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#113631 (Received by flhurricane at: 4:40 PM 29.Sep.2006) TCDAT4 TROPICAL STORM ISAAC DISCUSSION NUMBER 9 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL092006 500 PM EDT FRI SEP 29 2006 CONVECTION HAS BEEN INTENSIFYING AND ATTEMPTING TO FORM A RING AROUND THE CENTER OF ISAAC. IN ADDITION THE OUTFLOW HAS BEEN EXPANDING AND BECOMING MORE SYMMETRIC. THESE ARE ALL SIGNS OF AN INTENSIFYING SYSTEM. DVORAK T-NUMBERS HAVE INCREASED MARKEDLY... AND SINCE AN EARLIER QUIKSCAT PASS SUPPORTED WINDS OF 40-45 KT TWELVE HOURS AGO... THE INITIAL INTENSITY IS RAISED TO 50 KT. ISAAC IS LEAVING THE COOLER WATERS PREVIOUSLY UPWELLED BY GORDON AND HELENE AND MOVING INTO WARMER SSTS. IN ADDITION... THE DRY AIR IN THE MIDDLE TO UPPER LEVELS NEAR THE SYSTEM'S CORE HAS BEEN REDUCED AND THERE IS LITTLE SHEAR APPARENT ON SATELLITE IMAGES. THESE FACTORS SUPPORT INTENSIFICATION FOR THE NEXT 36 HOURS OR SO. THUS THE INTENSITY FORECAST IS RAISED FROM THE PREVIOUS ONE AND SHOWS ISAAC BECOMING A HURRICANE. THE FORECAST IS HIGHER THAN THE PREVIOUS ONE AND IN BEST AGREEMENT WITH A BLEND OF THE SHIPS/FSSE. THE INITIAL MOTION IS VIRTUALLY UNCHANGED FROM EARLIER AND IS ABOUT 295/6. MIDDLE-LEVEL RIDGING OVER THE CENTRAL ATLANTIC SHOULD BE THE PRIMARY STEERING MECHANISM FOR THE NEXT ONE OR TWO DAYS... SENDING THE STORM ON A WEST-NORTHWEST TO NORTHWESTWARD HEADING. THEREAFTER ISSAC WILL REACH THE WESTERN PERIPHERY OF THE RIDGE AND LIKELY ACCELERATE TO THE NORTH IN FRONT OF A MASSIVE TROUGH FORECAST TO PROGRESS EASTWARD ACROSS THE WESTERN ATLANTIC. THE UKMET HAS SHIFTED ITS TRACK WESTWARD FROM EARLIER BUT IS STILL ON THE EASTERN SIDE OF THE MODEL ENVELOPE. OTHER MODELS ARE MOSTLY UNCHANGED OR SLIGHTLY FARTHER WEST OF THE PREVIOUS GUIDANCE. THE OFFICIAL FORECAST IS NUDGED WESTWARD BUT STILL LIES JUST EAST OF THE MODEL CONSENSUS. GLOBAL MODELS SUGGEST THAT ISAAC WILL BECOME RAPIDLY EXTRATROPICAL BETWEEN 48 AND 72 HOURS FOLLOWED BY ABSORPTION INTO A LARGER EXTRATROPICAL LOW IN 4 TO 5 DAYS. FORECAST POSITIONS AND MAX WINDS INITIAL 29/2100Z 29.7N 56.8W 50 KT 12HR VT 30/0600Z 30.1N 57.8W 55 KT 24HR VT 30/1800Z 31.1N 59.1W 60 KT 36HR VT 01/0600Z 32.5N 60.3W 65 KT 48HR VT 01/1800Z 34.7N 61.0W 60 KT 72HR VT 02/1800Z 43.5N 57.0W 55 KT...EXTRATROPICAL 96HR VT 03/1800Z 50.0N 48.0W 50 KT...EXTRATROPICAL 120HR VT 04/1800Z...ABSORBED $$ FORECASTER BLAKE/STEWART |