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| #112664 (Received by flhurricane at: 10:40 PM 23.Sep.2006) TCDAT3 HURRICANE HELENE DISCUSSION NUMBER 48 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082006 1100 PM EDT SAT SEP 23 2006 HELENE CONTINUES TO HANG ON TO TROPICAL CHARACTERISTICS. THE OVERALL APPEARANCE IS VERY SIMILAR TO SIX HOURS AGO...AND CONVECTION HAS REMAINED CLOSE TO CENTER ACROSS THE NORTHWESTERN SEMICIRCLE. AN AMSU OVERPASS AT 2025 UTC...STILL SHOWED THE SYSTEM MAINTAINING A WELL-DEFINED MID- TO UPPER-LEVEL WARM CORE WHICH SUPPORTS THE ANALYSIS OF THE FSU CYCLONE PHASE SPACE DIAGRAMS. THUS...WE ARE KEEPING HELENE A 75 KT HURRICANE FOR THIS ADVISORY. ANALYSIS OF A 2132 UTC QUIKSCAT PASS REVEALED THAT THE WIND RADII HAVE CHANGED IN ORIENTATION SINCE THE PREVIOUS PASS... AND THE INITIAL WIND RADII REFLECT THESE WINDS. THE BIGGEST CHALLENGE WITH THIS FORECAST HAS BEEN DETERMINING WHEN EXTRATROPICAL TRANSITION WILL COMPLETE. BASED UPON THE CURRENT APPEARANCE...THIS SHOULD OCCUR WITHIN 6 TO 18 HOURS...WITH THE HELP OF AN APPROACHING SHORT-WAVE TROUGH. AFTER WHICH...SLOW WEAKENING IS ANTICIPATED IN THE FIRST 36 HOURS FOLLOWED BY A FASTER WEAKENING RATE DUE TO INCREASING SHEAR AND DECREASED BAROCLINIC ENERGY. THE INITIAL MOTION IS ESTIMATED AT 070/20...BUT A SHORTER 6-HOUR MOTION COULD BE CLOSER TO 25 KT. THE OVERALL FORECAST TRACK PHILOSOPHY HAS NOT CHANGED. HELENE SHOULD MAINTAIN AN EAST- NORTHEAST TO NORTHEAST MOTION THROUGH 4 DAYS. THE MODEL GUIDANCE IS WELL CLUSTERED FOR THE FIRST 36 HOURS...THEN THE GFS AND ECMWF TAKE A SOUTHWARD TURN. SINCE THE GFS MODEL LACKS RUN-TO-RUN CONSISTENCY...IT HAS BEEN GIVEN VERY LITTLE WEIGHT. THE OFFICIAL FORECAST IS CLOSE TO THE PREVIOUS FORECAST BUT SLIGHTLY SLOWER IN THE LATER PERIODS AND JUST SOUTH OF THE GFDL. FORECAST POSITIONS AND MAX WINDS INITIAL 24/0300Z 39.1N 41.7W 75 KT 12HR VT 24/1200Z 40.5N 38.1W 70 KT...EXTRATROPICAL 24HR VT 25/0000Z 42.5N 34.2W 70 KT...EXTRATROPICAL 36HR VT 25/1200Z 43.8N 31.0W 65 KT...EXTRATROPICAL 48HR VT 26/0000Z 45.0N 27.5W 55 KT...EXTRATROPICAL 72HR VT 27/0000Z 47.0N 20.0W 45 KT...EXTRATROPICAL 96HR VT 28/0000Z 48.5N 15.0W 35 KT...EXTRATROPICAL 120HR VT 29/0000Z...DISSIPATED $$ FORECASTER MAINELLI/BEVEN |
