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| #112389 (Received by flhurricane at: 10:43 AM 22.Sep.2006) TCDAT3 HURRICANE HELENE DISCUSSION NUMBER 41 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082006 1100 AM EDT FRI SEP 22 2006 THE SATELLITE PRESENTATION OF HELENE HAS DEGRADED SLIGHTLY OVER THE LAST SEVERAL HOURS...WITH INNER CORE CONVECTION STARTING TO SPUTTER AND FAVOR THE NORTHEASTERN QUADRANT. ACCORDINGLY...DVORAK DATA T NUMBERS HAVE DECREASED...BUT THE INITIAL INTENSITY IS MAINTAINED AT 75 KT FOR NOW IN ACCORDANCE WITH THE DVORAK CI NUMBERS. INCREASING VERTICAL SHEAR AND DECREASING SSTS APPEAR TO BE TAKING THEIR TOLL ON HELENE...AND THE CYCLONE SHOULD GRADUALLY WEAKEN AS IT SLOWLY LOSES TROPICAL CHARACTERISTICS DURING THE NEXT COUPLE OF DAYS. ON DAYS 2 AND 3...STRONG BAROCLINIC ENERGY IS EXPECTED TO LIMIT FURTHER WEAKENING...WITH HELENE FORECAST BY ALL THE GLOBAL MODELS TO BE A LARGE AND POWERFUL EXTRATROPICAL CYCLONE OVER THE OPEN WATERS OF THE CENTRAL NORTH ATLANTIC. IT SHOULD BE NOTED THAT THE PRECISE TIMING OF EXTRATROPICAL TRANSITION IS DIFFICULT TO DISCERN GIVEN SOME DIFFERENCES IN STRUCTURAL EVOLUTION AMONG THE MODELS...AS SHOWN BY THE FSU PHASE SPACE DIAGRAMS. TOWARD THE END OF THE FORECAST...GLOBAL MODELS SUGGEST THAT HELENE WILL LOSE SOME OF ITS BAROCLINIC SUPPORT WHICH WOULD INDUCE A WEAKENING TREND. THE OFFICIAL FORECAST SHOWS A LITTLE MORE WEAKENING DURING THE FIRST 24 HOURS GIVEN THE RECENT TRENDS IN THE CONVECTIVE PATTERN. BEYOND 24 HOURS...THE OFFICIAL INTENSITY FORECAST IS ESSENTIALLY AN UPDATE OF THE PREVIOUS ONE. HELENE IS STARTING TO ACCELERATE NORTHEASTWARD AS IT ROUNDS THE WESTERN PERIPHERY OF A MID-TROPOSPHERIC RIDGE...AND THE INITIAL MOTION ESTIMATE IS NOW 040/18. THE HURRICANE IS EMBEDDED WITHIN STRONG SOUTHWESTERLY FLOW AHEAD OF A MIDDLE TO UPPER-LEVEL TROUGH OVER THE WESTERN ATLANTIC. THE GUIDANCE REMAINS TIGHTLY CLUSTERED THROUGH 72 HOURS...AND MODELS ARE NOW IN BETTER AGREEMENT AT DAYS 4 AND 5. THE OFFICIAL FORECAST IS IN BEST AGREEMENT WITH THE NOGAPS MODEL AND IS A LITTLE SLOWER AND TO THE SOUTH OF THE PREVIOUS ADVISORY. FORECAST POSITIONS AND MAX WINDS INITIAL 22/1500Z 34.4N 54.2W 75 KT 12HR VT 23/0000Z 36.2N 51.6W 70 KT 24HR VT 23/1200Z 37.9N 47.4W 65 KT 36HR VT 24/0000Z 40.0N 42.8W 65 KT 48HR VT 24/1200Z 42.4N 37.5W 65 KT...EXTRATROPICAL 72HR VT 25/1200Z 47.5N 29.0W 65 KT...EXTRATROPICAL 96HR VT 26/1200Z 52.5N 20.0W 55 KT...EXTRATROPICAL 120HR VT 27/1200Z 56.5N 15.0W 50 KT...EXTRATROPICAL $$ FORECASTER RHOME/KNABB |
