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| #152785 (Received by flhurricane at: 10:52 PM 27.Sep.2007) TCDAT2 TROPICAL STORM KAREN DISCUSSION NUMBER 13 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL122007 1100 PM EDT THU SEP 27 2007 A LARGE BURST OF DEEP CONVECTION FORMED NEAR THE CENTER OF KAREN TONIGHT. HOWEVER...THUNDERSTORMS ARE ALREADY DIMINISHING AND SOUTHWESTERLY WIND SHEAR CONTINUES TO HINDER ANY DEVELOPMENT. DVORAK SATELLITE ESTIMATES ARE A LITTLE HIGHER THAN EARLIER...BUT ARE STILL BELOW THE PREVIOUS INITIAL INTENSITY. WINDS WILL BE KEPT AT 50 KT FOR THIS ADVISORY. THE INITIAL MOTION ESTIMATE OF 295/12 IS A BLEND OF THE EXTRAPOLATED MOVEMENT FROM THE LAST AVAILABLE VISIBLE IMAGES AND A RECENT SSMIS MICROWAVE PASS. A SMALL RIGHTWARD BEND TO THE TRACK IS EXPECTED OVER THE NEXT COUPLE OF DAYS AS LOW TO MID-LEVEL RIDGING TO THE NORTH OF THE SYSTEM SLOWLY WEAKENS. THE RIDGE IS EXPECTED TO BE THE WEAKEST IN ABOUT 48-72 HOURS DUE TO A MIDDLE-LATITUDE TROUGH IN THE CENTRAL ATLANTIC AND COULD STEER THE STORM MORE TO THE NORTH-NORTHWEST AT THAT TIME. HOWEVER...THIS TROUGH IS NOT EXPECTED TO BE STRONG ENOUGH TO TURN THE STORM OUT TO SEA AND ALL MODELS BUILD RIDGING TO THE NORTH OF THE STORM...FORCING A WEST-NORTHWEST TURN AT THE END OF THE FORECAST PERIOD. THE OFFICIAL FORECAST IS SHIFTED WESTWARD WITH THIS PACKAGE IN BEST AGREEMENT WITH THE GFS/ECMWF MODELS AND IS ON THE LEFT SIDE OF THE GUIDANCE ENVELOPE. IF THE STORM WEAKENS MORE THAN EXPECTED...THIS FORECAST MAY HAVE TO BE ADJUSTED FARTHER TO THE WEST..ESPECIALLY IN THE SHORT-TERM. KAREN SHOULD BE IN AN ENVIRONMENT OF STRONG WESTERLY SHEAR CAUSED BY THE MID-OCEANIC TROUGH FOR AT LEAST THE NEXT 48-72 HOURS. THIS PATTERN FAVORS AT LEAST SLOW WEAKENING DURING THAT TIME. HOWEVER... IF THE STORM SURVIVES THE SHEAR...GLOBAL MODELS SUGGEST THAT AN OPPORTUNITY FOR RESTRENGTHENING COULD OCCUR IN THE LONG-RANGE AS THE TROUGH WEAKENS AND IS REPLACED BY RIDGING. RELIABLE INTENSITY GUIDANCE RESPONDS TO THIS CHANGE IN SHEAR AND SHOWS AN INCREASE IN STRENGTH BEYOND 3 DAYS. IN FACT...THE GFDL/HWRF MODELS ACTUALLY MAKE KAREN A HURRICANE IN 5 DAYS. DUE TO LARGE UNCERTAINTY AT THE LONGER-RANGES...THE OFFICIAL FORECAST WILL ONLY BE SLIGHTLY INCREASED...AND COULD BE CONSERVATIVE IF THE STORM ENDURES THE NEXT 48 HOURS OR SO. FORECAST POSITIONS AND MAX WINDS INITIAL 28/0300Z 14.8N 49.5W 50 KT 12HR VT 28/1200Z 15.7N 50.9W 45 KT 24HR VT 29/0000Z 16.9N 52.5W 45 KT 36HR VT 29/1200Z 18.0N 53.8W 40 KT 48HR VT 30/0000Z 19.2N 54.8W 40 KT 72HR VT 01/0000Z 21.0N 56.0W 40 KT 96HR VT 02/0000Z 22.5N 57.5W 45 KT 120HR VT 03/0000Z 23.5N 59.5W 50 KT $$ FORECASTER BLAKE |
