Tropical Storm BILL

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TROPICAL STORM BILL DISCUSSION NUMBER   6
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL032009
500 PM AST SUN AUG 16 2009
 
SATELLITE IMAGES SHOW THAT BILL HAS DEVELOPED A CENTRAL DENSE
OVERCAST ALONG WITH IMPRESSIVE BANDING AND OUTFLOW.  THESE FEATURES
SUGGEST THAT THE STORM CONTINUES TO INTENSIFY.  SATELLITE INTENSITY
ESTIMATES FROM TAFB/SAB HAVE INCREASED TO T3.5...55 KT ON THE
DVORAK SCALE AND THIS VALUE WILL BE USED FOR THE INITIAL WIND
SPEED.

BILL IS MOVING TOWARD THE WEST-NORTHWEST OR 285/14.  THIS GENERAL
MOTION IS EXPECTED FOR THE NEXT COUPLE OF DAYS AS A SUBTROPICAL
RIDGE SHOULD REMAIN FIRM TO THE NORTH OF THE TROPICAL CYCLONE. 
THEREAFTER...A COUPLE OF EVOLUTIONS TO THE SYNOPTIC PATTERN SEEM
POSSIBLE.  THE FIRST...FAVORED BY THE GFS/ECMWF/GFDL/HWRF...SHOWS A
MID-LATITUDE SHORTWAVE DIVING INTO THE SUBTROPICAL RIDGE...CAUSING
IT TO BREAK AROUND 65W.  THIS LEADS TO A NORTHWEST MOTION OF BILL
IN ABOUT THREE DAYS.  THE OTHER SOLUTION...SHOWN BY THE
NOGAPS/UKMET...IS THAT THE HURRICANE BYPASSES THE FIRST BREAK AND
CONTINUES MOVING TOWARD THE WEST-NORTHWEST...EVENTUALLY TURNING
NORTHWEST AT THE END OF THE FORECAST DUE TO A LARGE MID-LATITUDE
TROUGH APPROACHING THE EASTERN UNITED STATES.  THE OFFICIAL
FORECAST IS SHIFTED A LITTLE BIT TO THE RIGHT AT THE END...IN
DEFERENCE TO THE HIGHLY-SKILLED MODELS IN THE FORMER SOLUTION.

WATER VAPOR IMAGES SHOW THAT THE HURRICANE IS ON THE SOUTH SIDE OF
AN UPPER HIGH WITH LITTLE SHEAR LIKELY FOR THE NEXT SEVERAL DAYS. 
IN COMBINATION WITH WARM SSTS...THERE IS NO REASON NOT TO FORECAST
CONTINUED STRENGTHENING OF BILL.  IN FACT...THE SHIPS RAPID
INTENSIFICATION INDEX SHOWS A 38 PERCENT CHANCE OF A 30 KT INCREASE
IN WINDS DURING THE NEXT 24 HOURS.  ALL RELIABLE MODELS CONTINUE TO
SHOW THIS SYSTEM BECOMING A MAJOR HURRICANE...WITH THE
GFDL/HWRF/GFDN MODELS FORECASTING BILL TO REACH CATEGORY FOUR
STRENGTH BY DAY 5.  HOWEVER...THERE IS SOME EVIDENCE IN THE GLOBAL
MODEL FIELDS THAT THE UPPER-LEVEL WINDS WILL BECOME LESS FAVORABLE
AFTER 72 HOURS WITH SOME SOUTHWESTERLY SHEAR DEVELOPING AS BILL
BECOMES DISPLACED FROM AN UPPER-LEVEL HIGH.  THE OFFICIAL FORECAST
WILL SHOW THE INTENSITY LEVELING OFF AFTER THAT TIME DUE TO THAT
POSSIBILITY. THE NHC INTENSITY FORECAST IS CLOSE TO THE PREVIOUS
FORECAST AND IS SIMILAR TO THE SHIPS MODEL.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      16/2100Z 12.8N  40.0W    55 KT
 12HR VT     17/0600Z 13.4N  42.1W    65 KT
 24HR VT     17/1800Z 14.2N  45.0W    75 KT
 36HR VT     18/0600Z 15.0N  47.7W    85 KT
 48HR VT     18/1800Z 16.0N  50.4W    95 KT
 72HR VT     19/1800Z 18.5N  55.5W   105 KT
 96HR VT     20/1800Z 22.0N  60.5W   105 KT
120HR VT     21/1800Z 26.5N  65.0W   105 KT
 
$$
FORECASTER BLAKE
 
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