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| #293005 (Received by flhurricane at: 10:49 AM 02.Sep.2009) TCDAT1 TROPICAL STORM ERIKA DISCUSSION NUMBER 4 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL062009 1100 AM AST WED SEP 02 2009 VISIBLE SATELLITE IMAGES AND AIR FORCE RESERVE HURRICANE HUNTER DATA SHOW THAT ERIKA IS A POORLY ORGANIZED SYSTEM. THERE ARE MULTIPLE SWIRLS APPARENT ON THE SATELLITE IMAGERY ROTATING IN THE LARGER CIRCULATION ENVELOPE WITH A MEAN CENTER TO THE WEST OF THE DEEP CONVECTION. THE SFMR EARLIER REPORTED WINDS OF ABOUT 35 KT...WHICH IS IN GOOD AGREEMENT WITH SATELLITE INTENSITY ESTIMATES...SO THAT VALUE WILL BE USED AS THE INITIAL INTENSITY. VERTICAL WIND SHEAR HAS INCREASED EARLIER THAN EXPECTED...WITH CIMSS DIAGNOSING ABOUT 15-20 KT OF SHEAR. AT FIRST GLANCE...THE SHEAR DOES NOT APPEAR TO BE TOO STRONG OVER THE SYSTEM WITH SOUTH OR SOUTHEASTERLY WINDS REPORTED AT 200 MB OVER THE NORTHERN LEEWARD ISLANDS. HOWEVER...UPPER-AIR DATA FROM GUADELOUPE AND ST. MAARTEN SUGGEST THAT THE SHEAR IS PRIMARILY COMING FROM SOUTHWESTERLY WINDS BETWEEN 300 MB-250 MB...AND THIS IS APPARENTLY UNDERCUTTING THE OUTFLOW LAYERS AT 200 MB AND ABOVE. MOST OF THE GLOBAL MODELS DO NOT SHOW MUCH RELAXATION OF THIS SHEAR...AND ACTUALLY SHOW IT INCREASING SIGNIFICANTLY IN A COUPLE OF DAYS. DESPITE ALL THIS SHEAR...ALL RELIABLE GUIDANCE RESTRENGTHENS THIS SYSTEM TO NEAR HURRICANE STRENGTH IN A FEW DAYS. THIS REINTENSIFICATION DOES NOT SEEM VERY LIKELY AND THE NHC FORECAST IS MUCH LOWER THAN THE GUIDANCE. GIVEN THE POSSIBLE SHEAR DURING THE FORECAST PERIOD...IT WOULD NOT BE SURPRISING IF ERIKA DISSIPATED. A 24-HOUR MOTION IS ABOUT 275/9. A WEST-NORTHWESTWARD TRACK IS PREDICTED TO RESUME AS THE STORM MOVES TO THE SOUTH OF A WEAK SUBTROPICAL RIDGE FOR A COUPLE OF DAYS. THIS RIDGE IS FORECAST TO WEAKEN SOMEWHAT IN A FEW DAYS...AND A NORTHWESTWARD MOTION IS POSSIBLE BY THE END OF THE FORECAST. THE OFFICIAL FORECAST HAS BEEN SHIFTED LEFT OF THE PREVIOUS ONE AND IS LEFT OF THE DYNAMICAL CONSENSUS DUE TO THE ANTICIPATION OF ERIKA REMAINING WEAK. IT SHOULD BE EMPHASIZED THAT...OVERALL...THIS IS A LOW-CONFIDENCE TRACK FORECAST. TROPICAL STORM WATCHES COULD BE REQUIRED FOR THE U.S. AND BRITISH VIRGIN ISLANDS AS WELL AS PUERTO RICO LATER TODAY. FORECAST POSITIONS AND MAX WINDS INITIAL 02/1500Z 16.5N 60.4W 35 KT 12HR VT 03/0000Z 16.9N 61.5W 35 KT 24HR VT 03/1200Z 17.4N 62.8W 40 KT 36HR VT 04/0000Z 18.0N 64.2W 45 KT 48HR VT 04/1200Z 18.6N 65.5W 40 KT 72HR VT 05/1200Z 20.0N 68.5W 35 KT 96HR VT 06/1200Z 21.5N 71.5W 30 KT 120HR VT 07/1200Z 23.5N 73.5W 30 KT $$ FORECASTER BLAKE/FRANKLIN |
