Isaac lashing the Lesser Antilles; TD 10 forms

By: Dr. Jeff Masters , 3:06 PM GMT on August 22, 2012

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Tropical Storm Isaac is lashing the entire Lesser Antilles chain of islands with heavy rains this morning, and the winds will be on the increase this afternoon as the storm heads west at 19 mph. Isaac is still a weak and disorganized tropical storm, but that is changing. The Hurricane Hunters are in the storm, and have thus far found no increase in the storm's top surface winds, which remain near 45 mph. In their 7:44 am EDT center fix, an Air Force Reserve aircraft measured a rather unimpressive center pressure of 1007 mb, and top winds at their 1000 foot flight altitude of 56 mph. The plane 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 it is unlikely Isaac will become a hurricane today. Satellite loops, however, show that Isaac has developed a Central Dense Overcast (CDO) of high cirrus clouds, the hallmark of a developing storm. These clouds have very cold cloud tops, indicating that the updrafts creating them are quite strong. An analysis of upper level winds from the University of Wisconsin CIMSS shows that upper-level outflow is becoming well-established to the southwest and northwest, but outflow is restricted on the southeast side. A large clump of heavy thunderstorms several hundred miles southeast of the center continues to compete for moisture, and is interfering with the low level inflow and upper level outflow of the storm. The intensification rate of Isaac will increase if the storm is able to integrate this clump of heavy thunderstorms into its circulation, which satellite loops this morning suggest is now beginning to happen. Radar imagery from Barbados and Martinique show plenty of heavy rain showers, mostly on the south side of Isaac where it is moister, but little spiral banding. Hurricane hunter missions are scheduled for Isaac every six hours, and a NOAA hurricane hunter research aircraft will also be in the storm, with missions scheduled every 12 hours. The NOAA jet is first scheduled to fly into the storm on Thursday afternoon, to do a large-scale dropsonde mission to aid model forecasts.


Figure 1. Morning radar image of Isaac taken from the Barbados radar at 10:15 am EDT. Image credit: Barbados Weather Service.

Intensity forecast for Isaac
The latest 8 am EDT run of the SHIPS model shows that wind shear is moderate, 10 - 15 knots, but is expected to relax by this evening to a low 5 - 10 knots. Ocean temperatures have increased to a very warm 29°C, and the storm is now over waters with a high total heat content. The lowering wind shear and warm waters should allow the storm to wall off the dry air that has been interfering with development, and also allow the storm to integrate the thunderstorm clump on its southeast side that has been interfering with low level inflow and upper level outflow. It will take some time for the increase in organization to result in an increase in Isaac's winds, and I still expect top winds of 45 - 60 mph in the Lesser Antilles Islands this evening when the core of the storm moves through. On Thursday, when Isaac will be in the Eastern Caribbean, conditions should be favorable enough to allow steady strengthening to a Category 1 hurricane. The 11 am EDT wind probability forecast from NHC predicted a 47% chance that Isaac will become a hurricane by Friday morning, and a 16% chance it will be a Category 2 or stronger hurricane then. This is a reduction in the odds given in the 5 am advisory.

Impact of Isaac on the Islands
The entire Lesser Antilles Islands chain will have a three-day period of heavy weather Wednesday through Friday. Sustained tropical storm-force winds extend out about 50 miles to the north of the center and 30 miles to the south, so an 80-mile wide swath of the Lesser Antilles will potentially see tropical storm-force winds of 45 - 60 mph this Wednesday evening. Guadaloupe, Antigua and Barbuda, Dominica, Montserrat, and St. Kitts and Nevis at highest risk of these winds.

Winds in St. Croix in the Virgin Islands will likely rise above tropical storm force on Thursday morning, and the south coast of Puerto Rico should see tropical storm-force winds by Thursday afternoon. The San Juan airport may be able to stay open Thursday afternoon and evening, but I think it is more likely they will be forced to shut down.

