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| #378670 (Received by flhurricane at: 10:47 AM 25.Sep.2010) TCDAT4 TROPICAL STORM LISA DISCUSSION NUMBER 20 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL142010 1100 AM AST SAT SEP 25 2010 ALTHOUGH OUTER BANDING FEATURES IN THE NORTHERN AND WESTERN QUADRANTS HAVE ACTUALLY IMPROVED SINCE THE LAST ADVISORY... SATELLITE IMAGERY INDICATES THE CENTRAL DEEP CONVECTION HAS ERODED SOME DUE TO MODERATE WESTERLY SHEAR IMPINGING ON LISA. THE INITIAL INTENSITY OF 60 KT IS BASED ON A BLEND OF SATELLITE INTENSITY ESTIMATES T4.5/77 KT FROM TAFB... T2.4/35 KT FROM SAB...AND AN ADT VALUE OF T3.8/61 KT FROM UW-CIMSS....AND THE CYCLONE HAS BEEN DOWNGRADED TO TROPICAL STORM STATUS AGAIN. LISA IS ON THE PREVIOUS FORECAST TRACK AND THE INITIAL MOTION ESTIMATE IS NOW 350/10. THROUGHOUT THE FORECAST PERIOD...LISA IS EXPECTED TO MOVE NORTH TO NORTH-NORTHWESTWARD AROUND THE WESTERN PERIPHERY OF THE BROAD SUBTROPICAL RIDGE SITUATED OVER NORTHWESTERN AFRICA. ALL OF THE GLOBAL AND REGIONAL MODELS ARE IN MUCH BETTER AGREEMENT ON THIS SCENARIO THAN IN PREVIOUS MODEL RUNS...ASSUMING THAT LISA MAINTAINS VERTICAL COHERENCE THROUGH AT LEAST 72 HOURS. THE OFFICIAL FORECAST TRACK IS SIMILAR TO THE PREVIOUS ADVISORY AND IS CLOSE TO THE CONSENSUS MODEL...TVCN. WESTERLY TO SOUTHWESTERLY VERTICAL SHEAR IS FORECAST TO INCREASE TO ABOVE 25 KT BY 24 HOURS AND CONTINUE INCREASE TO NEAR 35 KT BY 36-48 HOURS. THIS UNFAVORABLE UPPER-LEVEL FLOW PATTERN SHOULD CONTINUE THE CURRENT WEAKENING TREND. ALTHOUGH LISA IS PRESENTLY LOCATED OVER RELATIVELY COOL SSTS OF 25.5C...THE CYCLONE IS NOT FORECAST TO MOVE OVER WATER ANY COOLER THAN THOSE CURRENT OCEAN TEMPERATURES. HOWEVER...THE GFS-BASED SHIPS MODEL INDICATES THAT UPPER-LEVEL TEMPERATURES ARE FORECAST TO COOL BY AT LEAST 3 DEGREES CELSIUS...WHICH WOULD RESULT IN GREATER INSTABILITY AHEAD OF LISA THAN WHAT THE CYCLONE IS CURRENTLY INGESTING. THE RESULT IS THAT DEEP CONVECTION COULD PERSIST NEAR THE CENTER...ALBEIT SHEARED TO THE EAST...AND PREVENT LISA FROM BECOMING A NON-CONVECTIVE REMNANT LOW PRESSURE SYSTEM AS EARLY AS INDICATED BY THE OFFICIAL FORECAST. FORECAST POSITIONS AND MAX WINDS INITIAL 25/1500Z 22.7N 28.2W 60 KT 12HR VT 26/0000Z 23.9N 28.4W 50 KT 24HR VT 26/1200Z 25.3N 28.6W 45 KT 36HR VT 27/0000Z 26.6N 28.8W 40 KT 48HR VT 27/1200Z 27.9N 29.1W 35 KT 72HR VT 28/1200Z 30.1N 29.6W 30 KT...POST-TROP/REMNT LOW 96HR VT 29/1200Z 32.0N 30.0W 25 KT...POST-TROP/REMNT LOW 120HR VT 30/1200Z...DISSIPATED $$ FORECASTER STEWART |
