Tropical Storm BILL

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TROPICAL STORM BILL DISCUSSION NUMBER   7
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL032009
1100 PM AST SUN AUG 16 2009
 
INFRARED SATELLITE IMAGERY REVEALS AN IMPRESSIVE MASS OF DEEP
CONVECTIVE TOPS TO -80C WITHIN A CENTRAL DENSE OVERCAST...ALONG
WITH AN EXPANDING UPPER-LEVEL OUTFLOW.  A LONG...WELL-DEFINED BAND
CONNECTED TO THE ITCZ IS NOTED IN THE SOUTHERN SEMICIRCLE...WHILE
LESSER BANDING COILS AROUND THE NORTHERN HALF OF THE CIRCULATION.
DVORAK T-NUMBERS ARE 3.5 AND 4.0 FROM SAB AND TAFB...RESPECTIVELY.
TAKING THE MEAN OF THESE INTENSITY ESTIMATES...THE ADVISORY WIND
SPEED IS SET AT 60 KT.
 
RECENT SATELLITE CENTER FIXES ARE SOMEWHAT UNCERTAIN BUT YIELD AN
INITIAL MOTION ESTIMATE OF 290/17...REPRESENTING A SLIGHTLY FASTER
FORWARD SPEED THAN BEFORE. UNDER THE INFLUENCE OF SUBTROPICAL
RIDGING TO THE NORTH...BILL IS EXPECTED TO MAINTAIN A GENERAL
WEST-NORTHWEST HEADING FOR THE NEXT 48-72 HOURS.  WITH LITTLE
SPREAD IN THE CURRENT MODEL CYCLE THROUGH THIS TIME...THE OFFICIAL
TRACK DEVIATES LITTLE FROM THE PREVIOUS ADVISORY. THEREAFTER...
MODEL GUIDANCE IS IN GOOD AGREEMENT THAT A RATHER SUBSTANTIAL BREAK
IN THE SUBTROPICAL RIDGE SHOULD DEVELOP ROUGHLY ALONG 60W...WHICH
SHOULD RESULT IN A MORE NORTHWESTWARD COURSE.  MOST OF THE MODEL
SOLUTIONS HAVE SHIFTED FARTHER NORTH FROM THE PREVIOUS RUNS...AND
SO HAS THE MODEL CONSENSUS.  ONLY THE UKMET FAVORS A MORE
WEST-NORTHWESTWARD PATH AND APPEARS AS THE LONE...SOUTHERN OUTLIER
AMONGST THE REMAINDER OF THE GLOBAL MODELS. THE OFFICIAL FORECAST
IS ADJUSTED NORTH OF THE PREVIOUS ONE...ESPECIALLY BEYOND 96
HOURS...BASED ON THE LATEST GUIDANCE.
 
EVEN THOUGH ENVIRONMENTAL SHEAR IS QUITE LOW...BILL HAS BEEN
TRACKING OVER MARGINAL SEA SURFACE TEMPERATURES AND WATERS ARE NOT
FORECAST TO WARM FURTHER UNTIL AFTER 24 HOURS.  INTERESTINGLY...THE
GFDL/HWRF SHOW A NEARLY FLAT INTENSIFICATION RATE FOR THE NEXT DAY
OR TWO...PERHAPS UNDERSCORING THE TEPID THERMODYNAMIC CONDITIONS. IT
SHOULD BE NOTED...HOWEVER...THAT THE SHIPS RAPID INTENSITY INDEX
SHOWS A 41% CHANCE OF A 30 KT INTENSITY INCREASE DURING THE NEXT 24
HOURS.  THEREAFTER...THE GUIDANCE IS IN GOOD AGREEMENT THAT LOW
SHEAR AND WARMING WATERS SHOULD PREVAIL FOR BILL...ALLOWING THE
CYCLONE TO EVOLVE INTO A MAJOR HURRICANE OVER THE FORECAST PERIOD.
THE OFFICIAL FORECAST IS CLOSEST TO THE SHIPS MODEL GUIDANCE...
WHICH PREDICTS A FAIRLY STEADY RATE OF DEVELOPMENT OVER THE NEXT
FEW DAYS.  TOWARD THE END OF THE FORECAST PERIOD...THE GLOBAL
MODELS HAVE BEEN HINTING AT INCREASING WESTERLY SHEAR AS BILL
OUTRUNS FAVORABLE ANTICYCLONIC FLOW TO THE SOUTHEAST.
ACCORDINGLY...THE OFFICIAL INTENSITY FORECAST SHOWS SLIGHT
WEAKENING NEAR THE END OF THE PERIOD.
  
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      17/0300Z 13.4N  41.7W    60 KT
 12HR VT     17/1200Z 14.1N  43.7W    70 KT
 24HR VT     18/0000Z 14.9N  46.5W    80 KT
 36HR VT     18/1200Z 15.8N  49.2W    85 KT
 48HR VT     19/0000Z 16.8N  51.8W    95 KT
 72HR VT     20/0000Z 19.7N  56.8W   105 KT
 96HR VT     21/0000Z 23.5N  61.5W   105 KT
120HR VT     22/0000Z 28.1N  65.5W   100 KT
 
$$
FORECASTER KIMBERLAIN/PASCH
 
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