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Hurricane ISABEL


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HURRICANE ISABEL DISCUSSION NUMBER  39
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
5 PM EDT MON SEP 15 2003
 
AIRCRAFT RECONNAISSANCE OBSERVATIONS CONFIRM THAT ISABEL HAS
WEAKENED TO CATEGORY THREE STATUS.  PEAK FLIGHT LEVEL WINDS OF 120
KT SUPPORT SURFACE WINDS OF ABOUT 110 KT...WHICH IS THE ADVISORY
INTENSITY...ALTHOUGH DROPSONDES IN THE EYEWALL SUGGEST THE WINDS
MIGHT BE EVEN A LITTLE BIT LESS.  THERE IS NOT MUCH EVIDENCE OF
SHEAR AND THE OUTFLOW PATTERN STILL LOOKS STRONG.  HOWEVER...THE
EYEWALL CONVECTION REMAINS ASYMMETRIC AND THE WEST SIDE OF THE
CIRCULATION LOOKS A LITTLE DRY.  ALTHOUGH ISABEL HAS BEEN
WEAKENING...AND MAY WEAKEN FURTHER...THERE IS SOME MODEL GUIDANCE
THAT SUGGESTS THAT A MORE FAVORABLE UPPER WIND PATTERN...WITH
ENHANCED ANTICYCLONIC OUTFLOW...MAY DEVELOP IN THE LAST 24 HOURS
BEFORE LANDFALL.  HOWEVER...DYNAMICAL MODELS HAVE LIMITED SKILL IN
PREDICTING SUCH PATTERNS...IN PARTICULAR THEY OFTEN OVER-DEVELOP
UPPER-LEVEL ANTICYCLONES OVER TROPICAL CYCLONES.  THE OFFICIAL
FORECAST SIMPLY CALLS FOR LITTLE CHANGE IN STRENGTH...AND
ANTICIPATES A MAJOR HURRICANE...CATEGORY THREE OR HIGHER...AT
LANDFALL.

THE INITIAL MOTION IS NOW 305/7...OR NORTHWEST. MODEL GUIDANCE
REMAINS VERY TIGHTLY CLUSTERED...EVEN MORE SO THAN THIS MORNING AS
THE 12Z GFS IS A LITTLE FASTER THAN ITS EARLIER RUN. THERE HAS BEEN
NO SIGNIFICANT CHANGE TO THE FORECAST TRACK.  HOWEVER...SOME OF THE
GUIDANCE IS SUGGESTING THAT THE MOTION OVER THE NEXT DAY OR TWO MAY
BE A LITTLE MORE NORTHWARD BEFORE ISABEL TURNS BACK TO THE
NORTHWEST...AFTER THE UPPER TROUGH CURRENTLY OVER THE GREAT LAKES
MOVES OUT AND HIGH PRESSURE BUILDS AGAIN NORTH OF THE HURRICANE. 
BY 48 HOURS OR SO...AN APPROACHING MID- TO UPPER-LEVEL TROUGH FROM
THE WEST WILL HELP ACCELERATE ISABEL NORTH-NORTHWESTWARD AROUND THE
WEST SIDE OF THE DEEP-LAYER RIDGE ANCHORED NEAR 35N AND 60 W. 
 
FORECASTER FRANKLIN/PASCH
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      15/2100Z 25.6N  70.0W   110 KT
 12HR VT     16/0600Z 26.3N  70.8W   110 KT
 24HR VT     16/1800Z 27.5N  71.6W   105 KT
 36HR VT     17/0600Z 29.0N  72.3W   105 KT
 48HR VT     17/1800Z 30.8N  73.2W   105 KT
 72HR VT     18/1800Z 35.5N  76.0W   100 KT...INLAND
 96HR VT     19/1800Z 42.0N  79.0W    50 KT...INLAND
120HR VT     20/1800Z 52.0N  80.0W    30 KT...EXTRATROPICAL
 
 
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