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| #34525 (Received by flhurricane at: 6:20 PM 21.Jul.2005) TCDAT1 TROPICAL DEPRESSION SIX DISCUSSION NUMBER 1 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 5 PM EDT THU JUL 21 2005 AIR FORCE RECON AIRCRAFT HAS BEEN INVESTIGATING THE STRONG TROPICAL WAVE JUST EAST OF THE BAHAMAS AND HAVE FOUND WINDS THAT SUPPORT AT LEAST A 30-KT TROPICAL DEPRESSION...AND ADVISORIES HAVE BEEN INITIATED ON TD-6. CONVECTION HAS DECREASED SOMEWHAT NEAR THE CENTER OVER THE PAST COUPLE OF HOURS...BUT THIS IS TYPICAL FOR THIS TIME OF THE DAY. UPPER-LEVEL OUTFLOW HAS CONTINUED TO IMPROVE DURING THE DAY...INCLUDING THE RESTRICTED SOUTHWEST QUADRANT. THE INITIAL MOTION ESTIMATE IS 300/11...BASED ON THE PAST 3 HOURS OF SATELLITE POSITIONS. THE FORECAST TRACK IS SOMEWHAT PROBLEMATIC. THE GLOBAL MODELS TAKE THE SURFACE TO 850 MB CIRCULATION NORTHWARD AND THEN RECURVE IT NORTHEASTWARD THROUGH THE SUBTROPICAL RIDGE. HOWEVER...ALL OF THE MODELS GENERALLY AGREE THAT THE MID-LEVEL/500 MB CIRCULATION WILL MOVE INTO THE RIDGE AXIS AND STALL IN ABOUT 48 HOURS AS A SHORTWAVE TROUGH MOVES NORTHEAST OF THE DEPRESSION AND ALLOWS THE RIDGE TO BUILD BACK IN TO THE WEST. AFTER THAT...SOME OF THE MODELS MEANDER THE SYSTEM SLOWLY EAST OR SOUTHWARD...WHILE THE GFS AND GFDL MODELS TAKE THE MID-LEVEL CIRCULATION SOUTHWESTWARD TOWARD FLORIDA. THE MEDIUM AND DEEP BAM MODELS ACTUALLY TAKE THE DEPRESSION SLOWLY WESTWARD AFTER 48 HOURS ACROSS CENTRAL FLORIDA. GIVEN THE LARGE SPREAD IN THE MODEL GUIDANCE...WHICH TYPICALLY SUGGESTS SLOW MOTION...THE FORECAST TRACK SHOWS A GRADUAL TURN TO THE NORTH AFTER 24 HOURS AND THEN TURNS THE CYCLONE SLOWLY EASTWARD AND MAKING A SMALL CLOCKWISE LOOP WELL EAST OF THE CENTRAL FLORIDA COAST. THE WATER AHEAD OF THE DEPRESSION IS QUITE WARM...AROUND 29C...AND THE SHEAR IS FORECAST REMAIN LOW WITH A FAVORABLE OUTFLOW PATTERN FOR THE NEXT 36 TO 48 HOURS. THEREFORE...THE DEPRESSION COULD REACH HURRICANE STRENGTH WITHIN 48 TO 72 HOURS. NOTE...THE AIRCRAFT HAS JUST REPORTED AT 2114Z FLIGHT-LEVEL WINDS OF 48 KT...WHICH WOULD NORMALLY SUPPORT UPGRADING THE DEPRESSION TO A TROPICAL STORM. HOWEVER...CONVECTION HAS BEEN WEAKENING AND WE WOULD PREFER TO SEE DEEP CONVECTION PERSIST BEFORE UPGRADING THIS SYSTEM TO A TROPICAL STORM. FORECASTER STEWART FORECAST POSITIONS AND MAX WINDS INITIAL 21/2100Z 25.3N 75.4W 30 KT 12HR VT 22/0600Z 26.3N 76.8W 35 KT 24HR VT 22/1800Z 27.5N 77.5W 40 KT 36HR VT 23/0600Z 28.5N 77.6W 45 KT 48HR VT 23/1800Z 29.0N 77.0W 50 KT 72HR VT 24/1800Z 29.0N 76.5W 60 KT 96HR VT 25/1800Z 28.5N 76.0W 60 KT 120HR VT 26/1800Z 28.0N 76.5W 60 KT |
