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| #112604 (Received by flhurricane at: 4:34 PM 23.Sep.2006) TCDAT3 HURRICANE HELENE DISCUSSION NUMBER 47 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082006 500 PM EDT SAT SEP 23 2006 HELENE CURRENTLY DISPLAYS BOTH TROPICAL AND EXTRATROPICAL CHARACTERISTICS. THE CYCLONE HAS A FRONTAL-LIKE AND ASYMMETRIC APPEARANCE IN SATELLITE IMAGERY WITH LIMITED DEEP CONVECTION. HOWEVER...FSU CYCLONE PHASE SPACE DIAGRAMS DIAGNOSE HELENE TO STILL HAVE A DEEP WARM CORE WHICH IS CONSISTENT WITH ITS ABILITY TO STILL PRODUCE CONVECTION...ALBEIT LIMITED...NEAR THE CENTER. HELENE IS SOMEWHERE IN PROCESS OF EXTRATROPICAL TRANSITION AND IT IS NOT CLEAR AT WHAT POINT THE TRANSFORMATION WILL BE COMPLETE. SINCE THE OVERALL STRUCTURE APPEARS UNCHANGED SINCE THE EARLIER 0915 UTC QUIKSCAT DATA...WHICH SHOWED 80 KT MAXIMUM WINDS...THE INITIAL INTENSITY WILL REMAIN AT 80 KT FOR THIS ADVISORY. WATER VAPOR IMAGERY SHOWS A MIDDLE TO UPPER-LEVEL SHORTWAVE TROUGH QUICKLY APPROACHING HELENE FROM THE WEST RESULTING IN THE UPPER-LEVEL OUTFLOW BECOMING INCREASINGLY RESTRICTED. IF THIS TREND CONTINUES...AN INCREASINGLY SHEARED HELENE COULD LOSE WHAT REMAINS OF ITS INNER CORE CONVECTION AND THEREFORE LOSE WHAT TROPICAL CHARACTERISTICS IT HAS LEFT. ASSUMING EXTRATROPICAL TRANSITION COMPLETES IN ABOUT 12 HOURS...AS CURRENTLY FORECAST...THE CYCLONE SHOULD GRADUALLY WEAKEN OVER THE NEXT DAY OR SO. MORE RAPID WEAKENING IS FORECAST THEREAFTER AS THE CYCLONE LOSES SOME OF ITS BAROCLINIC SUPPORT. HELENE HAS BEEN MOVING A LITTLE SOUTH OF THE PREVIOUS FORECAST TRACK WITH AN INITIAL MOTION ESTIMATE OF 070/19. THERE HAS OTHERWISE 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 BUT NOT NEARLY AS MUCH AS BEFORE. THE NOGAPS MODEL...WHICH WAS PREVIOUSLY THE NORTHERN OUTLIER...HAS SHIFTED SIGNIFICANTLY SOUTH AND IS NOW ALONG THE SOUTHERN SIDE OF THE GUIDANCE SUITE. THE UKMET MODEL...WHICH PREVIOUSLY SHOWED A TURN DUE SOUTH WITH THE ECMWF...HAS SHIFTED NORTH AND IS NOW IN GOOD AGREEMENT WITH THE OFFICIAL FORECAST. THE ECMWF CONTINUES TO SHOW THE SOUTHWARD TURN...BUT THIS IS CONSIDERED A LESS LIKELY SCENARIO GIVEN THE NOW BETTER AGREEMENT AMONG THE OTHER DYNAMICAL MODELS. THE OFFICIAL FORECAST IS SHIFTED JUST A LITTLE SOUTH OF THE PREVIOUS ONE. FORECAST POSITIONS AND MAX WINDS INITIAL 23/2100Z 38.0N 44.4W 80 KT 12HR VT 24/0600Z 39.1N 40.9W 75 KT...EXTRATROPICAL 24HR VT 24/1800Z 41.3N 36.4W 70 KT...EXTRATROPICAL 36HR VT 25/0600Z 43.2N 32.4W 65 KT...EXTRATROPICAL 48HR VT 25/1800Z 44.6N 28.8W 55 KT...EXTRATROPICAL 72HR VT 26/1800Z 47.0N 21.0W 45 KT...EXTRATROPICAL 96HR VT 27/1800Z 49.0N 15.0W 35 KT...EXTRATROPICAL 120HR VT 28/1800Z 51.0N 11.0W 30 KT...EXTRATROPICAL $$ FORECASTER RHOME/KNABB |