On Thursday night, heavy rains and tropical storm-force winds should arrive on the southern coast of the Dominican Republic, and all airports in the D.R. will probably be closed on Friday. Rainfall amounts of 8 - 12 inches will likely affect the Dominican Republic Thursday through Saturday, creating dangerous flash floods and mudslides.

Isaac is potentially a very dangerous storm for Haiti, where 400,000 people still live outside underneath tarps in the wake of the 2010 earthquake. Heavy rains from Isaac will begin on Friday morning in Haiti, and last through Sunday. Rainfall amounts of 8 - 12 inches are possible, which will be capable of causing extreme flooding on the vegetation-denuded slopes of Haiti. It will be a major challenge to keep those Haitians living outside safe, if rainfall amounts of 5 - 10 inches occur.

Impact on Florida, Cuba, Jamaica, and the Bahamas
Florida, Cuba, Jamaica, and the Bahamas are all at high risk of receiving hurricane conditions from Isaac. The latest set of 06Z (2 am EDT) model runs for Isaac are fairly unified for the coming three days, showing a westward track to a point on the south coast of Hispaniola. All of the models then predict a more west-northwest track across the island and into eastern Cuba, as Isaac responds to a trough of low pressure over the Southeast U.S. Most of the models then predict a path for Isaac along the spine of Cuba, then into the Florida Straits off the coast of Miami by five days from now. A notable exception is our best-performing model, the ECMWF, which keeps Isaac just south of Cuba, and takes the storm more to the west between Jamaica and Cuba on Saturday, then into the Yucatan Channel between Mexico and Cuba by Monday. 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. Some models predict a more easterly exit point, allowing Isaac to move up the east coast of Florida, and potentially make landfall in the Southeast U.S. The latest 06Z GFS model run predicts a more westerly track, which would potentially allow Isaac to move up the west coast of Florida towards Tampa. Keep in mind that the average error in a 5-day forecast is 260 miles. The two most recent runs of the GFS model, at 00Z and 06Z (8 pm and 2 am EDT), gave positions for Isaac that were about 250 miles apart--the earlier run putting the center near West Palm Beach, and the more recent run giving a location between Key West and Havana, Cuba. While passage over the high mountains of Hispaniola and then Cuba will substantially disrupt Isaac and probably reduce it below hurricane strength, the storm is quite large, and should be able to re-intensify once it emerges over the Florida Straits. Waters will be very warm, near 30°C, wind shear is predicted to be light, and forecasts of the upper-level winds show the possibility of an upper-level outflow pattern very favorable for intensification. If Isaac spends a day over water, that should be enough time for it to intensify into a Category 1 hurricane, and if the storm takes a longer 2-day track over water up either the east or west coast of Florida, a Category 2 or stronger storm is possible.

Isaac is a threat to affect Tampa during the Republican National Convention, August 27 - 30, and the official NHC forecast now has Tampa in the 5-day cone of uncertainly. The latest 11 am EDT wind probability forecast from NHC gives Tampa a 9% chance of receiving tropical storm-force winds for the 24-hour period ending on the morning of first day of the convention (Monday). I blogged about the climatological chances of a hurricane causing an evacuation of Tampa during the convention in a post last week, putting the odds at 0.2%. I put the odds of an evacuation occurring during the convention in the current situation at 3%.


Figure 2. Morning satellite image of Tropical Depression Ten.

Tropical Depression Ten forms in the Eastern Atlantic
The large tropical wave in the Central Atlantic, midway between Africa and the Lesser Antilles Islands, has become Tropical Depression Ten. The depression has an impressive amount of spin and heavy thunderstorm activity, as seen on visible satellite loops, and should be Tropical Storm Joyce by Thursday. None of the models show that TD 10 will be a threat to the Lesser Antilles Islands.

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? Well, 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 3, 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 3. 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 this afternoon.

Jeff Masters

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hey guys I don't know but I don't take the 11am fix for Isaac to be at 15.9 N
well Sattelite radar and surface obs do kinda suggest that the LLC is quite abit S of 16N more like near 15.5N or even further S

anyway we should have a much better idea when recon flys in
by the way recon takeoff in about 3 hours I think
Member Since: June 13, 2009 Posts: 2 Comments: 12147
Quoting BTRWeather:

Anyone have a link to past NHC official tracks? If not, I would like to copy these images if you don't mind?

