000
WTNT43 KNHC 180837
TCDAT3
HURRICANE BILL DISCUSSION NUMBER  12
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL032009
500 AM AST TUE AUG 18 2009
 
TRMM DATA FROM 0223 UTC AND RECENT INFRARED IMAGERY SUGGEST THAT
BILL IS UNDERGOING AN EYEWALL REPLACEMENT CYCLE...WITH THE COLDEST
CLOUD TOPS CURRENTLY IN THE OUTER CONVECTIVE RING.  SATELLITE
INTENSITY ESTIMATES REMAIN 90 KT FROM SAB AND 77 KT FROM TAFB.  THE
INITIAL INTENSITY IS 85 KT...AND THIS COULD BE GENEROUS DUE TO THE
EYEWALL REPLACEMENT.

THE INITIAL MOTION IS 285/15.  BILL IS ON THE SOUTH SIDE OF A
LOW/MID-LEVEL RIDGE...WHICH SHOULD STEER THE HURRICANE GENERALLY
WEST-NORTHWESTWARD FOR THE NEXT COUPLE OF DAYS.  AFTER THAT...THE
LARGE-SCALE MODELS FORECAST THE RIDGE TO WEAKEN AS A SERIES OF
TROUGHS MOVE THROUGH THE WESTERN ATLANTIC AND THE EASTERN UNITED
STATES.  ONE MID/UPPER-LEVEL TROUGH IS CURRENTLY NORTHEAST OF
BERMUDA...MOVING SOUTHWARD.  A SECOND MID/UPPER-LEVEL TROUGH IS
ALONG THE EAST COAST OF THE UNITED STATES...FORECAST TO MOVE
EASTWARD AND THEN SOUTHWARD.  FINALLY...A STRONG DEEP-LAYER TROUGH
IS FORECAST TO DEVELOP OVER THE EASTERN UNITED STATES BETWEEN
72-120 HR.  ALL TRACK GUIDANCE INDICATES THAT BILL SHOULD TURN
NORTHWARD IN RESPONSE TO THE TROUGHS...BUT THERE IS SOME NOTABLE
SPREAD IN WHERE AND WHEN.  THE ECMWF...GFS...GFS ENSEMBLE MEAN...
AND THE BAM MODELS TURN BILL NORTHWARD BETWEEN 62W-65W IN RESPONSE
TO THE FIRST TWO TROUGHS.  THE REMAINDER OF THE TRACK GUIDANCE
CALLS FOR THE NORTHWARD TURN BETWEEN 65W-70W...DUE TO LESS RESPONSE
TO THE FIRST TWO TROUGHS AND THE EVENTUAL EFFECT OF THE THIRD
TROUGH.  THE PREVIOUS FORECAST TRACK WAS IN THE WESTERN CLUSTER...
AND THERE IS NO OBVIOUS REASON TO JUMP THE FORECAST TO THE EASTERN
CLUSTER.  THEREFORE...THE NEW TRACK IS DOWN THE MIDDLE OF THE
WESTERN GUIDANCE CLUSTER AND SIMILAR TO THE PREVIOUS FORECAST.

BILL IS IN A LIGHT VERTICAL WIND SHEAR ENVIRONMENT...AND THE SEA
SURFACE TEMPERATURES INCREASE ALONG THE FORECAST TRACK.  THE
HURRICANE SHOULD INTENSIFY AFTER THE EYEWALL REPLACEMENT CYCLE IS
OVER...WITH ALL GUIDANCE FORECASTING IT TO BECOME A MAJOR
HURRICANE.  THE INTENSITY FORECAST CALLS FOR A PEAK INTENSITY OF
110 KT IN 48 HR.  AFTER 48 HR...THE LARGE-SCALE MODELS FORECAST
BILL TO ENCOUNTER MODERATE SOUTHERLY OR SOUTHWESTERLY VERTICAL WIND
SHEAR...AND THE INTENSITY FORECAST THUS CALLS FOR GRADUAL WEAKENING
DURING THAT TIME.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      18/0900Z 15.5N  49.7W    85 KT
 12HR VT     18/1800Z 16.2N  51.7W    90 KT
 24HR VT     19/0600Z 17.4N  54.2W   100 KT
 36HR VT     19/1800Z 18.9N  56.7W   105 KT
 48HR VT     20/0600Z 20.5N  59.0W   110 KT
 72HR VT     21/0600Z 24.0N  63.5W   110 KT
 96HR VT     22/0600Z 29.5N  67.5W   105 KT
120HR VT     23/0600Z 35.5N  68.0W   100 KT
 
$$
FORECASTER BEVEN