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| #289601 (Received by flhurricane at: 4:52 AM 20.Aug.2009) TCDAT3 HURRICANE BILL DISCUSSION NUMBER 20 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL032009 500 AM AST THU AUG 20 2009 DURING THE LAST PASS THROUGH THE NORTHEASTERN EYEWALL...AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT REPORTED 123 KT WINDS AT 700 MB...WHICH IS DOWN ALMOST 20 KT FROM EARLIER. AN EYEWALL DROPSONDE SUGGESTS THE SURFACE WINDS IN THAT AREA WERE 105-110 KT. BASED ON THIS AND A SLIGHT WEAKENING OF THE SATELLITE CLOUD PATTERN...THE INITIAL INTENSITY IS REDUCED TO 110 KT. ANALYSES FROM CIMSS AT THE UNIVERSITY OF WISCONSIN SHOW THAT BILL HAS BEEN EXPERIENCING 10-15 KT OF WESTERLY VERTICAL WIND SHEAR. THIS MAY BE STARTING TO DIMINISH...AS THE CIRRUS OUTFLOW IS INCREASING IN THE WESTERN SEMICIRCLE. THE INITIAL MOTION IS 305/16. BILL IS MOVING TOWARD A WEAKNESS IN THE SUBTROPICAL RIDGE CAUSED BY TWO MID/UPPER-LEVEL TROUGHS...ONE TO THE EAST-NORTHEAST OF BERMUDA AND ONE BETWEEN BERMUDA AND THE UNITED STATES. THE SECOND TROUGH IS FORECAST TO MOVE SOUTHWARD AND WESTWARD DURING THE NEXT 24-48 HR...CAUSING BILL TO GRADUALLY TURN MORE NORTHWARD AS THE SYSTEMS INTERACT. AFTER 48 HR...ALL GUIDANCE FORECASTS A LARGE DEEP-LAYER TROUGH TO MOVE INTO THE EASTERN UNITED STATES...WITH A LARGE MID/UPPER-LEVEL RIDGE BUILDING OVER THE CENTRAL ATLANTIC. THIS COMBINATION SHOULD STEER BILL NORTHWARD AND EVENTUALLY NORTHEASTWARD INTO THE WESTERLIES. THE TRACK GUIDANCE IS TIGHTLY CLUSTERED FOR THE FIRST 48-72 HR...THEN BECOME SOMEWHAT DIVERGENT IN BOTH DIRECTION AND SPEED. THE GFDN AND NOGAPS ARE ON THE LEFT SIDE OF THE GUIDANCE ENVELOPE...BRINGING THE CENTER OF BILL JUST SOUTHEAST OF NEW ENGLAND AND THEN OVER NOVA SCOTIA AND NEWFOUNDLAND. THE CORRECTED CONSENSUS MODELS CALL FOR A MUCH SHARPER EASTWARD TURN...KEEPING THE CENTER WELL AWAY FROM NEW ENGLAND AND CANADA. THE TRACK FORECAST IS BETWEEN THESE EXTREMES AND DOWN THE MIDDLE OF THE GUIDANCE ENVELOPE IN BEST OVERALL AGREEMENT WITH THE GFDL. IT SHOULD BE NOTED THAT BILL WILL PROBABLY NOT TURN AS SHARPLY BETWEEN 72-96 HR AS IMPLIED BY THE TRACK GRAPHIC...AND THUS IS LIKELY TO PASS CLOSER TO NEW ENGLAND THAN THE GRAPHIC WOULD SUGGEST. THE LARGE-SCALE MODELS SUGGEST THE CURRENT SHEAR SHOULD DIMINISH DURING THE NEXT 24 HR...AND THIS BASIS SOME RE-INTENSIFICATION IS EXPECTED. THE INTENSITY FORECAST IS MOST UNCERTAIN DURING THE 24-72 HR TIME FRAME AS BILL INTERACTS WITH THE CURRENT WESTERN ATLANTIC TROUGH. THIS TROUGH COULD CAUSE SHEAR TO WEAKEN THE SYSTEM. HOWEVER...SOME OF THE LARGE-SCALE MODELS FORECAST INCREASED UPPER-LEVEL DIVERGENCE OVER BILL...WHICH COULD STRENGTHEN THE HURRICANE. FOR NOW...THE INTENSITY FORECAST CALLS FOR LITTLE CHANGE IN STRENGTH AS A COMPROMISE. AFTER 72 HR...BILL SHOULD WEAKEN RAPIDLY AS IT MOVES OVER COLDER SEA SURFACE TEMPERATURES AND INTO STRONG VERTICAL WIND SHEAR. EXTRATROPICAL TRANSITION SHOULD BEGIN AROUND 96 HR AND BE COMPLETE BY 120 HR. LARGE SWELLS GENERATED BY THIS HURRICANE WILL AFFECT THE NORTHERN LEEWARD ISLANDS...THE BAHAMAS...BERMUDA AND MOST OF THE EASTERN U.S. COAST DURING THE NEXT FEW DAYS. THESE SWELLS WILL LIKELY CAUSE EXTREMELY DANGEROUS SURF AND LIFE-THREATENING RIP CURRENTS. PLEASE CONSULT STATEMENTS ISSUED BY YOUR LOCAL WEATHER OFFICE FOR MORE DETAILS. FORECAST POSITIONS AND MAX WINDS INITIAL 20/0900Z 21.6N 60.3W 110 KT 12HR VT 20/1800Z 23.0N 62.3W 115 KT 24HR VT 21/0600Z 25.2N 64.7W 120 KT 36HR VT 21/1800Z 27.8N 66.5W 120 KT 48HR VT 22/0600Z 30.8N 68.1W 120 KT 72HR VT 23/0600Z 38.5N 68.0W 105 KT 96HR VT 24/0600Z 46.6N 59.0W 70 KT 120HR VT 25/0600Z 53.5N 36.0W 55 KT...EXTRATROPICAL $$ FORECASTER BEVEN |
