Hurricane ISABEL

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HURRICANE ISABEL DISCUSSION NUMBER  30
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
11 AM EDT SAT SEP 13 2003
 
ISABEL HAS COMPLETED AN EYEWALL REPLACEMENT CYCLE BASED ON A
13/1102Z SSMI OVERPASS SHOWING A CLOSED 35 NMI DIAMETER EYE. THE
INTENSITY IS MAINTAINED AT 130 KT BASED ON CONTINUITY WITH THE
PREVIOUS DATA...A CONSENSUS SATELLITE INTENSITY ESTIMATE OF 127
KT...RAW ODT VALUES OF T6.5...OR 127 KT...AND THE FACT THAT THE EYE
HAS CLEARED OUT NICELY AND SURROUNDING EYEWALL CLOUD TOPS HAVE
COOLED TO -70C AND COLDER ONCE AGAIN. NEXT RECON MISSION IS AT 18Z.
 
THE INITIAL MOTION IS 280/08. OVER THE PAST 72 HOURS...ISABEL HAS
BEEN MOVING AT 275 DEGREES. THE HURRICANE MADE A SLIGHT NORTHWARD
WOBBLE DURING THE EARLIER EYEWALL REPLACEMENT CYCLE...BUT IT NOW
APPEARS TO HAVE RESUMED A 275-280 DEGREES MOTION. ALL OF THE GLOBAL
AND REGIONAL MODELS HAVE HAD A DISTINCT RIGHT-OF-TRACK BIAS BY AS
MUCH AS 10 DEGREES...AND EACH MODEL RUN HAS BEEN A LITTLE FARTHER
WEST AND SLOWER. THE EXCEPTION IS THE 06Z GFDL RUN WHICH MADE A
MAJOR SHIFT WESTWARD BY MORE 200 NMI IN THE LATER FORECAST PERIODS
...WHILE THE 06Z NOGAPS MODEL ACCELERATED ISABEL AND HAS THE
HURRICANE ONSHORE THE SOUTHEAST COAST OF NORTH CAROLINA IN 120
HOURS. HOWEVER...THE MODEL CONSENSUS IS SLOWER AND FARTHER WEST
BEFORE A TURN TO THE NORTHWEST OCCURS IN THE LATER PERIODS. THE
MODELS OBVIOUSLY HAVE NOT BEEN HANDLING THE STRONG RIDGING TO THE
NORTH OF ISABEL VERY WELL. THIS MAY BE DUE TO THE OUTFLOW ON THE
NORTH SIDE BEING DRAWN NORTHWARD AND SINKING ALONG 30N-33N...WHICH
MAY HAVE HELPED TO STRENGTHEN THE SUBTROPICAL RIDGE. WATER VAPOR
IMAGERY INDICATES THIS PATTERN HAS PERSISTED FOR MORE THAN 72 HOURS
AND CONTINUES TO SHIFT WESTWARD WITH ISABEL. THEREFORE...A GENERAL
WESTWARD MOTION SHOULD CONTINUE FOR ANOTHER 36 HOURS OR SO.
AFTERWARDS...THERE REMAINS CONSIDERABLE DIVERGENCE AMONG THE MODELS
AND HOW THEY HANDLE THE CURRENT SHORTWAVE TROUGH/LOW OVER THE
SOUTHEASTERN U.S. WATER VAPOR IMAGERY INDICATES THE TROUGH HAS
BECOME NEGATIVELY TILTED IN AN EAST-WEST ORIENTATION. THIS WOULD
SUGGEST THAT THE LOW SHOULD BEGIN TO LIFT OUT AND ALLOW THE RIDGE
NORTH OF ISABEL TO GRADUALLY BUILD WESTWARD...WHICH IS SIMILAR TO
THE 06Z GFS...GFDL...AND ETA...AND 00Z UKMET SOLUTIONS. THE KEY
WILL BE HOW THE CENTRAL U.S. TROUGH EVOLVES AND HOW DEEP IT DIGS
INTO THE SOUTHERN U.S. AND GULF OF MEXICO. THE OFFICIAL FORECAST
WAS ADJUSTED SLIGHTLY TO THE LEFT OF AND SLOWER THAN THE PREVIOUS
FORECAST...WHICH IS CONSISTENT WITH THE GUNA MODEL CONSENSUS.
HOWEVER...IT IS STILL TOO EARLY TO DETERMINE WHEN OR WHERE ISABEL
WILL MAKE LANDFALL. HOPEFULLY...THE NOAA GULFSTREAM-IV SYNOPTIC
DATA MISSION AT 14/00Z WILL PROVIDE BETTER MODEL CONVERGENCE.
 
ALTHOUGH ISABEL HAS A RATHER LARGE EYE DIAMETER...CONDITIONS APPEAR
FAVORABLE FOR AT LEAST SOME SLIGHT ADDITIONAL STRENGTHENING OVER
THE NEXT COUPLE OF DAYS AS THE HURRICANE MOVES OVER WARMER WATER.
 
FORECASTER STEWART
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      13/1500Z 22.2N  61.5W   130 KT
 12HR VT     14/0000Z 22.6N  63.0W   135 KT
 24HR VT     14/1200Z 23.0N  64.8W   135 KT
 36HR VT     15/0000Z 23.7N  66.7W   130 KT
 48HR VT     15/1200Z 24.5N  68.7W   130 KT
 72HR VT     16/1200Z 26.0N  71.0W   125 KT
 96HR VT     17/1200Z 28.5N  73.0W   120 KT
120HR VT     18/1200Z 31.5N  75.0W   110 KT
 
 
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