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| #112435 (Received by flhurricane at: 4:46 PM 22.Sep.2006) TCDAT3 HURRICANE HELENE DISCUSSION NUMBER 42 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082006 500 PM EDT FRI SEP 22 2006 THE SATELLITE PRESENTATION OF HELENE CONTINUES TO DEGRADE AS THE INNER CORE CONVECTION HAS WEAKENED CONSIDERABLY DURING THE LAST 6 HOURS AND THE LOW-LEVEL CIRCULATION IS NOW PARTIALLY EXPOSED. THE INITIAL INTENSITY OF 65 KT IS BASED ON A BLEND OF DVORAK INTENSITY ESTIMATES OF 77 KT...65 KT...AND 55 KT FROM TAFB...AFWA...AND SAB RESPECTIVELY. THE OVERALL SATELLITE EVOLUTION DURING THE DAY INDICATES THAT HELENE IS ENTERING THE EARLY STAGES OF EXTRATROPICAL TRANSITION. IN ADDITION TO DRY MIDDLE TO UPPER-LEVEL AIR WRAPPING AROUND THE SOUTHERN SIDE OF THE CYCLONE...THE OVERALL CLOUD SHIELD IS BECOMING INCREASINGLY DISPLACED NORTH OF THE CENTER...AND VISIBLE IMAGERY SUGGESTS THAT HELENE IS NOW STARTING TO INTERACT WITH A BAROCLINIC ZONE JUST TO ITS NORTHWEST. HOWEVER...DETERMINING WHEN EXTRATROPICAL TRANSITION WILL BE COMPLETE CONTINUES TO BE PROBLEMATIC. DESPITE THE RAPIDLY CHANGING STRUCTURE SEEN IN SATELLITE IMAGERY...MOST OF THE GLOBAL MODELS MAINTAIN A SHALLOW WARM CORE CYCLONE...ALBEIT ASYMMETRIC...FOR ANOTHER COUPLE OF DAYS. GIVING MORE WEIGHT TO THE RECENT TRENDS...THE OFFICIAL FORECAST NOW CALLS FOR EXTRATROPICAL TRANSITION BY 24 HOURS...A LITTLE SOONER THAN BEFORE. ONCE HELENE TRANSFORMS INTO AN EXTRATROPICAL CYCLONE...BAROCLINIC ENERGY COULD PRODUCE A LITTLE RESTRENGTHENING...AND HELENE IS FORECAST BY ALL THE GLOBAL MODELS TO BE A LARGE AND POWERFUL EXTRATROPICAL CYCLONE OVER THE OPEN WATERS OF THE CENTRAL NORTH ATLANTIC. BY DAYS 4 AND 5...HELENE IS EXPECTED TO LOSE SOME OF ITS BAROCLINIC SUPPORT AND GRADUALLY WEAKEN. HELENE CONTINUES TO ROUND THE WESTERN PERIPHERY OF A MID-LEVEL RIDGE INTO THE MIDDLE LATITUDE WESTERLIES...RESULTING IN ACCELERATION NORTHEASTWARD. THE INITIAL MOTION ESTIMATE IS NOW 050/18. TRACK GUIDANCE HAS SHIFTED NOTICEABLY TO THE SOUTH. HOWEVER...THE GUIDANCE ANTICIPATED A MORE NORTHERLY TRACK A COUPLE OF DAYS AGO THAT NEVER MATERIALIZED. THE OFFICIAL FORECAST IS MORE CONSERVATIVELY SHIFTED A LITTLE SOUTH...NOT AS FAR SOUTH AS THE DYNAMICAL MODEL CONSENSUS. FORECAST POSITIONS AND MAX WINDS INITIAL 22/2100Z 35.4N 53.0W 65 KT 12HR VT 23/0600Z 36.8N 50.1W 60 KT 24HR VT 23/1800Z 38.4N 45.3W 65 KT...EXTRATROPICAL 36HR VT 24/0600Z 40.3N 40.5W 65 KT...EXTRATROPICAL 48HR VT 24/1800Z 42.2N 36.1W 60 KT...EXTRATROPICAL 72HR VT 25/1800Z 47.0N 28.0W 55 KT...EXTRATROPICAL 96HR VT 26/1800Z 51.0N 20.0W 45 KT...EXTRATROPICAL 120HR VT 27/1800Z 55.0N 14.0W 40 KT...EXTRATROPICAL $$ FORECASTER RHOME/KNABB |
