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Hurricane BILL


ZCZC MIATCDAT3 ALL
TTAA00 KNHC DDHHMM
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
 
NNNN


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Page last modified: Thursday, 31-Dec-2009 12:09:05 UTC