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| #112156 (Received by flhurricane at: 10:58 PM 20.Sep.2006) TCDAT3 HURRICANE HELENE DISCUSSION NUMBER 35 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082006 1100 PM EDT WED SEP 20 2006 OVERALL...HELENE HAS CHANGED LITTLE IN STRUCTURE DURING THE PAST SEVERAL HOURS. SEVERAL MICROWAVE IMAGES DURING THE AFTERNOON AND EVENING SHOWED MINIMAL CONVECTION SOUTH AND SOUTHEAST OF THE CENTER...POSSIBLY DUE TO ENTRAINMENT OF DRY AIR SEEN IN DATA FROM NOAA RESEARCH AIRCRAFT. SINCE THE LAST MICROWAVE IMAGE...THERE HAS BEEN SOME COOLING OF THE CLOUD TOPS NEAR THE CENTER...ALTHOUGH THE EYE REMAINS POORLY DEFINED. THE INITIAL INTENSITY IS REDUCED TO 85 KT BASED ON FURTHER EXAMINATION OF DATA FROM THE NOAA P-3 MISSION THIS AFTERNOON. THIS AGREES WITH SATELLITE INTENSITY ESTIMATES OF 90 KT FROM SAB AND AFWA...AND 77 KT FROM TAFB. HELENE HAS TURNED TO THE RIGHT DURING THE PAST 6-12 HR WITH THE INITIAL MOTION NOW 360/8. THE HURRICANE IS ROUNDING THE WESTERN END OF THE SUBTROPICAL RIDGE AND WILL RECURVE INTO THE WESTERLIES DURING THE NEXT 24-36 HR. ALL TRACK GUIDANCE AGREES WITH THIS SCENARIO...ALTHOUGH THERE REMAINS SOME SPREAD IN BOTH DIRECTION AND SPEED AFTER 24-36 HR. THE OVERALL MODEL CONSENSUS IS SLOWER THAN 6 HR AGO. THE NEW TRACK FORECAST IS SHIFTED A LITTLE TO THE EAST OF...AND IS SLOWER THAN...THE PREVIOUS FORECAST. IT IS IN BEST AGREEMENT WITH THE FSU SUPERENSEMBLE AND THE GFS ENSEMBLE MEAN. IN THE SHORT TERM...ALL GUIDANCE FORECASTS HELENE TO STRENGTHEN A LITTLE...AND THIS REMAINS IN THE INTENSITY FORECAST. AFTER RECURVATURE...INCREASING SHEAR AND COOLING SEA SURFACE TEMPERATURES SHOULD CAUSE GRADUAL WEAKENING. THE LATEST LARGE-SCALE MODEL RUNS SEND MIXED SIGNALS ON EXTRATROPICAL TRANSITION. THE MODEL SURFACE FIELDS FORECAST A COLD FRONT TO INTERACT WITH HELENE AT ABOUT 48 HR. HOWEVER...FSU CYCLONE PHASE DIAGRAMS BASED ON THOSE MODELS SUGGEST THAT HELENE MAY NOT BECOME A CLASSIC COLD CORE CYCLONE. INSTEAD...THEY SUGGEST THAT HELENE WILL BECOME AN ASYMMETRIC WARM CORE SYSTEM AS TRANSITION BEGINS...THEN REGAIN A SYMMETRIC WARM CORE. THIS WILL LIKELY BE DUE TO A NON-TROPICAL SECLUSION PROCESS RATHER THAN THE NORMAL CONVECTIVE PROCESS OF A TROPICAL CYCLONE. GIVEN THE UNCERTAINTY...THE ONSET OF EXTRATROPICAL TRANSITION WILL BE DELAYED 24 HR FROM THE PREVIOUS FORECASTS. REGARDLESS OF THE EXACT STRUCTURAL EVOLUTION...HELENE SHOULD REMAIN A LARGE AND POWERFUL CYCLONE AS IT CROSSES THE NORTH ATLANTIC. EVEN THOUGH HELENE IS FORECAST TO RECURVE OVER THE OPEN WATERS OF THE CENTRAL ATLANTIC...SOME LONG PERIOD SWELLS COULD AFFECT PORTIONS OF THE EAST COAST OF NORTH AMERICA AND BERMUDA. FORECAST POSITIONS AND MAX WINDS INITIAL 21/0300Z 27.1N 57.0W 85 KT 12HR VT 21/1200Z 28.6N 57.0W 90 KT 24HR VT 22/0000Z 31.2N 56.3W 90 KT 36HR VT 22/1200Z 34.0N 54.5W 85 KT 48HR VT 23/0000Z 37.2N 51.2W 75 KT 72HR VT 24/0000Z 43.0N 42.0W 70 KT...BECOMING EXTRATROPICAL 96HR VT 25/0000Z 48.0N 32.0W 65 KT...EXTRATROPICAL 120HR VT 26/0000Z 52.0N 24.0W 55 KT...EXTRATROPICAL $$ FORECASTER BEVEN |