Go ahead. The first one I pulled off a google search. The other one I pulled directly from the "printer friendly" version at the NHC website.
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Quoting LargoFl:
OK GUYS..WHY DO THEY HAVE TWO WIND CONES UP???
one for TS Isaac, and one for TD10?
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Quoting BahaHurican:

I can't help but think about how much people would be freaking out if Isaac's center was a mere 60 or so miles south of where it is currently estimated. That would be one good looking storm.
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12Z GFS Init:



About right.
Member Since: August 14, 2008 Posts: 4 Comments: 4977
Quoting Patrap:


Got dat from EmmyRose on FB.

So I'll relay it to her hanna.

: )

thank ya kindly...always go to the source I say :)
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Quoting jeffs713:
Prolly St. Croix. Maybe Barbados.

OK, I wondered if it was BDos.
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The CDO seems to be moving through the islands largely south of Guadaloupe. Yet the track has been consistently predicting the the centerline crosses that island. Seems confusing.
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Quoting 954Soxfan:
Looking better and better for Sfla!!!! Keep that cone moving away!!!


it would be nice if you posted some reasoning behind your statement. The track is still on s fl....
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Quoting jeffs713:
For people freaking out about the cone basically destroying Florida... Just remember last year:

IRENE


ISSAC


Things can, and will change. How they will change, we don't know. But they will change.

Anyone have a link to past NHC official tracks? If not, I would like to copy these images if you don't mind?
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46. Dr. Jeff Masters, Director of Meteorology (Admin)
11:21 AM EDT on August 22, 2012


Thanks Dr.
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Quoting hurricanehanna:

stop that ;P


Got dat from EmmyRose on FB.

So I'll relay it to her hanna.

: )
Member Since: July 3, 2005 Posts: 426 Comments: 128636
Quoting pottery:

Where they flying from ?
Prolly St. Croix. Maybe Barbados.
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Quoting kshipre1:
can someone give their best educated answer and guess if they think it is possible this storm could miss florida to it's east or west? I think the intentsity forecast seems better for Florida compared to last night. someone please chime in on their predicted path east or west and possible intensity. thanks!

At 4-5 days out, absolutely it could miss FL to either side, especially based on a 300+ mile cone radii at that far out, and the angle Issac is expected to come in. If it was coming from the east, the chance would be smaller, but we'd have a whole different scenario in that case.

Right now the models are basically figuring that Issac would hit FL like an arrow striking another arrow directly in the bullseye. (also known as rather unlikely)
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TD10 and Isaac seem to establish a Fujiwhara pathway.
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Quoting Methurricanes:
Is the Super Bowl Doomed?


No more than the RNC Circus one could say with absolute Faith, easily.

; )
Member Since: July 3, 2005 Posts: 426 Comments: 128636
He isn't an open wave lol!
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Oh boy here it comes central florida
Member Since: August 6, 2011 Posts: 4 Comments: 39116
Quoting VAbeachhurricanes:


they should be leaving in about 30 minutes

Where they flying from ?
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12z gfs initialization

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Thank you Dr..The different path scenario,s for Haiti are frightening. The unlikely path to the north would probably be the most forgiving, a pass to the south would still bring high winds and heavy rain, but not as bad as a direct hit. It appears that Hispaniola is to big for Isaac to miss, and there will be some type of tragedy. Not to sound prophetic, just a quick look at this regions history provides one with enough reason to be troubled.
Member Since: September 27, 2007 Posts: 1 Comments: 21413
Quoting Patrap:
Fer snits and Giggles maybe?



stop that ;P
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Quoting kshipre1:
can someone give their best educated answer and guess if they think it is possible this storm could miss florida to it's east or west? I think the intentsity forecast seems better for Florida compared to last night. someone please chime in on their predicted path east or west and possible intensity. thanks!


