Hurricane BILL

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HURRICANE BILL DISCUSSION NUMBER  11
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL032009
1100 PM AST MON AUG 17 2009
 
BILL CONTINUES TO SLOWLY STRENGTHEN OVER THE TROPICAL ATLANTIC.  THE
HURRICANE IS GENERATING VERY DEEP...COLD-TOPPED CONVECTION...THOUGH
THERE ARE SOME EVIDENT ASYMMETRIES.  SATELLITE PICTURES ALSO REVEAL
AN EXPANSIVE CIRRUS CANOPY...SUGGESTIVE OF A WELL-ESTABLISHED
UPPER-LEVEL OUTFLOW.  EARLIER MICROWAVE PASSES INDICATED AN ABSENCE
OF INNER CORE FEATURES...BUT A 2229 SSMIS PASS SUGGESTS THAT BILL
MAY BE FINALLY FORMING AN INNER CORE.  DVORAK T-NUMBERS ARE 4.5 AND
5.0 FROM TAFB AND SAB...RESPECTIVELY. BASED UPON THESE ESTIMATES...
THE INITIAL INTENSITY IS RAISED TO 85 KT.
 
THERE ARE NO OBVIOUS REASONS TO PREVENT BILL FROM INTENSIFYING
FURTHER DURING THE SEVERAL DAYS...AS THE INTENSITY GUIDANCE AGREES
ON AN ENVIRONMENT CHARACTERIZED BY LOW SHEAR AND WARM SEA SURFACE
TEMPERATURES.  PERHAPS THE ONLY LIMITING FACTOR WILL BE
FLUCTUATIONS WITHIN THE INNER CORE FOR WHICH THERE IS VIRTUALLY NO
FORECAST SKILL.  THE GUIDANCE IS IN GENERAL AGREEMENT THAT BILL
SHOULD REACH A PEAK IN INTENSITY IN ABOUT 72 HOURS.  TOWARD THE END
OF THE PERIOD...GLOBAL MODELS HAVE BEEN PREDICTING A GRADUAL
INCREASE IN SOUTHWESTERLY SHEAR AS BILL BEGINS TO BECOME MORE
DISPLACED TO THE NORTH OF AN UPPER-LEVEL ANTICYCLONE.
CONSEQUENTLY...THE OFFICIAL FORECAST SHOWS A WEAKENING TREND AFTER
72 HOURS.
 
RECENT CENTER FIXES YIELD AN INITIAL MOTION ESTIMATE OF 285/15.  THE
FORECAST RATIONALE REMAINS UNCHANGED.  A WEAKNESS ALONG 50-55W IS
FORECAST TO ERODE A LARGE PORTION OF THE SUBTROPICAL RIDGE OVER THE
CENTRAL ATLANTIC BEGINNING AS SOON AS 24 HOURS FROM NOW.  AS A
RESULT...THE GENERAL WEST-NORTHWESTWARD TRACK THAT BILL HAS
MAINTAINED FOR SEVERAL DAYS SHOULD BEGIN TO BEND MORE TOWARD THE
NORTHWEST BY 48-72 HOURS.  BEYOND 72 HOURS...BILL SHOULD BE STEERED
ON A NORTH-NORTHWESTERLY TO NORTHERLY TRACK WHERE IT SHOULD BEGIN
ENCOUNTERING THE EFFECTS OF A DIGGING LONGWAVE TROUGH OVER THE
EASTERN UNITED STATES.  THE OFFICIAL FORECAST IS NOT ALL THAT
DIFFERENT FROM THE PREVIOUS ADVISORY PACKAGE...WITH ONLY COSMETIC
CHANGES MADE TO THE FORECAST TRACK.
 
THE WIND RADII WERE ADJUSTED SLIGHTLY BASED ON THE MOST RECENT
QUIKSCAT OVERPASS.  BILL IS FORECAST TO GROW A LITTLE MORE IN
SIZE DURING THE FORECAST PERIOD...ALTHOUGH SOME GUIDANCE SUCH AS THE
GFS SUGGESTS THAT IT MAY BECOME EVEN LARGER THAN INDICATED IN THE
NHC FORECAST.
  
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      18/0300Z 15.0N  48.3W    85 KT
 12HR VT     18/1200Z 15.7N  50.3W    95 KT
 24HR VT     19/0000Z 16.9N  52.8W   100 KT
 36HR VT     19/1200Z 18.1N  55.3W   105 KT
 48HR VT     20/0000Z 19.7N  57.8W   110 KT
 72HR VT     21/0000Z 23.5N  62.5W   110 KT
 96HR VT     22/0000Z 28.5N  66.0W   105 KT
120HR VT     23/0000Z 34.5N  68.5W   100 KT
 
$$
FORECASTER KIMBERLAIN/PASCH
 
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