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| #111130 (Received by flhurricane at: 5:00 AM 16.Sep.2006) TCDAT3 TROPICAL STORM HELENE DISCUSSION NUMBER 16 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082006 500 AM EDT SAT SEP 16 2006 A SERIES OF MICROWAVE OVERPASSES REVEALS THAT HELENE HAS A WELL-DEFINED CIRCULATION...BUT WITH DEEP CONVECTION THAT DOES NOT PERSIST IN THE SAME QUADRANT FOR VERY LONG. THE MOST RECENT IMAGE FROM AMSR-E AT 0420 UTC INDICATED THAT DEEP CONVECTION WAS PRESENT ONLY TO THE NORTHEAST OF THE CENTER...AND GOES IMAGERY SINCE THEN SHOWS THAT THE CONVECTION HAS WRAPPED AROUND TO THE NORTHWEST SIDE. MEANWHILE...THERE IS VERY LIMITED DEEP CONVECTION IN THE SOUTHEASTERN SEMICIRCLE. EVEN THOUGH DVORAK T-NUMBERS REMAIN AT 4.0 USING AN EMBEDDED CENTER PATTERN...CORRESPONDING TO AN INTENSITY OF 65 KT...THE PERCOLATING BEHAVIOR OF THE CONVECTION LEADS ME TO LEAVE THE ADVISORY INTENSITY AT 60 KT FOR NOW. NEVERTHELESS...THE ENVIRONMENT APPEARS CONDUCIVE ENOUGH FOR HELENE TO BECOME A HURRICANE LATER TODAY AND SUBSEQUENTLY STRENGTHEN DURING THE NEXT FEW DAYS. THE SHIPS AND GFDL BOTH FORECAST GRADUAL INTENSIFICATION FOR ABOUT 48 HOURS. THEREAFTER THE SHIPS LEVELS OFF NEAR 80 KT... BUT THE GFDL ANTICIPATES HELENE LATER APPROACHING MAJOR HURRICANE STATUS. THE NEW OFFICIAL INTENSITY FORECAST IS A BLEND OF THOSE TWO MODELS EARLY ON...THEN FOLLOWS THE GFDL...AND IS VERY CLOSE TO THE PREVIOUS ADVISORY. DUE TO THE PULSATING CONVECTIVE PATTERN...I WOULD NOT HAVE A VERY GOOD IDEA WHERE THE LOW-LEVEL CENTER IS WITHOUT THE BENEFIT OF THE MICROWAVE IMAGERY...WHICH INDICATES THE INITIAL MOTION IS ESSENTIALLY UNCHANGED AT 300/13. ALL OF THE TRACK GUIDANCE SHOWS A MOTION BETWEEN WEST-NORTHWEST OR NORTHWEST FOR THE NEXT 3-4 DAYS INTO A WEAKNESS IN THE SUBTROPICAL RIDGE OVER THE CENTRAL ATLANTIC. THAT WEAKNESS IS CURRENTLY ASSOCIATED WITH HURRICANE GORDON AND SHOULD BE REINFORCED BY A SHORT-WAVE MID- TO UPPER-LEVEL LOW THAT WILL MOVE EASTWARD FROM THE EAST COAST OF THE UNITED STATES DURING THE NEXT FEW DAYS. THAT FEATURE WILL PROBABLY NOT BE ENOUGH TO TURN HELENE NORTHWARD...BUT THE LONGWAVE TROUGH CURRENTLY OVER THE WESTERN UNITED STATES PROBABLY WILL...WHEN IT GETS INTO THE WESTERN ATLANTIC IN SEVERAL DAYS. NEARLY ALL OF THE DYNAMICAL MODELS TURN HELENE NORTH-NORTHWESTWARD AHEAD OF THAT TROUGH ON DAY 5...AND THAT TURN IS NOW INTRODUCED IN THE OFFICIAL FORECAST...WHICH IS OTHERWISE VERY SIMILAR TO THE PREVIOUS ADVISORY. FORECAST POSITIONS AND MAX WINDS INITIAL 16/0900Z 17.9N 44.7W 60 KT 12HR VT 16/1800Z 18.8N 46.1W 65 KT 24HR VT 17/0600Z 19.7N 47.6W 75 KT 36HR VT 17/1800Z 20.6N 49.1W 80 KT 48HR VT 18/0600Z 21.4N 50.4W 85 KT 72HR VT 19/0600Z 23.0N 53.0W 95 KT 96HR VT 20/0600Z 25.0N 56.0W 95 KT 120HR VT 21/0600Z 28.0N 58.0W 90 KT $$ FORECASTER KNABB |
