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| #288865 (Received by flhurricane at: 10:55 PM 16.Aug.2009) TCDAT3 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 |
