ZCZC MIATCDEP5 ALL TTAA00 KNHC DDHHMM TROPICAL STORM DARBY DISCUSSION NUMBER 16 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 8 AM PDT FRI JUL 30 2004 DARBY HAS SIGNIFICANTLY DECREASED IN ORGANIZATION DURING THE PAST SEVERAL HOURS. THE LOW-LEVEL CENTER HAS BECOME EXPOSED TO THE SOUTH OF THE CONVECTION...AND THE CONVECTION HAS DECREASED SIGNIFICANTLY IN COVERAGE. SATELLITE INTENSITY ESTIMATES ARE 65 KT FROM AFWA AND SAB...AND 45 KT FROM TAFB. BASED ON THIS...DARBY IS DOWNGRADED TO A 55 KT TROPICAL STORM. THE INITIAL MOTION IS 280/12...AND OVER THE LAST FEW HOURS THE CYCLONE MAY BE MOVING DUE WEST. DARBY IS ON THE SOUTH SIDE OF A STRONG LOW/MID LEVEL RIDGE...WHILE A MID/UPPER LEVEL TROUGH IS LOCATED NORTH OF THE HAWAIIAN ISLANDS. THE RIDGE SHOULD KEEP DARBY ON A BASICALLY WESTWARD COURSE THROUGH ABOUT 72 HR. AFTER THAT... THERE IS A CHANCE THE CYCLONE COULD TURN MORE NORTHWARD IF THE CYCLONE STILL HAS ANY VERTICAL DEPTH. THE NOGAPS...WHICH KEEPS DARBY STRONG THROUGH THE FORECAST PERIOD...TURNS IT NORTHWARD IN RESPONSE TO THE CENTRAL PACIFIC TROUGH. OTHER LARGE-SCALE MODELS...WHICH FORECAST A WEAKER SYSTEM...SHOW A MORE WESTERLY MOTION. THE OFFICIAL FORECAST CALLS FOR A WESTWARD TRACK FOR 72 HR... FOLLOWED BY A WEST-NORTHWESTWARD MOTION. THE TRACK IS NUDGED A LITTLE SOUTH OF THE PREVIOUS PACKAGE AND IS IN BEST AGREEMENT WITH THE GFDN AND BAMM. THE INTENSITY FORECAST IS PROBLEMATIC. NORMALLY A CYCLONE OF THIS ORGANIZATION AT THIS LOCATION CONTINUES WEAKENING UNTIL DISSIPATION...AS INDICATED BY THE GFS...GFDL...AND SHIPS MODELS. HOWEVER...SEA SURFACE TEMPERATURES ARE A LITTLE WARMER THAN NORMAL IN THE CENTRAL PACIFIC...AND THE FORECAST TRACK MOVES THE CYCLONE OVER INCREASING SSTS AFTER IT CROSSES 140W. THIS MAY BE REFLECTED IN THE NOGAPS FORECAST. THE INTENSITY FORECAST TENDS TO COMPROMISE...CALLING FOR DARBY TO WEAKEN TO A DEPRESSION IN 36 HR THEN LINGER THROUGH THE REST OF THE FORECAST PERIOD. ALTERNATIVE SCENARIOS INCLUDE DARBY COULD DISSIPATE BEFORE IT REACHES THE WARMER WATER...OR THAT INCREASING VERTICAL SHEAR ASSOCIATED WITH THE TROUGH COULD DISSIPATE IT DESPITE THE WARMER WATER. FORECASTER BEVEN FORECAST POSITIONS AND MAX WINDS INITIAL 30/1500Z 18.6N 132.2W 55 KT 12HR VT 31/0000Z 18.9N 134.1W 45 KT 24HR VT 31/1200Z 19.1N 136.8W 35 KT 36HR VT 01/0000Z 19.3N 139.7W 30 KT 48HR VT 01/1200Z 19.4N 142.5W 25 KT 72HR VT 02/1200Z 19.5N 147.5W 25 KT 96HR VT 03/1200Z 20.5N 152.0W 25 KT 120HR VT 04/1200Z 22.0N 155.0W 25 KT $$ NNNN
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Page last modified: Friday, 30-Jul-2004 14:42:29 UTC