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| #112050 (Received by flhurricane at: 10:40 AM 20.Sep.2006) TCDAT3 HURRICANE HELENE DISCUSSION NUMBER 33 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082006 1100 AM EDT WED SEP 20 2006 THE SATELLITE PRESENTATION ASSOCIATED WITH HELENE HAS DEGRADED THIS MORNING AS THE EYE IS NO LONGER EVIDENT AND THE CENTRAL CORE CONVECTION HAS BECOME LESS SYMMETRIC. WHILE SATELLITE INTENSITY ESTIMATES ARE UNCHANGED AT 102 KT FROM BOTH TAFB AND SAB...THE ACTUAL DATA T VALUES ARE MUCH LOWER AT 77 KT. THE INITIAL INTENSITY IS LOWERED TO 90 KT GIVEN THESE DATA. THE REASONS FOR THE CURRENT WEAKENING ARE NOT READILY APPARENT. MICROWAVE DATA FROM YESTERDAY SUGGESTED THAT HELENE WAS ENTERING AN EYEWALL REPLACEMENT CYCLE WHICH MAY HAVE NOT YET COMPLETED. ADDITIONALLY...WATER VAPOR IMAGERY SHOWS THAT UPPER LEVEL OUTFLOW IS NOW RESTRICTED TO THE SOUTHWEST INDICATING THAT VERTICAL SHEAR AND MIDDLE TO UPPER LEVEL DRY AIR IS IMPINGING UPON THE CYCLONE. GIVEN THE AFOREMENTIONED UNCERTAINTIES...THE INTENSITY FORECAST IS TRICKY. IF THE EYEWALL REPLACEMENT CYCLE DOES NOT COMPLETE OR THE SHEAR DOES NOT RELAX... ADDITIONAL WEAKENING IS POSSIBLE. FURTHER COMPLICATING MATTERS IS A LARGE MIDDLE TO UPPER TROPOSPHERIC TROUGH CURRENTLY MOVING OFF THE EAST COAST OF THE UNITED STATES WHICH MAY PROVIDE A TEMPORARY SHOT OF DIFFLUENT FLOW ALOFT TOMORROW FOLLOWED BY INCREASING SHEAR. THEREAFTER...THE CYCLONE IS EXPECTED TO START INTERACTING WITH THE ATTENDANT BAROCLINIC ZONE ULTIMATELY RESULTING IN THE SYSTEM BEGINNING EXTRATROPICAL TRANSITION IN 72 HOURS. DURING THE EXTRATROPICAL TRANSITION...HELENE WILL BE SITUATED NEAR THE RIGHT ENTRANCE REGION OF A STRONG UPPER-LEVEL JET WHICH MAY PROVIDE ENOUGH ENERGY TO PREVENT WEAKENING. HELENE IS MOVING A LITTLE LEFT OF THE PREVIOUS ADVISORY WITH AN INITIAL MOTION ESTIMATE OF 315/10. THE LONG ANTICIPATED TURN TOWARD THE NORTHWEST APPEARS TO BE UNDERWAY. GLOBAL MODELS ARE TIGHTLY CLUSTERED ABOUT A GRADUAL NORTHWARD TURN DURING THE NEXT 36 HOURS FOLLOWED BY AN ACCELERATION NORTHEASTWARD AHEAD OF A LARGE TROUGH MOVING INTO THE WESTERN ATLANTIC. THE OFFICIAL FORECAST IS NUDGED A LITTLE TO THE LEFT AND IS SLIGHTLY SLOWER THAN THE PREVIOUS ADVISORY THROUGH 72 HOURS AND IS LITTLE CHANGED THEREAFTER. 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 20/1500Z 26.0N 56.3W 90 KT 12HR VT 21/0000Z 27.3N 57.0W 90 KT 24HR VT 21/1200Z 29.4N 57.3W 95 KT 36HR VT 22/0000Z 32.3N 56.4W 90 KT 48HR VT 22/1200Z 35.0N 54.5W 85 KT 72HR VT 23/1200Z 40.5N 47.0W 85 KT...BECOMING EXTRATROPICAL 96HR VT 24/1200Z 46.0N 36.5W 65 KT...EXTRATROPICAL 120HR VT 25/1200Z 50.0N 28.0W 55 KT...EXTRATROPICAL $$ FORECASTER RHOME/AVILA |
