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Hurricane ISABEL


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HURRICANE ISABEL DISCUSSION NUMBER  31
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
5 PM EDT SAT SEP 13 2003
 
THERE HAS BEEN A PLETHORA OF WIND DATA FROM BOTH THE AIR FORCE
RESERVE AND NOAA HURRICANE HUNTERS THIS AFTERNOON. IN SUMMARY...THE
AIRCRAFT FOUND MAXIMUM 700 MB FLIGHT-LEVEL WINDS OF 157 AND 158 KT
...ALONG WITH A DROPSONDE WIND VALUE OF 178 KT...OR 205 MPH...AT
879 MB...ALL OF WHICH CORRESPONDS TO SURFACE WIND ESTIMATES OF
141-142 KT. THEREFORE...THE INITIAL INTENSITY HAS BEEN INCREASED TO
140 KT/160 MPH...SO ISABEL IS A CATEGORY 5 HURRICANE ONCE AGAIN.
 
THE INITIAL MOTION ESTIMATE IS NOW 285/11. THIS MAY BE A SHORT-TERM
ACCELERATION DUE TO THE EYE CONTRACTING DOWN IN SIZE AND
RE-ADJUSTING WITHIN THE CENTRAL DEEP CONVECTION. HOWEVER...THE 285
DEGREES MOTION NOW APPEARS TO BE A STEADY TREND. THE NEW 12Z GFS
FORECAST TRACK HAS COME IN ALMOST ON TOP OF THE PREVIOUS OFFICAL
FORECAST THROUGH 48 HOURS AND THEN IS LEFT OF THE TRACK BY 72 HOURS
...BEFORE HOOKING BACK ACROSS THE PREVIOUS FORECAST TRACK AND
ACCELERATING ISABEL NORTHWARD. THIS SCENARIO IS ALSO IDENTICAL TO
THE 12Z ETA MODEL RUN. SIMILARLY BUT TO THE RIGHT OF THE PREVIOUS
FORECAST ARE THE GFDL...NOGAPS...UKMET...AND CANADIAN MODELS.
INTERESTINGLY ENOUGH...ALL OF THE MODELS AGREE ON A SHARP DECREASE
IN FORWARD SPEED OR A STALL IN 60-72 HOURS...AND THEN RAPIDLY
ACCELERATING ISABEL NORTH OR NORTH-NORTHWESTWARD AFTER 96 HOURS AS
THE CYCLONE IS FORECAST TO GET PICKED UP BY AN APPROACHING
SHORTWAVE TROUGH FROM THE WEST AND A BUILDING RIDGE TO THE EAST.
HOWEVER...THERE IS STILL SOME UNCERTAINTY ON THE CURRENT STRENGTH
OF THE SUBTROPICAL RIDGE TO THE NORTH OF ISABEL...SO IT IS PRUDENT
TO WAIT UNTIL ALL OF THE NOAA GULFSTREAM-IV DROPSONDE DATA MAKE IT
INTO THE 14/00Z MODELS. THE OFFICIAL FORECAST TRACK IS JUST AN
UPDATE OF THE PREVIOUS TRACK THROUGH 96 HOURS...WITH A LITTLE MORE
NORTHWARD ACCELERATION INDICATED AT 120 HOURS AS A COURSE OF LEAST
REGRET. NOTE -- NONE OF THE NHC MODEL GUIDANCE HAS ISABEL OVER LAND
BY 120 HOURS.

WHILE SOME SLIGHT STRENGTHENING IS POSSIBLE IN THE NEXT 12 HOURS...
THERE IS NO WAY TO REALLY TIME THE DEVELOPMENT OF ANOTHER EYEWALL
REPLACEMENT CYCLE. IN THE LONG RUN...UPPER-LEVEL CONDITIONS ARE
FORECAST TO REMAIN FAVORABLE FOR AT LEAST THE NEXT 48-72 HOURS...
WITH THE HURRICANE ALSO MOVING OVER WARMER WATER. THEREFORE...A
STEADY INTENSITY OR ONLY VERY SLOW WEAKENING IS EXPECTED FOR THE
NEXT COUPLE OF DAYS. BY 120 HOURS...ISABEL MAY BEGIN TO
EXPERIENCING UPPER-LEVEL SOUTH TO SOUTHWESTERLY SHEAR...WHICH
SHOULD BRING ABOUT MORE RAPID WEAKENING. HOWEVER...ISABEL IS STILL
EXPECTED TO REMAIN A MAJOR HURRICANE THOUGHOUT THE FORECAST PERIOD.
 
FORECASTER STEWART
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      13/2100Z 22.6N  62.6W   140 KT
 12HR VT     14/0600Z 23.0N  64.2W   140 KT
 24HR VT     14/1800Z 23.7N  66.2W   140 KT
 36HR VT     15/0600Z 24.4N  68.0W   135 KT
 48HR VT     15/1800Z 25.2N  69.4W   135 KT
 72HR VT     16/1800Z 26.5N  71.5W   130 KT
 96HR VT     17/1800Z 29.5N  73.0W   125 KT
120HR VT     18/1800Z 33.5N  75.0W   105 KT
 
 
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