Best advice from me, follow the NHC official Cone and Guidance.

They are the Word up one could say.

: )
Member Since: July 3, 2005 Posts: 426 Comments: 128636
Quoting Patrap:
Fer snits and Giggles maybe?


Is the Super Bowl Doomed?
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Quoting VAbeachhurricanes:


If the storm was sitting still then yes, however since the storm is moving at 21mph the storms on the south side would have to be moving at >21 to appear to be moving east.


So it is moving so fast it can't spin properly, hence slow to no intensification... I'll buy that for now until the next RECON.
Member Since: July 4, 2005 Posts: 204 Comments: 8896
11AM is not good for S FL at all
Quoting 954Soxfan:
Looking better and better for Sfla!!!! Keep that cone moving away!!!
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Quoting shfr173:
Question? Did HH visit storm this AM? If so will data be in this model run?


they should be leaving in about 30 minutes
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SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE TAMPA BAY AREA - RUSKIN FL
1105 AM EDT WED AUG 22 2012

FLZ039-042-043-048-049-051-221600-
SUMTER-LEVY-CITRUS-HERNANDO-PASCO-HILLSBOROUGH-
1105 AM EDT WED AUG 22 2012

...STRONG THUNDERSTORMS WILL AFFECT CITRUS...NORTHWESTERN
HILLSBOROUGH...NORTHWESTERN SUMTER...SOUTHERN LEVY...WESTERN HERNANDO
AND WESTERN PASCO COUNTIES...

NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATES THUNDERSTORMS
LOCATED ALONG A LINE EXTENDING FROM CRYSTAL MANOR TO 29 MILES WEST
OF PINE ISLAND TO 20 MILES WEST OF PORT RICHEY TO 42 MILES WEST OF
HOLIDAY...OR ALONG A LINE EXTENDING FROM 7 MILES NORTH OF CRYSTAL
RIVER TO 33 MILES NORTHWEST OF HUDSON TO 20 MILES WEST OF HUDSON TO
38 MILES WEST OF DUNEDIN...MOVING EAST AT 20 MPH...WILL AFFECT
CRYSTAL RIVER...OZELLO...CRYSTAL RIVER AIRPORT AND BEVERLY
HILLS...UNTIL NOON EDT.

GUSTY WINDS 35 TO 45 MPH WILL OCCUR. FREQUENT LIGHTNING IS EXPECTED.
TO BE SAFE GO INDOORS IMMEDIATELY. IF CAUGHT OUTSIDE...FIND A LOW
SPOT...AND STAY AWAY FROM TALL OBJECTS. WHEN DRIVING THROUGH HEAVY
RAIN...SLOW DOWN. ALWAYS LEAVE A SAFE DISTANCE BETWEEN YOU AND OTHER
VEHICLES.

&&

LAT...LON 2942 8301 2928 8240 2921 8241 2920 8253
2905 8253 2896 8219 2867 8238 2826 8246
2817 8265 2818 8286 2822 8285 2822 8278
2845 8268 2869 8268 2878 8276 2890 8270
2907 8283 2915 8282 2916 8298 2909 8308
TIME...MOT...LOC 1457Z 270DEG 17KT 2899 8259 2866 8308
2835 8300 2808 8338

$$
Member Since: August 6, 2011 Posts: 4 Comments: 39116
can someone give their best educated answer and guess if they think it is possible this storm could miss florida to it's east or west? I think the intentsity forecast seems better for Florida compared to last night. someone please chime in on their predicted path east or west and possible intensity. thanks!
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Quoting shfr173:
Question? Did HH visit storm this AM? If so will data be in this model run?

Probably 00z. Slight chance of 18z, but unlikely.
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Doom-inator?
Member Since: July 3, 2005 Posts: 426 Comments: 128636
Quoting LargoFl:
OK GUYS..WHY DO THEY HAVE TWO WIND CONES UP???

Isn't this picking up TD10 too ?
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Quoting Patrap:
Fer snits and Giggles maybe?




