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| #377929 (Received by flhurricane at: 4:44 AM 22.Sep.2010) TCDAT4 TROPICAL STORM LISA DISCUSSION NUMBER 6 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL142010 500 AM EDT WED SEP 22 2010 INFRARED SATELLITE IMAGES INDICATE THAT THERE HAS BEEN LITTLE SIGNIFICANT CHANGE ASSOCIATED WITH THE CLOUD PATTERN OF LISA. THE CYCLONE CONSISTS OF A CENTRAL MASS OF PULSATING DEEP CONVECTION... ALTHOUGH WITH FEWER BANDING FEATURES THAN YESTERDAY. SEVERAL RECENT MICROWAVE OVERPASSES SUGGEST THAT THE LOW- AND MID-LEVEL CENTERS ARE NOT CO-LOCATED...A SIGN THAT THE CYCLONE IS BEING IMPACTED BY VERTICAL WIND SHEAR. DVORAK CI NUMBERS AT 0600 UTC ARE AT 2.5 AND 3.0 FROM SAB AND TAFB...RESPECTIVELY. A BLEND OF THESE ESTIMATES IS THE BASIS FOR KEEPING THE INITIAL INTENSITY AT 40 KT FOR THIS ADVISORY. THE CENTER HAS BEEN DIFFICULT TO LOCATE BUT RECENT MICROWAVE AND SATELLITE FIXES SUGGEST THAT IT BE A LITTLE FARTHER SOUTH THAN PREVIOUSLY INDICATED. A SHORT-TERM INITIAL MOTION FROM THESE FIXES YIELDS AN ESTIMATE OF 060/4. THE TRACK GUIDANCE CONTINUES TO BE FAIRLY DIVERGENT BUT GENERALLY SHOWS LISA DRIFTING SOUTH OR EAST IN A REGION OF WEAK STEERING DURING THE 48 HOURS. AFTER THAT...A TURN TOWARD WEST-NORTHWEST IS EXPECTED WITH A FASTER FORWARD SPEED AS A WEAK LOW- TO MID-LEVEL RIDGE BUILDS NORTH OF THE STORM. GIVEN THE LARGE SPREAD IN THE GLOBAL MODELS...THE NEW NHC TRACK FORECAST REMAINS OF LOW CONFIDENCE AND IS SLOWED FURTHER UNTIL A MORE ESTABLISHED TREND IS EVIDENT IN THE GUIDANCE. WATER VAPOR IMAGERY SHOWS A SHARP UPPER-LEVEL TROUGH NORTH OF LISA THAT IS FORECAST TO LIFT OUT IN 24-48 HOURS. THIS FEATURE HAS BEEN IMPARTING A NORTHWESTERLY SHEAR OVER THE STORM...BUT SHIPS MODEL OUTPUT INDICATES THAT THIS SHEAR SHOULD NOT BE PARTICULARLY STRONG DURING THIS TIME. AFTER ABOUT 60-72 HOURS...HOWEVER...GLOBAL MODELS FORECAST A STRENGTHENING OF UPPER-LEVEL WESTERLY FLOW OVER LISA...AND THE INTENSITY GUIDANCE SHOWS WEAKENING LATER IN THE PERIOD. OF COURSE...THE INTENSITY GUIDANCE IS HIGHLY DEPENDENT ON WHERE LISA IS LOCATED AT THAT TIME...AND THE GREATER THAN NORMAL UNCERTAINTY REGARDING ITS FORECAST TRACK MAKES THE CURRENT INTENSITY FORECAST DIFFICULT. THE NEW NHC INTENSITY FORECAST HAS BEEN ADJUSTED A BIT DOWNWARD BEFORE 72 HOURS BUT REMAINS AT OR ABOVE THE STATISTICAL-DYNAMICAL OUTPUT. FORECAST POSITIONS AND MAX WINDS INITIAL 22/0900Z 18.0N 30.5W 40 KT 12HR VT 22/1800Z 18.0N 30.5W 45 KT 24HR VT 23/0600Z 17.9N 30.5W 50 KT 36HR VT 23/1800Z 17.8N 30.5W 50 KT 48HR VT 24/0600Z 17.8N 30.7W 55 KT 72HR VT 25/0600Z 18.5N 32.5W 50 KT 96HR VT 26/0600Z 19.5N 35.0W 45 KT 120HR VT 27/0600Z 21.0N 38.0W 35 KT $$ FORECASTER KIMBERLAIN |
