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| #377777 (Received by flhurricane at: 10:47 AM 21.Sep.2010) TCDAT4 TROPICAL STORM LISA DISCUSSION NUMBER 3 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL142010 1100 AM EDT TUE SEP 21 2010 INTERMITTENT BURSTS OF DEEP CONVECTION HAVE BEEN OCCURRING NEAR THE WELL-DEFINED CIRCULATION CENTER NOTED IN A 21/0932Z SSMIS OVERPASS. UPPER-LEVEL OUTFLOW HAS ALSO GRADUALLY BEEN IMPROVING TO THE NORTH AND SOUTH OF LISA. THE INITIAL INTENSITY HAS BEEN INCREASED TO 40 KT BASED ON A BLEND OF DVORAK SATELLITE INTENSITY ESTIMATES OF T2.5/35 KT FROM BOTH TAFB AND SAB...AND UW-CIMSS ADT CURRENT INTENSITY ESTIMATES OF T3.5/45 KT. AFTER A BRIEF JOG TO THE NORTHEAST A FEW HOURS AGO...LISA HAS RESUMED A MOTION ESTIMATE OF 360/4. LISA IS EXPECTED TO MOVE SLOWLY NORTHWARD FOR THE NEXT 24 HOURS OR SO DUE TO THE COMBINATION OF WEAK STEERING CURRENTS ASSOCIATED WITH A BREAK IN THE SUBTROPICAL RIDGE AND MODERATE NORTHWESTERLY FLOW ON THE WEST SIDE OF A DEEP-LAYER TROUGH SITUATED TO THE NORTH OF THE CYCLONE. AS THE AFOREMENTIONED TROUGH LIFTS OUT TO THE NORTHEAST BY 36-48 HOURS... THE RIDGE TO THE NORTH OF LISA IS EXPECTED TO BUILD BACK IN AND FORCE THE CYCLONE ON A SLOW WEST-NORTHWESTWARD TRACK. THE MODEL GUIDANCE IS WIDELY DIVERGENT WITH THE HWRF/GFDL MODELS TAKING LISA DUE NORTH THROUGHOUT THE FORECAST PERIOD...WHILE THE NOGAPS/GFDN MODELS MOVE LISA DUE WEST. THE REST OF THE NHC MODEL GUIDANCE IS SPREAD OUT EVENLY BETWEEN THESE TWO EXTREMES. SUCH A LARGE SPREAD IN THE MODEL GUIDANCE TYPICALLY SUGGESTS THAT THE FORWARD MOTION WILL BE RELATIVELY SLOW...AND THAT IS REFLECTED IN THIS FORECAST PACKAGE. THE OFFICIAL FORECAST TRACK IS SIMILAR THE PREVIOUS ADVISORY TRACK AND LIES A LITTLE NORTH OF THE MODEL CONSENSUS. ALTHOUGH THE VERTICAL SHEAR IS EXPECTED TO REMAIN LOW FOR ABOUT THE NEXT 72 HOURS...THE MID-LEVEL MOISTURE ENVIRONMENT SURROUNDING LISA IS FORECAST TO DRY OUT CONSIDERABLY AND SSTS ARE EXPECTED TO DROP BELOW 27C. THE DRIER AIR AND COOLER WATER MAY PUT THE BREAKS ON THE INTENSIFICATION PROCESS AS THE CYCLONE NEARS HURRICANE STRENGTH IN 48-72 HOURS. AFTER 72 HOURS...LISA IS EXPECTED TO BEGIN FEELING THE EFFECTS OF INCREASING WESTERLY SHEAR AS ANOTHER UPPER-LEVEL TROUGH DIGS SOUTHWARD AND AMPLIFIES TO THE NORTHWEST OF THE CYCLONE. IT SHOULD BE NOTED THAT THE ECMWF MODEL DOES NOT MOVE LISA NORTH OF 20N LATITUDE...WHICH KEEPS THE CYCLONE IN A LOWER SHEAR ENVIRONMENT BENEATH AN UPPER-LEVEL ANTICYCLONE FROM 72-120 HOURS. THE OFFICIAL INTENSITY FORECAST IS A BLEND OF THE SHIPS AND LGEM INTENSITY MODELS...AND LEANS A LITTLE TOWARD THE STRONGER ECMWF SOLUTION. FORECAST POSITIONS AND MAX WINDS INITIAL 21/1500Z 18.1N 31.7W 40 KT 12HR VT 22/0000Z 18.3N 31.7W 45 KT 24HR VT 22/1200Z 18.7N 31.8W 50 KT 36HR VT 23/0000Z 19.0N 32.2W 55 KT 48HR VT 23/1200Z 19.4N 33.0W 60 KT 72HR VT 24/1200Z 20.1N 34.5W 55 KT 96HR VT 25/1200Z 20.9N 35.8W 50 KT 120HR VT 26/1200Z 21.0N 37.5W 45 KT $$ FORECASTER STEWART |
