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| #112188 (Received by flhurricane at: 4:43 AM 21.Sep.2006) TCDAT3 HURRICANE HELENE DISCUSSION NUMBER 36 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082006 500 AM EDT THU SEP 21 2006 THE OVERALL CONVECTIVE PATTERN HAS DECREASED IN AREAL COVERAGE AND THE EYE HAS RECENTLY BECOME CLOUD-OBSCURED. SATELLITE INTENSITY ESTIMATES ARE 77 KT FROM ALL THREE AGENCIES...SO THE ADVISORY INTENSITY HAS BEEN DECREASED TO 80 KT. THE UPPER-LEVEL OUTFLOW PATTERN REMAINS EXCELLENT IN ALL QUADRANTS...BUT WATER VAPOR AND INFRARED SATELLITE IMAGERY INDICATE SOME MID-LEVEL SHEAR IS NOW UNDERCUTTING THE FAVORABLE OUTFLOW LAYER. THE INITIAL MOTION ESTIMATE IS 010/11. HELENE IS BEGINNING TO ROUND THE WESTERN PERIPHERY OF THE SUBTROPICAL RIDGE SITUATED EAST-WEST ALONG 29N-30N LATITUDE. A DEEP-LAYER TROUGH MOVING OFF THE U.S. EAST COAST THAT EXTENDS FROM NEW ENGLAND SOUTHWARD TO THE SOUTHERN FLORIDA PENINSULA IS PROGRESSING EASTWARD AT 20-25 KT AND SHOULD GRADUALLY TURN HELENE NORTHEASTWARD DURING THE NEXT 24-36 HOURS... WITH MORE RAPID ACCELERATION EXPECTED AFTER THAT. THE OFFICIAL FORECAST TRACK HAS BEEN SHIFTED SLIGHTLY EASTWARD...BUT NOT NEARLY AS FAR EAST AND SOUTH AS THE UKMET AND ECMWF MODELS...WHICH BRING HELENE VERY CLOSE TO THE NORTHWESTERN AZORES IN 72-96 HOURS. THOSE TWO MODELS HAD A DISTINCT RIGHT BIAS DURING THE RECENT EVOLUTION OF FORMER HURRICANE GORDON...SO LESS WEIGHT WAS PLACED ON THOSE SOLUTIONS AND THE OFFICIAL TRACK IS SIMILAR TO THE MODEL CONSENSUS. AS HELENE BEGINS TO ACCELERATE NORTHEASTWARD...THE MID-LEVEL SHEAR COULD ABATE ENOUGH TO ALLOW FOR SOME SLIGHT INTENSIFICATION WHILE THE CYCLONE IS STILL OVER 27C SSTS. BY 72 HOURS...HOWEVER...HELENE IS EXPECTED TO COME UNDER THE INFLUENCE OF STRONG CYCLONIC MID- TO UPPER-LEVEL FLOW AND INCREASING BAROCLINIC EFFECTS...CAUSING THE CYCLONE TO TRANSITION OVER TO A POWERFUL EXTRATROPICAL LOW PRESSURE SYSTEM SIMILAR TO WHAT FORMER HURRICANE GORDON DID YESTERDAY. 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 21/0900Z 28.4N 56.8W 80 KT 12HR VT 21/1800Z 30.1N 56.5W 85 KT 24HR VT 22/0600Z 32.8N 55.3W 90 KT 36HR VT 22/1800Z 35.6N 52.7W 85 KT 48HR VT 23/0600Z 38.5N 48.7W 75 KT...BECOMING EXTRATROPICAL 72HR VT 24/0600Z 43.9N 39.1W 65 KT...EXTRATROPICAL 96HR VT 25/0600Z 48.0N 30.0W 60 KT...EXTRATROPICAL 120HR VT 26/0600Z 53.0N 22.0W 55 KT...EXTRATROPICAL $$ FORECASTER STEWART |
