ZCZC MIATCDAT3 ALL TTAA00 KNHC DDHHMM HURRICANE ISABEL DISCUSSION NUMBER 41 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 5 AM EDT TUE SEP 16 2003 SATELLITE IMAGERY AND AIR FORCE RESERVE HURRICANE HUNTER DATA INDICATE THAT ISABEL HAS BECOME QUITE DISORGANIZED DURING THE PAST 6-12 HR. CENTER FIXES FROM IR IMAGERY ARE 20-25 NM EAST OF THE FIXES FROM THE AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT AND MICROWAVE DATA...SUGGESTING WESTERLY SHEAR IS AFFECTING THE SYSTEM. THE AIRCRAFT REPORTS THAT THE WIND FIELD HAS SPREAD OUT...WITH AN INNER RADIUS OF MAXIMUM WIND OF 40 NM AND SEVERAL MAXIMA PRESENT OUTSIDE OF THAT. THE MAXIMUM FLIGHT-LEVEL WINDS HAVE BEEN 101 KT...AND THE LATEST CENTRAL PRESSURE IS 956 MB. THE MAXIMUM WINDS ARE REDUCED TO 100 KT...AND THIS IS LIKELY GENEROUS. THE INITIAL MOTION IS NOW 325/6. THE TRACK FORECAST IS REASONABLY STRAIGHT FORWARD. ISABEL IS ON THE WEST SIDE OF A DEEP-LAYER RIDGE EXTENDING FROM THE CANADIAN MARITIMES SOUTH-SOUTHEASTWARD TO EAST OF THE HURRICANE. A BROAD AREA OF WESTERLY FLOW IS OVER THE CENTRAL AND EASTERN UNITED STATES...WITH A SHORTWAVE TROUGH LIFTING NORTHEASTWARD THROUGH THE GREAT LAKES. LARGE-SCALE MODELS INDICATE THAT THE RIDGE SHOULD BUILD SOMEWHAT WESTWARD AS THE SHORTWAVE LIFTS OUT...WHICH SHOULD CAUSE ISABEL TO MOVE IN A GENERAL NORTHWEST OR NORTH-NORTHWESTERLY DIRECTION THROUGH 72 HR. AFTER THAT...THE STORM SHOULD TURN NORTHWARD AND NORTH-NORTHEASTWARD AS IT MERGES WITH THE NEXT STRONG SHORTWAVE TROUGH MOVING EASTWARD ACROSS THE NORTHERN UNITED STATES. ALL DYNAMICAL MODELS AGREE WITH THIS SCENARIO...AS DOES THE OFFICIAL FORECAST WHICH IS MOSTLY AN UPDATE OF THE PREVIOUS PACKAGE. IT SHOULD BE NOTED THAT WHILE THE FORECAST CURRENTLY CALLS FOR LANDFALL IN NORTH CAROLINA IN ABOUT 60 HR...IT WOULD ONLY TAKE A 10 DEGREE CHANGE TO THE RIGHT FOR ISABEL TO MAKE LANDFALL ON THE DELMARVA PENINSULA AS SHOWN IN THE CANADIAN MODEL. THE INTENSITY FORECAST IS VERY PROBLEMATIC. LARGE-SCALE MODELS SUGGEST THAT THE CURRENT SHEAR COULD CONTINUE FOR ANOTHER 12-24 HR. BEYOND THAT TIME...THE MODELS ALL FORECAST THE DEVELOPMENT OF A NEGATIVELY-TILTED SHORTWAVE TROUGH OVER THE SOUTHEASTERN UNITED STATES. WHILE THIS TROUGH MAY NOT REDUCE THE SHEAR...IT COULD PROVIDE A BETTER OUTFLOW PATTERN AND DYNAMICAL FORCING TO SUSTAIN OR STRENGTHEN THE STORM. ON THE OTHER HAND...ISABEL'S BROAD WIND FIELD SUGGESTS THAT THE STORM MIGHT BE SLOW TO RESPOND TO A MORE FAVORABLE ENVIRONMENT...AND THUS MIGHT MOVE ASHORE BEFORE SIGNIFICANT STRENGTHENING COULD OCCUR. THERE ARE THREE POSSIBLE SCENARIOS. FIRST...CONTINUED WEAKENING DUE TO SHEAR AS FORECAST BY THE SHIPS MODEL. SECOND...CONTINUED SHORT-TERM WEAKENING FOLLWED BY RE-INTENSIFICATION AS ISABEL APPROACHES THE COAST. THIRD...A RELATIVELY STEADY-STATE HURRICANE UNTIL LANDFALL. THE INTENSITY FORECAST WILL GO WITH THE THIRD SCENARIO AT THIS TIME. FORECASTER BEVEN FORECAST POSITIONS AND MAX WINDS INITIAL 16/0900Z 26.6N 70.7W 100 KT 12HR VT 16/1800Z 27.6N 71.2W 100 KT 24HR VT 17/0600Z 29.0N 72.1W 100 KT 36HR VT 17/1800Z 30.7N 73.2W 100 KT 48HR VT 18/0600Z 33.0N 74.7W 100 KT 72HR VT 19/0600Z 38.0N 78.5W 55 KT...INLAND 96HR VT 20/0600Z 45.5N 79.0W 40 KT...INLAND EXTRATROPICAL 120HR VT 21/0600Z 55.0N 76.0W 30 KT...INLAND EXTRATROPICAL NNNN
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Page last modified: Monday, 07-Feb-2005 16:49:58 UTC