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| #219473 (Received by flhurricane at: 10:37 AM 30.Sep.2008) TCDAT2 TROPICAL STORM LAURA DISCUSSION NUMBER 6 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL122008 1100 AM EDT TUE SEP 30 2008 WHILE SOMEWHAT OF A PARADOX...LAURA CAN NOW BE DESIGNATED AS A TROPICAL CYCLONE DESPITE THE FACT THAT IT CONTINUES TO MOVE NORTHWARD OVER PROGRESSIVELY COLDER WATERS. THE ONE ASPECT WHICH WAS HOLDING US BACK FROM DECLARING IT TROPICAL WAS THE UPPER LOW COLLOCATED WITH THE SYSTEM. WATER VAPOR IMAGERY INDICATES THAT THIS FEATURE HAS BECOME OFFSET AND STRETCHED TO THE SOUTHWEST OF LAURA...AND UPPER LEVEL OUTFLOW IS NOW EXPANDING TO THE NORTH OF THE CYCLONE. IN ADDITION...SCATTEROMETER DATA INDICATE THAT THE RADIUS OF MAXIMUM WINDS HAS CONTINUED TO CONTRACT TO ABOUT 60 NM. CLOUD TOP TEMPERATURES ARE NOT ALL THAT COLD...BUT THAT IS QUITE NORMAL FOR HIGH-LATITUDE TROPICAL CYCLONES DUE TO THE LOWER TROPOPAUSE. FRANKLY...LAURA NOW LOOKS LIKE A TROPICAL STORM. THE INITIAL INTENSITY IS HELD AT 50 KT BASED ON AN 0827 UTC QUIKSCAT PASS ALTHOUGH THE AREA OF STRONGEST WINDS HAS BEEN SHRINKING. LAURA SHOULD BE ABLE TO HOLD THAT INTENSITY FOR ONLY ANOTHER 12 HOURS OR SO BEFORE MOVING OVER SEA SURFACE TEMPERATURES LESS THAN 18C. GLOBAL MODEL FIELDS INDICATE THAT THERMAL ADVECTION AROUND THE SYSTEM BECOMES MORE PROMINENT LATER TONIGHT...AND LAURA IS EXPECTED TO BE EXTRATROPICAL IN 24 HOURS OR SO AS IT MIGRATES INTO THE COLDER AIR MASS. SOME GRADUAL WEAKENING THROUGH 48 HOURS IS EXPECTED DUE TO THE COLDER OCEAN WATERS AND MARGINAL BAROCLINIC FORCING. THEREAFTER...SOME RE-STRENGTHENING AND AN EXPANSION OF THE WIND FIELD COULD OCCUR AS THE EXTRATROPICAL LOW RACES EAST WITHIN THE MID-LATITUDE WESTERLIES TOWARDS THE NORTHERN BRITISH ISLES. EXCEPT FOR A SLIGHT WESTWARD SHIFT IN THE TRACK GUIDANCE AT 36-48 HOURS...THERE HAS BEEN NO SIGNIFICANT CHANGE IN THE GLOBAL MODELS FOR THIS CYCLE AND THE OFFICIAL FORECAST LIES VERY CLOSE TO THE MODEL CONSENSUS. FORECAST POSITIONS AND MAX WINDS INITIAL 30/1500Z 41.2N 48.8W 50 KT 12HR VT 01/0000Z 42.8N 48.4W 50 KT 24HR VT 01/1200Z 45.5N 47.4W 45 KT...EXTRATROPICAL 36HR VT 02/0000Z 48.7N 45.6W 40 KT...EXTRATROPICAL 48HR VT 02/1200Z 51.7N 43.6W 40 KT...EXTRATROPICAL 72HR VT 03/1200Z 56.0N 35.0W 45 KT...EXTRATROPICAL 96HR VT 04/1200Z 57.0N 19.0W 50 KT...EXTRATROPICAL 120HR VT 05/1200Z 58.0N 3.5W 50 KT...EXTRATROPICAL $$ FORECASTER BERG/PASCH |
