810 
 WTNT43 KNHC 200849
 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