ZCZC MIATCDAT3 ALL TTAA00 KNHC DDHHMM 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 NNNN
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