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| #289844 (Received by flhurricane at: 4:46 AM 21.Aug.2009) TCDAT3 HURRICANE BILL DISCUSSION NUMBER 24 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL032009 500 AM AST FRI AUG 21 2009 NEAR 06Z UTC..AN AIR FORCE RESERVE RECONNAISSANCE AIRCRAFT REPORTED A 700 MB FLIGHT-LEVEL WINDS OF 128 KT...AND AN EYEWALL DROPSONDE INDICATED SURFACE WINDS NEAR 105 KT. SINCE THAT TIME...THE SATELLITE APPEARANCE OF BILL HAS WEAKENED SIGNIFICANTLY...WITH THE EYE NOW BARELY APPARENT. THE INITIAL INTENSITY WILL BE A POSSIBLY GENEROUS 105 KT BASED ON THE AIRCRAFT DATA. ANALYSES FROM CIMSS AT THE UNIVERSITY OF WISCONSIN SHOW 15 KT OF SOUTHERLY VERTICAL WIND SHEAR AFFECTING BILL...AND THIS MAY BE ALLOWING DRY AIR SEEN IN WATER VAPOR IMAGERY AND TOTAL PRECIPITABLE WATER DATA TO REACH THE CORE OF THE HURRICANE. THE INITIAL MOTION IS 315/15. THERE IS NO CHANGE IN THE FORECAST PHILOSOPHY SINCE THE PREVIOUS ADVISORY. BILL IS MOVING TOWARD AN INCREASING WEAKNESS IN THE SUBTROPICAL RIDGE...AND IS EXPECTED TO RECURVE AHEAD OF A LARGE DEEP-LAYER TROUGH MOVING INTO THE EASTERN UNITED STATES. THE TIGHTLY-CLUSTERED TRACK GUIDANCE HAS CHANGED ONLY SLIGHTLY SINCE THE PREVIOUS ADVISORY...WITH SOME NORTHWARD SHIFT IN THE GUIDANCE ENVELOPE AFTER 48 HR. THE NEW FORECAST TRACK IS SIMILAR TO THE PREVIOUS TRACK THROUGH 48 HR...THEN IS ADJUSTED A LITTLE TO THE NORTH AT SUBSEQUENT TIMES TO ALIGN WITH THE DYNAMICAL MODEL CONSENSUS. THE INTENSITY FORECAST IS BECOMING INCREASINGLY PROBLEMATIC. THE SHIPS MODEL IS FORECASTING BILL TO BE IN A LIGHT VERTICAL SHEAR ENVIRONMENT THROUGH 72 HR...BUT THE MODEL HAS BEEN UNDERESTIMATING THE ONGOING SHEAR FOR THE PAST 24 HR. BILL IS CURRENTLY OVER SEA SURFACE TEMPERATURES OF 28-29C...BUT THE SSTS ALONG THE FORECAST TRACK DECREASE DRAMATICALLY AFTER 24 HR. ANOTHER COMPLICATION IS THE POSSIBLE IMPACT OF THE JET STREAM ASSOCIATED WITH THE EASTERN U. S. TROUGH...WHICH COULD HELP THE HURRICANE MAINTAIN ITS STRENGTH OR EVEN INTENSIFY IT A LITTLE. AT THIS TIME...IT LOOKS LIKE THE MOST FAVORABLE JET STREAM DYNAMICS WILL REMAIN WEST OF BILL. BASED ON THE CURRENT TRENDS...THE FIRST PART OF THE INTENSITY FORECAST HAS BEEN NUDGED DOWNWARD...WITH ONLY A MODEST RE-INTENSIFICATION BETWEEN 12-24 HR AS SHOWN IN THE SHIPS MODEL. THEREAFTER...BILL SHOULD STEADILY WEAKEN OVER COLDER WATERS...WITH EXTRATROPICAL TRANSITION EXPECTED AT ABOUT 96 HR. AN ALTERNATIVE SCENARIO IS THAT BILL WEAKENS FURTHER EVEN BEFORE THE CENTER REACHES THE COLDER WATER...AND WEAKENS FASTER THAN CURRENTLY FORECAST AFTER THAT. LARGE SWELLS GENERATED BY THIS HURRICANE WILL AFFECT A LARGE PORTION OF THE WESTERN NORTH ATLANTIC COASTAL AREA 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 21/0900Z 26.2N 65.4W 105 KT 12HR VT 21/1800Z 28.1N 66.9W 105 KT 24HR VT 22/0600Z 31.1N 68.4W 110 KT 36HR VT 22/1800Z 34.6N 68.8W 100 KT 48HR VT 23/0600Z 38.6N 67.6W 90 KT 72HR VT 24/0600Z 46.5N 58.5W 70 KT 96HR VT 25/0600Z 52.5N 33.5W 55 KT...EXTRATROPICAL 120HR VT 26/0600Z 56.5N 15.0W 50 KT...EXTRATROPICAL $$ FORECASTER BEVEN |
