000 WTNT43 KNHC 181500 TCDAT3 HURRICANE BILL DISCUSSION NUMBER 13 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL032009 1100 AM AST TUE AUG 18 2009 VISIBLE SATELLITE IMAGES REVEAL A VERY SYMMETRIC HURRICANE WITH OUTFLOW WELL ESTABLISHED IN ALL QUADRANTS. THE EYE OF BILL IS QUITE LARGE...MEASURING ABOUT 35-40 NM IN DIAMETER. SUBJECTIVE DVORAK INTENSITY ESTIMATES ARE NOW T5.0 FROM BOTH TAFB AND SAB... AND THE INITIAL INTENSITY IS INCREASED TO 90 KT. A HURRICANE HUNTER AIRCRAFT THAT WAS ORIGINALLY SCHEDULED TO INVESTIGATE THE REMNANTS OF ANA HAS BEEN TASKED TO FLY INTO BILL THIS AFTERNOON. DATA FROM THE AIRCRAFT WILL PROVIDE A BETTER ASSESSMENT OF THE INTENSITY AND SIZE OF THE HURRICANE. THE INITIAL MOTION IS 285/14. BILL IS CURRENTLY BEING STEERED WEST-NORTHWESTWARD BY THE SUBTROPICAL RIDGE TO THE NORTH OF THE HURRICANE. A COUPLE OF TROUGHS SEEN IN WATER VAPOR IMAGERY TO THE NORTH AND NORTHWEST OF THE HURRICANE ARE FORECAST TO WEAKEN THE RIDGE DURING THE NEXT COUPLE OF DAYS. THIS SHOULD ALLOW BILL TO TURN NORTHWESTWARD AND THE MODELS ARE IN GOOD AGREEMENT ON THIS SCENARIO DURING THROUGH 48 HOURS. AFTER THAT...A LARGE MID-LATITUDE TROUGH IS FORECAST BY THE GLOBAL MODELS TO MOVE INTO THE EASTERN UNITED STATES ON DAY 4 AND 5. THE MODELS RESPOND BY TURNING THE HURRICANE NORTHWARD AT THAT TIME...HOWEVER THERE REMAINS SOME SPREAD AS TO WHEN AND WHERE THAT WILL OCCUR. AS BEFORE...THE GFS AND ECMWF REMAIN ON THE RIGHT SIDE OF THE MODEL ENVELOPE...WHILE THE UKMET AND HWRF ARE ON THE LEFT. THE OFFICIAL FORECAST IS VERY SIMILAR TO THE PREVIOUS NHC TRACK AND NEAR THE MIDDLE OF THE GUIDANCE ENVELOPE THROUGH 72 HOURS. THEREAFTER...THE NEW TRACK HAS BEEN SHIFTED EASTWARD...BUT REMAINS TO THE WEST OF THE MODEL CONSENSUS. BILL IS IN A CONDUCIVE ATMOSPHERIC ENVIRONMENT FOR STRENGTHENING AND SSTS ALONG THE FORECAST TRACK WILL BE INCREASING DURING THE NEXT COUPLE OF DAYS. THE INTENSITY FORECAST SHOWS GRADUAL STRENGTHENING AND TOPS OUT AT 110 KT...WHICH IS CLOSE TO THE CONSENSUS OF THE INTENSITY GUIDANCE. LATER IN THE FORECAST PERIOD...MODERATE SOUTH TO SOUTHWESTERLY WIND SHEAR IS EXPECTED TO INCREASE...AND THE OFFICIAL FORECAST PREDICTS GRADUAL WEAKENING AT THAT TIME. THE WIND RADII WERE EXPENDED OUTWARD BASED ON AN EARLIER QUIKSCAT PASS. FORECAST POSITIONS AND MAX WINDS INITIAL 18/1500Z 15.9N 51.2W 90 KT 12HR VT 19/0000Z 16.8N 53.0W 95 KT 24HR VT 19/1200Z 18.0N 55.3W 105 KT 36HR VT 20/0000Z 19.6N 57.7W 110 KT 48HR VT 20/1200Z 21.5N 60.3W 110 KT 72HR VT 21/1200Z 25.9N 64.7W 110 KT 96HR VT 22/1200Z 31.5N 67.5W 105 KT 120HR VT 23/1200Z 38.0N 66.5W 95 KT $$ FORECASTER BROWN/CANGIALOSI