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| #377838 (Received by flhurricane at: 4:38 PM 21.Sep.2010) TCDAT4 TROPICAL STORM LISA DISCUSSION NUMBER 4 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL142010 500 PM EDT TUE SEP 21 2010 AFTER A SHORT-LIVED DECREASE IN THE CENTRAL CONVECTION...A BURST OF STRONG CONVECTION HAS RECENTLY DEVELOPED OVER THE WELL-DEFINED LOW-LEVEL CIRCULATION CENTER. RECENT TRMM AND AMSU MICROWAVE OVERPASSES INDICATE THE LOW- AND MID-LEVEL CIRCULATIONS HAVE TIGHTENED UP AND BECOME MORE VERTICALLY ALIGNED...SUGGESTING THAT THE VERTICAL SHEAR AFFECTING THE SYSTEM HAS DECREASED. UPPER-LEVEL OUTFLOW CONTINUES TO IMPROVE IN ALL QUADRANTS. THE INITIAL INTENSITY REMAINS AT 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 ESTIMATE OF T3.4/53 KT. THE INITIAL MOTION ESTIMATE IS 360/2. LISA HAS BEEN WOBBLING BETWEEN A NORTH AND NORTHEAST DIRECTION...PROBABLY DUE TO RE-ORGANIZATION OF THE SYSTEM CENTER. NOW THAT THE VERTICAL CIRCULATIONS APPEAR TO BE MORE VERTICALLY ALIGNED...HOPEFULLY THIS WILL RESULT IN LESS SCATTER IN THE SATELLITE FIX POSITIONS. LISA IS EXPECTED REMAIN IN VERY WEAK STEERING FLOW FOR THE NEXT 48-72 HOURS. AFTERWARDS...A MID-LEVEL TROUGH SITUATED JUST NORTH OF THE CYCLONE IS FORECAST TO SLOWLY LIFT OUT TO THE NORTHEAST...ALLOWING THE SUBTROPICAL RIDGE TO BUILD WESTWARD TO THE NORTH OF LISA...WHICH SHOULD NUDGE THE CYCLONE ON A SLOW WESTWARD OR WEST-NORTHWESTWARD TRACK. THE NHC MODEL GUIDANCE REMAINS WIDELY DIVERGENT BETWEEN THE NORTHWARD HWRF AND THE WESTWARD GFDN MODEL SOLUTIONS. THE OFFICIAL FORECAST TRACK IS SIMILAR THE PREVIOUS ADVISORY...AND REMAINS NORTH OF THE MODEL CONSENSUS AND BETWEEN THE GFS AND ECMWF TRACKS. THE VERTICAL SHEAR IS FORECAST TO REMAIN BELOW 10 KT FOR THE NEXT 48 HOURS...WHICH USUALLY FAVORS SIGNIFICANT INTENSIFICATION. HOWEVER...MID-LEVEL MOISTURE IS FORECAST TO DECREASE WITH HUMIDITY LEVELS BELOW 40 PERCENT AFTER 24 HOURS. DESPITE THE PRESENCE OF DRY AIR...JULIA IS FORECAST TO STRENGTHEN FAIRLY QUICKLY IN THE NEXT 24 HOURS DUE TO THE IMPROVED INNER-CORE STRUCTURE NOTED IN THE AFOREMENTIONED MICROWAVE IMAGERY. GIVEN THAT A 75-PERCENT EYEWALL FEATURE EXISTED 6 HRS AGO AND CLOUD TOPS HAVE RECENTLY COOLED TO BELOW -80C NEAR THE CENTER...A PERIOD OF RAPID INTENSIFICATION CAN NOT BE RULED OUT. THE ECMWF MODEL CONTINUES TO KEEP LISA SOUTH OF 20N LATITUDE AND BENEATH AN UPPER-LEVEL ANTICYCLONE. THE OFFICIAL INTENSITY FORECAST IS ABOVE THE OBJECTIVE INTENSITY GUIDANCE THROUGH 48 HOURS AND CONTINUES TO LEAN TOWARD THE STRONGER ECMWF SOLUTION. FORECAST POSITIONS AND MAX WINDS INITIAL 21/2100Z 18.3N 31.6W 40 KT 12HR VT 22/0600Z 18.6N 31.7W 50 KT 24HR VT 22/1800Z 19.0N 31.9W 60 KT 36HR VT 23/0600Z 19.3N 32.5W 65 KT 48HR VT 23/1800Z 19.7N 33.3W 65 KT 72HR VT 24/1800Z 20.4N 34.7W 55 KT 96HR VT 25/1800Z 21.0N 36.0W 45 KT 120HR VT 26/1800Z 21.0N 38.0W 40 KT $$ FORECASTER STEWART |
