000
WTNT43 KNHC 190241
TCDAT3
HURRICANE BILL DISCUSSION NUMBER  15
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL032009
1100 PM AST TUE AUG 18 2009
 
REPORTS FROM A NOAA HURRICANE HUNTER AIRCRAFT CONDUCTING A RESEARCH
MISSION IN BILL AND INFRARED SATELLITE IMAGERY INDICATE THAT BILL
HAS UNDERGONE A PERIOD OF RAPID INTENSIFICATION AND IS NOW A MAJOR
HURRICANE.  HIGHEST FLIGHT-LEVEL WINDS SO FAR HAVE BEEN 126 KT AT
700 MB...WITH THE LATEST MINIMUM PRESSURE OF 952 MB AT 0202 UTC.
THE HIGHEST SFMR-OBSERVED SURFACE WIND SPEED WAS 100 KT.  THE LATEST
DVORAK T-NUMBERS ARE 6.0 FROM BOTH TAFB AND SAB...WHILE UW CIMSS
OBJECTIVE ADVANCED DVORAK VALUES HAVE BEEN AS HIGH AS 7.0. 
BLENDING THE AIRCRAFT FLIGHT-LEVEL...SFMR...AND DROPSONDE WINDS
WITH THE VARIOUS SATELLITE-DERIVED INTENSITY ESTIMATES...THE
INITIAL INTENSITY IS SET AT 110 KT.  FAVORABLE ENVIRONMENTAL
CONDITIONS COULD ALLOW FOR FURTHER STRENGTHENING DURING THE NEXT
24 TO 36 HOURS.  HOWEVER...EYEWALL REPLACEMENT CYCLES...WHICH ARE
VERY DIFFICULT TO ANTICIPATE...MAY PRODUCE SOME FLUCTUATIONS IN
INTENSITY.  REGARDLESS...BILL IS EXPECTED TO REMAIN A LARGE AND
POWERFUL HURRICANE FOR SEVERAL DAYS AS IT MARCHES INTO THE CENTRAL
NORTH ATLANTIC.  TOWARD THE END OF THE PERIOD...SEA SURFACE
TEMPERATURES WILL BE DECREASING ALONG THE FORECAST TRACK AND
SOUTHERLY TO SOUTHWESTERLY SHEAR SHOULD BEGIN TAKING A TOLL.  AS A
RESULT...THE OFFICIAL INTENSITY FORECAST CALLS FOR WEAKENING AROUND
96 HOURS...WITH A MORE DECIDED RATE OF WEAKENING NEAR THE END OF
THE PERIOD.
 
BASED ON AIRCRAFT AND SATELLITE FIXES...THE INITIAL MOTION IS
ESTIMATED TO BE 295/13...REPRESENTING A SLIGHTLY MORE NORTHERLY
COMPONENT OF MOTION.  THE TRACK FORECAST PHILOSOPHY HAS NOT
CHANGED.  BILL IS FORECAST TO BE STEERED NORTHWESTWARD INTO A
SIGNIFICANT BREAK IN THE SUBTROPICAL RIDGE OVER THE NEXT FEW
DAYS...WHERE IT WILL COME UNDER THE INFLUENCE OF AN UNUSUALLY DEEP
LONGWAVE TROUGH DIGGING INTO THE EASTERN UNITED STATES.  THIS
STEERING FEATURE SHOULD CAUSE BILL TO TURN NORTHWARD AND THEN
NORTH-NORTHEASTWARD...WITH SOME INCREASE IN FORWARD SPEED.  THE
TRACK GUIDANCE IS IN REASONABLY GOOD AGREEMENT ON THIS
SCENARIO...WITH THE OFFICIAL TRACK IN THE MIDDLE OF GUIDANCE
ENVELOPE.
  
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      19/0300Z 17.2N  53.4W   110 KT
 12HR VT     19/1200Z 18.4N  55.4W   120 KT
 24HR VT     20/0000Z 20.0N  58.0W   125 KT
 36HR VT     20/1200Z 21.8N  60.5W   125 KT
 48HR VT     21/0000Z 23.8N  62.9W   120 KT
 72HR VT     22/0000Z 29.0N  67.0W   115 KT
 96HR VT     23/0000Z 35.0N  68.0W   100 KT
120HR VT     24/0000Z 42.0N  64.0W    85 KT
 
$$
FORECASTER KIMBERLAIN/PASCH