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| #112545 (Received by flhurricane at: 10:49 AM 23.Sep.2006) TCDAT3 HURRICANE HELENE DISCUSSION NUMBER 46 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082006 1100 AM EDT SAT SEP 23 2006 HELENE APPEARS WELL ON ITS WAY TO BECOMING AN EXTRATROPICAL CYCLONE. HOWEVER...SATELLITE IMAGERY SHOWS THAT A SMALL AREA OF DEEP CONVECTION CONTINUES OVER AND JUST NORTH OF THE CENTER. SINCE THE STRUCTURE APPEARS UNCHANGED SINCE THE 0915 UTC QUIKSCAT PASS THAT SHOWED 80 KT MAXIMUM WINDS...THE INITIAL INTENSITY IS HELD AT 80 KT. UNTIL THE INNER CORE CONVECTION DISSIPATES...WHICH COULD HAPPEN AT ANY TIME...HELENE WILL BE CLASSIFIED AS A TROPICAL CYCLONE. ONCE EXTRATROPICAL TRANSITION IS COMPLETE...WHICH WE ANTICIPATE WILL HAPPEN LATER TODAY...GRADUAL WEAKENING IS EXPECTED AS THE WIND FIELD SPREADS OUT. HOWEVER...THE OFFICIAL FORECAST STILL CALLS FOR EXTRATROPICAL HELENE TO PRODUCE SUSTAINED WINDS OF HURRICANE FORCE FOR THE NEXT COUPLE OF DAYS. EVEN THOUGH THE CENTER OF THE CYCLONE IS EXPECTED TO PASS NORTH OF THE AZORES DURING THE NEXT 2 DAYS...THE LARGE AND EXPANDING WIND FIELD MEANS THAT GALE FORCE WINDS COULD AFFECT THE AZORES BEGINNING ON SUNDAY. BEYOND 48 HOURS...WEAKENING COULD BECOME MORE RAPID AS GLOBAL MODELS SUGGEST THE CYCLONE WILL LOSE ITS BAROCLINIC SUPPORT. HELENE IS MOVING RIGHT ALONG THE PREVIOUS FORECAST TRACK WITH AN INITIAL MOTION ESTIMATE OF 065/20. THERE HAS BEEN LITTLE CHANGE IN THE TRACK FORECAST PHILOSOPHY IN THE SHORT-TERM...AND GUIDANCE REMAINS TIGHTLY CLUSTERED THROUGH 48 HOURS. THEREAFTER...THE TRACK GUIDANCE BEGINS TO DIVERGE WITH CONSIDERABLE SPREAD BY DAY 5. THE NOGAPS MODEL FORECASTS HELENE TO MERGE WITH ANOTHER EXTRATROPICAL SYSTEM RESULTING IN A NORTHWARD TURN. CONVERSELY...THE UKMET AND ECMWF MODELS FORECAST A LARGE UPPER-LEVEL CLOSED LOW TO DEVELOP EAST OF THE AZORES RESULTING IN EXTRATROPICAL HELENE TURNING SOUTH. AT THIS TIME...IT IS DIFFICULT TO DETERMINE IF EITHER OF THESE OUTLIER SCENARIOS IS MORE PLAUSIBLE. ACCORDINGLY...THE OFFICIAL FORECAST LIES IN BETWEEN THESE TWO EXTREMES AND IS NEAR BOTH THE DYNAMICAL MODEL CONSENSUS AND THE GFS ENSEMBLE MEAN. COMPARED TO THE PREVIOUS ADVISORY...THIS REPRESENTS A SOUTHWARD ADJUSTMENT AND A SLOWER FORWARD MOTION AT THE END OF THE FORECAST. GIVEN THE LARGE SPREAD OF THE DYNAMICAL MODELS...THERE IS CONSIDERABLE UNCERTAINTY IN THE OFFICIAL TRACK FORECAST. FORECAST POSITIONS AND MAX WINDS INITIAL 23/1500Z 37.7N 46.4W 80 KT 12HR VT 24/0000Z 38.9N 43.0W 80 KT...EXTRATROPICAL 24HR VT 24/1200Z 40.7N 38.5W 75 KT...EXTRATROPICAL 36HR VT 25/0000Z 42.5N 34.2W 70 KT...EXTRATROPICAL 48HR VT 25/1200Z 44.1N 30.7W 65 KT...EXTRATROPICAL 72HR VT 26/1200Z 47.0N 23.0W 55 KT...EXTRATROPICAL 96HR VT 27/1200Z 49.0N 18.0W 45 KT...EXTRATROPICAL 120HR VT 28/1200Z 50.5N 13.0W 40 KT...EXTRATROPICAL $$ FORECASTER RHOME/KNABB |
