Hurricane BILL

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HURRICANE BILL DISCUSSION NUMBER  16
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL032009
500 AM AST WED AUG 19 2009

BILL HAS A WELL-DEFINED 25-30 N MI WIDE EYE WITH EYEWALL CLOUD TOPS
TO NEAR -80C.  SATELLITE INTENSITY ESTIMATES FROM TAFB AND SAB WERE
115 KT AT 06Z...AND THE CURRENT SATELLITE SIGNATURE SUGGESTS A
SLIGHTLY STRONGER SYSTEM.  BASED ON THE SATELLITE ESTIMATES...THE
INITIAL INTENSITY IS INCREASED TO 115 KT.  A NOAA HURRICANE HUNTER
AIRCRAFT IS CURRENTLY ENROUTE TO INVESTIGATE BILL.

THE MOTION IS WOBBLING BETWEEN 295-300 DEGREES AT 14 KT.  BILL IS ON
THE SOUTHWEST SIDE OF THE ATLANTIC SUBTROPICAL RIDGE.  WATER VAPOR
IMAGERY SHOWS A MID/UPPER-LEVEL LOW NEAR 32N60W AND A MID/UPPER-
LEVEL TROUGH N OF 30N BETWEEN 69W-75W.  THESE FEATURES ARE
CURRENTLY CREATING A WEAKNESS IN THE WESTERN PORTION OF THE
SUBTROPICAL RIDGE.  THE TRACK GUIDANCE MODELS FORECAST BILL TO
GRADUALLY TURN NORTHWESTWARD TOWARDS THIS WEAKNESS DURING THE NEXT
48-72 HR...WITH THE ECMWF SHOWING THE EARLIEST TURN AND THE
NOGAPS/UKMET SHOWING THE LATEST TURN.  AFTER THAT...A LARGE
DEEP-LAYER TROUGH MOVING INTO THE EASTERN UNITED STATES SHOULD
CAUSE BILL TO RECURVE NORTHWARD AND NORTHEASTWARD.  THERE IS A
LARGE MODEL SPREAD IN BOTH DIRECTION AND SPEED AFTER 72 HR...WITH
THE NOGAPS ON ONE SIDE BRINGING BILL NEAR NEW ENGLAND AND THE
CORRECTED CONSENSUS MODELS ON THE OTHER SIDE CALLING FOR A SHARP
TURN OUT TO SEA.  OVERALL...THE ENVELOPE OF MODEL GUIDANCE HAS
SHIFTED A LITTLE TO THE WEST...SO THE NEW OFFICIAL FORECAST WILL DO
LIKEWISE.  THE NEW FORECAST TRACK IS NEAR THE MODEL CONSENSUS AND
THE MIDDLE OF THE GUIDANCE ENVELOPE.

BILL HAS STRENGTHENED IN SPITE OF BEING IN AN ENVIRONMENT OF
10-15 KT OF SOUTHWESTERLY VERTICAL WIND SHEAR.  THE LARGE-SCALE
MODELS FORECAST THIS SHEAR SHOULD DECREASE AFTER 12 HR...SO
ADDITIONAL STRENGTHENING IS FORECAST.  BY 48-72 HR...BILL SHOULD
INTERACT WITH THE UPPER-LEVEL TROUGH CURRENTLY JUST OFF THE NORTH
CAROLINA COAST.  THIS IS LIKELY TO CAUSE ANOTHER ROUND OF SHEAR...SO
THE INTENSITY FORECAST CALLS FOR SLOW WEAKENING DURING THAT TIME. 
AFTER 72 HR...BILL SHOULD BECOME EMBEDDED IN THE WESTERLIES AS IT
MOVES OVER COLDER SEA SURFACE TEMPERATURES...AND A FASTER WEAKENING
IS EXPECTED.  IT SHOULD BE NOTED THAT EYEWALL REPLACEMENT CYCLES...
WHICH ARE VERY DIFFICULT TO ANTICIPATE...MAY PRODUCE SOME
FLUCTUATIONS IN INTENSITY SUPERIMPOSED ON THE GENERAL TREND OF THE
INTENSITY FORECAST.
  
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      19/0900Z 18.0N  54.9W   115 KT
 12HR VT     19/1800Z 19.0N  56.8W   120 KT
 24HR VT     20/0600Z 20.6N  59.4W   125 KT
 36HR VT     20/1800Z 22.5N  62.0W   125 KT
 48HR VT     21/0600Z 24.4N  64.4W   120 KT
 72HR VT     22/0600Z 29.5N  68.0W   115 KT
 96HR VT     23/0600Z 36.0N  68.5W   100 KT
120HR VT     24/0600Z 44.0N  62.0W    80 KT
 
$$
FORECASTER BEVEN
 
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