ZCZC MIATCDEP5 ALL TTAA00 KNHC DDHHMM TROPICAL STORM DARBY DISCUSSION NUMBER 3 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 2 AM PDT TUE JUL 27 2004 TROPICAL STORM DARBY HAS BECOME MUCH BETTER ORGANIZED DURING THE PAST 6 HOURS AND HAS DEVELOPED A CENTRAL DENSE OVERCAST...OR CDO... FEATURE. THE LOW-LEVEL CENTER APPEARS TO BE EMBEDDED IN THE MIDDLE OF THE CDO AND UPPER-LEVEL OUTFLOW HAS ALSO IMPROVED AND BECOME MORE CIRCULAR. THE INITIAL INTENSITY OF 45 KT IS BASED ON A BLEND OF DVORAK SATELLITE INTENSITY ESTIMATES OF 55 KT...45 KT...AND 30 KT FROM SAB...TAFB...AND AFWA...RESPECTIVELY. THE INITIAL MOTION ESTIMATE REMAINS 280/12. THE OVERALL CONVECTIVE CLOUD PATTERN HAS BEEN TRANSLATING A LITTLE NORTH OF DUE WEST THE PAST 24 HOURS. IRONICALLY...THE DYNAMICAL MODELS HAVE ALL TAKEN DARBY OFF TO THE NORTHWEST FROM ITS INITIAL POSITIONS DURING THE SAME TIME FRAME. THE GFS...NOGAPS...AND THE GFDL MODELS HAVE BEEN THE WORSE AND HAVE HAD A 30 DEGREE RIGHT-OF-TRACK BIAS OVER THE LAST FEW MODEL RUNS. THE UKMET HAS HAD ONLY A SLIGHTLY LESS RIGHT BIAS. THIS APPEARS TO BE DUE TO THE GLOBAL MODELS HAVING POORLY INITIALIZED AN ALLEGED WEAKNESS IN THE SUBTROPICAL RIDGE TO THE NORTH OF DARBY. WATER VAPOR IMAGERY AND THE LATEST UW-CIMSS WATER VAPOR WIND ANALYSES INDICATE THE MID- TO UPPER-LEVEL FLOW NORTHWEST THROUGH NORTHEAST OF DARBY IS WESTERLY AT 10-15 KT. IN ADDITION... WATER VAPOR IMAGERY INDICATES THE UPPER-LEVEL LOW THAT HAD BEEN LOCATED ABOUT 500 NMI NORTH-NORTHWEST OF THE DARBY HAS BEEN MOVING RAPIDLY NORTHWESTWARD AWAY FROM THE CYCLONE...WHICH HAS LIKELY RESULTED IN THE ELIMINATION OF THE WEAKNESS IN THE SUBTROPICAL RIDGE. AS A RESULT...THE OFFICIAL FORECAST TRACK IS A LITTLE SOUTH OF THE PREVIOUS FORECAST TRACK AND IS CLOSE TO A BLEND OF THE STATISTICAL-DYNAMICAL MODEL...P91E...AND THE MEDIUM AND DEEP BAM. SATELLITE IMAGERY INDICATES THAT DARBY IS REALLY STARTING TO WIND UP A TIGHT CONVECTIVE BAND...SO A PERIOD OF RAPID INTENSIFICATION IS FORECAST FOR THE NEXT 24 HOURS OR SO. THIS IS SUPPORTED BY DARBY FORECAST TO REMAIN IN A VERY FAVORABLE UPPER-LEVEL FLOW REGIME WITH VERTICAL SHEAR GRADUALLY DECREASING TO LESS THAN 5 KT BY 36 HOURS. THE ONLY HINDERING INFLUENCE APPEARS TO BE THE COOLER WATER THAT DARBY WILL BE PASSING OVER AFTER 72 HOURS...AND THIS IS REFLECTED BY A WEAKENING FORECAST AT 96 AND 120 HOURS. HOWEVER...IF AN EYE DEVELOPS DURING THE NEXT 6-12 HOURS...THEN RAPID STRENGTHENING COULD LAST LONGER THAN CURRENTLY FORECAST AND DARBY COULD BECOME A MAJOR HURRICANE BY 36-48 HOURS WHILE STILL OVER 27-28C SSTS. FORECASTER STEWART FORECAST POSITIONS AND MAX WINDS INITIAL 27/0900Z 12.8N 117.3W 45 KT 12HR VT 27/1800Z 13.2N 118.9W 60 KT 24HR VT 28/0600Z 13.8N 120.9W 70 KT 36HR VT 28/1800Z 14.3N 122.8W 80 KT 48HR VT 29/0600Z 14.8N 124.9W 85 KT 72HR VT 30/0600Z 15.5N 128.7W 85 KT 96HR VT 31/0600Z 16.0N 132.5W 80 KT 120HR VT 01/0600Z 16.5N 137.0W 75 KT $$ NNNN
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Page last modified: Tuesday, 27-Jul-2004 08:42:26 UTC