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| #112269 (Received by flhurricane at: 4:40 PM 21.Sep.2006) TCDAT3 HURRICANE HELENE DISCUSSION NUMBER 38 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082006 500 PM EDT THU SEP 21 2006 A 1531 UTC TRMM MICROWAVE OVERPASS INDICATED THAT BANDING FEATURES WERE CONFINED TO THE NORTHEASTERN SEMICIRCLE OF HELENE. WITHIN THE PAST COUPLE OF HOURS...HELENE'S APPEARANCE IN CONVENTIONAL SATELLITE IMAGERY HAS IMPROVED A LITTLE WITH AN AREA OF COLD CLOUD TOPS EXPANDING OVER THE NORTHERN PART OF THE CIRCULATION. HOWEVER...DVORAK INTENSITY ESTIMATES HAVE NOT CHANGED AND THE INITIAL INTENSITY WILL REMAIN 70 KT FOR THIS ADVISORY. THE OFFICIAL INTENSITY FORECAST DOES NOT ANTICIPATE WEAKENING DURING THE NEXT DAY OR SO...AS HELENE IS EXPECTED TO REMAIN IN AN ENVIRONMENT OF ONLY MODEST SHEAR AND OVER 26 TO 27C WATER. THEREAFTER...HELENE IS FORECAST TO GRADUALLY WEAKEN AS SHEAR INCREASES AND SSTS COOL. THE NHC FORECAST CALLS FOR HELENE TO BECOME EXTRATROPICAL IN 2 TO 3 DAYS. THE INTENSITY FORECAST DURING THE EXTRATROPICAL PHASE IS A LITTLE HIGHER THAN THE SHIPS GUIDANCE AND FOLLOWS THE GLOBAL MODELS MORE CLOSELY. THE INITIAL MOTION ESTIMATE IS 010/12. HELENE CONTINUES TO MOVE NORTHWARD AROUND THE WESTERN PERIPHERY OF THE SUBTROPICAL RIDGE CENTERED WELL TO THE EAST OF THE HURRICANE. THE TRACK GUIDANCE SHOWS THAT HELENE SHOULD VERY SOON ROUND THE RIDGE AND TURN NORTH-NORTHEASTWARD AHEAD OF A LARGE DEEP-LAYERED TROUGH ALONG THE U.S. EAST COAST. THE DYNAMICAL MODELS ARE IN GOOD AGREEMENT ON THIS SCENARIO DURING THE NEXT COUPLE OF DAYS. AS HELENE BECOMES EXTRATROPICAL THE TRACK MODELS DIVERGE SIGNIFICANTLY. THE UKMET TURNS THE CYCLONE EASTWARD...WHILE THE GFS TAKE IT QUICKLY NORTHWARD MERGING IT WITH A LARGER EXTRATROPICAL CYCLONE IN ABOUT 5 DAYS. THE OFFICIAL FORECAST LIES IN BETWEEN THESE SOLUTIONS AND IS CLOSE TO THE GFS ENSEMBLE MEAN AND THE TRACK SHOWN BY THE ECMWF TRACK...BUT IS A LITTLE SLOWER THAN THE LATER MODEL. EVEN THOUGH HELENE IS FORECAST TO RECURVE OVER THE CENTRAL ATLANTIC OCEAN...SOME LONG PERIOD SWELLS COULD AFFECT PORTIONS OF THE EAST COAST OF NORTH AMERICA AND BERMUDA DURING THE NEXT COUPLE OF DAYS. FORECAST POSITIONS AND MAX WINDS INITIAL 21/2100Z 30.7N 56.8W 70 KT 12HR VT 22/0600Z 32.7N 55.9W 70 KT 24HR VT 22/1800Z 35.4N 53.2W 70 KT 36HR VT 23/0600Z 37.9N 49.1W 70 KT 48HR VT 23/1800Z 40.7N 43.7W 65 KT...EXTRATROPICAL 72HR VT 24/1800Z 46.5N 32.5W 60 KT...EXTRATROPICAL 96HR VT 25/1800Z 52.0N 23.0W 60 KT...EXTRATROPICAL 120HR VT 26/1800Z 58.0N 18.0W 55 KT...EXTRATROPICAL $$ FORECASTER BROWN |
