
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rides the northern coast of cuba this run.
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Quoting AllStar17:
When is the next recon flight?


They are in right now.
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Quoting chrisdscane:



sry to quote myself how do I post this as an image plzz help


Click on the IMAGE box... type or paste the image link and you should be good to go.
Member Since: September 18, 2007 Posts: 0 Comments: 1074
Quoting Levi32:
Also, it's not over until the lady sings (don't even know what that reference is from lol).


True dat. From opera. As in the opera isn't over until the fat lady sings. Opera sopranos are stereotyped as "fat" and, indeed, many have been.. er stocky.
Member Since: May 19, 2009 Posts: 0 Comments: 252

2373. MBSCEOCHam 3:47 AM GMT on August 23, 2012 +0

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.;;


MY Thoughts exactly. On track with Levi's solution as well. Not etched in stone, but just the general nature of storms in the past, when the curve happens. Seems to always trend to the right rather than to the west. 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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