000 WTPZ45 KNHC 230849 TCDEP5 TROPICAL STORM DARBY DISCUSSION NUMBER 2 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL EP052010 200 AM PDT WED JUN 23 2010 THE OVERALL CONVECTIVE STRUCTURE OF THE TROPICAL CYCLONE HAS CONTINUED TO BECOME MORE ORGANIZED. ALTHOUGH THE CIRCULATION CENTER APPEARS TO BE LOCATED ON THE WESTERN EDGE OF THE DEEP CONVECTION DUE TO SOME EASTERLY SHEAR...DVORAK T-NUMBERS ARE 2.5 FROM TAFB AND 2.0 FROM SAB. ASCAT DATA...WHICH IS KNOWN TO HAVE A LOW BIAS...SHOWED A MAXIMUM OF 30 KT OUTSIDE OF THE DEEP CONVECTION AT 0306 UTC. SINCE THE CLOUD PATTERN HAS IMPROVED OVER THE PAST FEW HOURS AND STRONGER WINDS ARE LIKELY OCCURRING IN THE DEEP CONVECTION...THE DEPRESSION IS UPGRADED TO TROPICAL STORM DARBY. THE ASCAT PASS HELPED IN LOCATING THE LOW-LEVEL CENTER...AND IT IS ESTIMATED THAT DARBY IS MOVING 305 DEGREES AT 8 KNOTS. A MID-LEVEL RIDGE LOCATED OVER THE SOUTHERN UNITED STATES AND MEXICO SHOULD STEER DARBY GENERALLY TOWARD THE WEST-NORTHWEST OVER THE NEXT 2 DAYS. THEREAFTER...STEERING CURRENTS ARE EXPECTED TO WEAKEN AS THE RIDGE BREAKS DOWN. IN FACT...SEVERAL GLOBAL MODELS SUGGEST THAT DARBY COULD STOP ALTOGETHER AND ATTAIN SOME EASTWARD MOTION WITHIN LOW-LEVEL SOUTHWESTERLY FLOW DURING THAT TIME. THE GLOBAL MODEL SOLUTIONS ARE COUNTER TO THAT OF THE GFDL AND HWRF...WHICH MAINTAIN A SLOW WEST-NORTHWESTWARD MOTION THROUGH THE FORECAST PERIOD. THIS UNCERTAINTY SUPPORTS THE NEGLIGIBLE MOTION SHOWN IN THE OFFICIAL FORECAST BY DAY 5. THE SHIPS MODEL INDICATES THAT THE EASTERLY SHEAR OVER DARBY SHOULD ABATE OVER THE NEXT 2 DAYS...GIVING THE SYSTEM A CHANCE TO INTENSIFY. THE NEW OFFICIAL FORECAST SHOWS A LITTLE MORE STRENGTHENING THAN THE PREVIOUS FORECAST...BUT THIS STILL MIGHT BE ON THE CONSERVATIVE SIDE SINCE THE SHIPS RI INDEX INDICATES A 1 IN 4 CHANCE OF RAPID INTENSIFICATION OVER THE NEXT 24 HOURS. WHATEVER THE MAXIMUM INTENSITY...INCREASED VERTICAL SHEAR AFTER DAY 3 IS EXPECTED TO INDUCE WEAKENING...ESPECIALLY IF DARBY BEGINS TO ATTAIN AN EASTERLY MOTION. FORECAST POSITIONS AND MAX WINDS INITIAL 23/0900Z 11.5N 94.0W 35 KT 12HR VT 23/1800Z 11.9N 95.1W 40 KT 24HR VT 24/0600Z 12.4N 96.6W 45 KT 36HR VT 24/1800Z 12.8N 98.2W 50 KT 48HR VT 25/0600Z 13.0N 99.4W 55 KT 72HR VT 26/0600Z 13.5N 101.0W 55 KT 96HR VT 27/0600Z 14.0N 101.5W 50 KT 120HR VT 28/0600Z 14.5N 101.5W 45 KT $$ FORECASTER BERG