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| #112105 (Received by flhurricane at: 4:34 PM 20.Sep.2006) TCDAT3 HURRICANE HELENE DISCUSSION NUMBER 34 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082006 500 PM EDT WED SEP 20 2006 A NOAA P-3 EQUIPPED WITH THE SFMR INSTRUMENT HAS BEEN SAMPLING HELENE THIS AFTERNOON. THUS FAR...THE DATA CONFIRMS THAT THE EYEWALL IS POORLY DEFINED AND THE SYSTEM HAS A LARGE WIND FIELD. AS A RESULT...THE INTENSITY SHOULD BE SLOW TO RESPOND TO CHANGES IN THE ENVIRONMENT. WHILE THE DATA FROM THE SFMR AND THE LARGE WIND FIELD ARGUE AGAINST A 90 KT HURRICANE...AN EYEWALL DROP REPORTED AN IMPRESSIVE 960 MB. IN LIGHT OF THE PRESSURE AND THE FACT THAT THE PLANE HAS NOT YET SAMPLED THE ENTIRE STORM...THE INITIAL INTENSITY IS HELD AT 90 KT. IF THE PLANE DOES NOT FIND HIGHER WINDS IN SUBSEQUENT PASSES...THE INTENSITY WILL NEED TO BE LOWERED LATER. THE INTENSITY FORECAST REMAINS HIGHLY PROBLEMATIC DUE TO THE CYCLONE INTERACTING WITH A STRONG MIDDLE TO UPPER-LEVEL TROUGH OVER THE NEXT TWO DAYS FOLLOWED BY EXTRATROPICAL TRANSITION. WHILE IT IS DIFFICULT TO ANTICIPATE TROUGH INTERACTIONS...GLOBAL MODELS INDICATE THAT HELENE MAY ENCOUNTER WEAKER SHEAR IN THE SHORT-TERM PRIOR TO THE ONSET OF STRONG SOUTHWESTERLY FLOW AHEAD OF THE TROUGH. ALL OF THE AVAILABLE INTENSITY GUIDANCE INDICATES HELENE MAY STRENGTHEN AS A RESULT OF THIS MORE FAVORABLE ENVIRONMENT. BY DAY 2...HELENE STARTS TO GET TANGLED UP WITH A BAROCLINIC ZONE ULTIMATELY RESULTING IN EXTRATROPICAL TRANSITION. SINCE BAROCLINIC FORCING IS EXPECTED DURING THE EXTRATROPICAL TRANSITION...ONLY SLOW WEAKENING IS FORECAST. INDEED HELENE MAY BE A LARGE AND POWERFUL EXTRATROPICAL STORM OVER THE NORTH CENTRAL ATLANTIC. THE OFFICIAL INTENSITY FORECAST IS LITTLE CHANGED FROM THE PREVIOUS ADVISORY. HELENE IS IN THE PROCESS OF A NORTHWARD TURN AND THE INITIAL MOTION ESTIMATE IS 315/08. GLOBAL MODELS CONTINUE TO BE TIGHTLY CLUSTERED THROUGH 72 HOURS AND ONLY MINOR CHANGES ARE REQUIRED TO THE PREVIOUS FORECAST. BEYOND 72 HOURS...MOST OF THE GLOBAL MODELS HAVE SHIFTED CONSIDERABLY NORTH WITH THE GFS AND GFDL ALONG THE NORTH SIDE OF THE MODEL SUITE AND THE NOGAPS NEAR THE SOUTH SIDE OF THE GUIDANCE. SINCE THE NOGAPS PERFORMED WELL DURING GORDON...THE OFFICIAL FORECAST IS ONLY SHIFTED A LITTLE TO THE NORTH AND CURRENTLY LIES JUST SOUTH OF THE DYNAMICAL MODEL CONSENSUS. EVEN THOUGH HELENE IS FORECAST TO RECURVE OVER THE OPEN WATERS OF THE CENTRAL ATLANTIC...LONG PERIOD SWELLS ARE ALREADY REGISTERING AT SEVERAL EAST COAST BUOYS. FORECAST POSITIONS AND MAX WINDS INITIAL 20/2100Z 26.6N 57.1W 90 KT 12HR VT 21/0600Z 27.9N 57.5W 90 KT 24HR VT 21/1800Z 30.6N 57.2W 95 KT 36HR VT 22/0600Z 33.9N 55.5W 90 KT 48HR VT 22/1800Z 37.5N 52.3W 85 KT...BECOMING EXTRATROPICAL 72HR VT 23/1800Z 43.5N 43.0W 75 KT...EXTRATROPICAL 96HR VT 24/1800Z 48.5N 35.0W 65 KT...EXTRATROPICAL 120HR VT 25/1800Z 52.0N 28.0W 55 KT...EXTRATROPICAL $$ FORECASTER RHOME/AVILA |