Doomcane (tm)
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If it misses Hispaniola's mountains it will be a:

Major Hurricane – wind speed greater than 110 MPH

Prior to impacting the SE US.


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Question? Did HH visit storm this AM? If so will data be in this model run?
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Quoting LargoFl:
OK GUYS..WHY DO THEY HAVE TWO WIND CONES UP???

Because they are expecting Joyce to develop.
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WxGeekVA forecast... From last blog, fixed to make it not look much like an official track:



UNOFFICIAL
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Quoting CybrTeddy:
This was Ernesto at the time it was struggling to find it's LLC, and looked generally a bad storm on satellite through most its life until it reached the Western Caribbean.


And here's Isaac at a similar longitude.

The first thing I noticed between the two images is the large size differential between the storms. Ernesto was tiny while Isaac is massive!
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Fer snits and Giggles maybe?


Member Since: July 3, 2005 Posts: 426 Comments: 128636
Quoting GetReal:
An observation with a question... After observing the Barbados radar loops it is clearly visible to anyone that the convection north of Barbados is moving EAST to
WEST. Now if the center of Isaac is currently located at 15.9N as NHC indicated on last advisory, shouldn't the convection on the Barbados radar be moving WEST to EAST. After all doesn't a cyclone rotate counter-clockwise in the N hemisphere?


Link hit the 400km loop.


If the low level center is that far north, Isaac is not stacked, and that is a stronger mid level circulation the Barbados radar is picking up.
I stated in the last Blogg that Isaac is probably a open wave but they keep it as a TS because of the lives that were lost with degrading TD 7 earlyer this month
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For people freaking out about the cone basically destroying Florida... Just remember last year:

IRENE


ISSAC


Things can, and will change. How they will change, we don't know. But they will change.
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Repost from previous blog, didn't catch that there was a new entry.

Things are starting to heat up in the tropics now. Very hard to say the exact track of Isaac now after 3 days time. I don't like historical comparisons much, dynamics are always different in some ways, but 1979's Hurricane David has a very similar track to that projected for Isaac. Models seem pretty on and in good agreement from what I'm seeing, but again, can't say for sure. It'll come down to his strength and the break strength on where Isaac will turn north. For now I'd take a middle of the road projection for him, taking him roughly to the Florida Bay.

Oh and I will say though, Isaac looks pretty good from satellite images for a 1006mb storm.
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...................Back to back tropical systems
Member Since: August 6, 2011 Posts: 4 Comments: 39116
Could TD 10 pose a threat to the East Coast, I know orginally, Isaac was suppose to recurve, and now it is suppose to go up the West Coast of Fl
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46. JeffMasters (Admin)
Quoting weathermanwannabe:
Thanks Dr. M. Basically, as I interpret your analysis, the storm is still having short-term intensity issues due to some dry air and the speed is probably contributing to a titled circulation. Also looks like the best conditions for rapid intensification might be, ironically, after passage over Cuba...........It's going to be a very touchy situation for the Keys/South Florida/Bahamas come next Monday if the storm really intensifies just offshore.


Yes, I'll talk in a later post, probably tomorrow, on how this storm reminds me of Tropical Storm Fay of 2008. We had some model runs saying that storm was going to go up the west coast of Florida and intensify into a hurricane after it popped off the coast of Cuba, but got lucky that it made landfall over the Everglades near Flamingo.

The low 3% chance of an evacuation is because we're talking about a 6-to-7-day forecast, which has an error of 400 miles.

Jeff Masters
And the Drama builds........
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Quoting txwcc:
From last blog...



That's my question too....Don't know what's going on.
my first guess is... the whole system is moving west at such a rate that spiraling, while rotating as expected around a fixed center, said center is moving at a greater rate than the respective spiraling. haven't viewed the Barbados radar, but visibly on satellite this appears to be the case.
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This was Ernesto at the time it was struggling to find it's LLC, and looked generally a bad storm on satellite through most its life until it reached the Western Caribbean.


And here's Isaac at a similar longitude.
Member Since: July 8, 2005 Posts: 259 Comments: 24164

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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.