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| #105922 (Received by flhurricane at: 5:00 PM 24.Aug.2006) TCDAT5 TROPICAL DEPRESSION FIVE DISCUSSION NUMBER 1 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL052006 500 PM EDT THU AUG 24 2006 AN AIR FORCE RECONNAISSANCE AIRCRAFT WORKED LONG AND HARD THIS AFTERNOON TO CLOSE OFF THE TROPICAL WAVE MOVING THROUGH THE WINDWARD ISLANDS...AND FOUND ENOUGH NORTHWESTERLY WINDS LATE THIS AFTERNOON TO CONSIDER THE SYSTEM A TROPICAL CYCLONE. THE AIRCRAFT JUST PASSED THROUGH THE CENTER AND REPORTED PEAK FLIGHT-LEVEL WINDS OF 39 KT...WHICH CORRESPONDS TO ABOUT 30 KT AT THE SURFACE. ALTHOUGH BARBADOS REPORTED SUSTAINED WINDS OF 33 KT EARLIER THIS AFTERNOON...THE DIRECTION OF THOSE WINDS SUGGESTED A CONVECTIVE OUTFLOW OR DOWNBURST NOT REPRESENTATIVE OF THE CYCLONE CIRCULATION. AIRCRAFT OBSERVATIONS IN THE BANDS WELL TO THE EAST OF THE CENTER INDICATE WINDS THERE ARE ALSO BELOW TROPICAL STORM STRENGTH. WITH THE CENTER ALREADY IN THE CARIBBEAN AND NOW MOVING AWAY FROM THE WINDWARD ISLANDS...NO WARNINGS ARE BEING ISSUED. HOWEVER... INTERESTS THERE SHOULD EXERCISE CAUTION UNTIL WINDS AND SEAS SUBSIDE. THE INITIAL MOTION IS 280/19. A LOW TO MID-LEVEL RIDGE OF HIGH PRESSURE IN THE SOUTHWESTERN ATLANTIC IS CURRENTLY PROVIDING A WESTWARD STEERING FLOW...AND THIS RIDGE SHOULD REMAIN ROUGHLY IN PLACE OVER THE NEXT FEW DAYS. THE OTHER KEY PLAYER IS AN UPPER-LEVEL TROUGH IN THE WESTERN CARIBBEAN. AS THE CYCLONE MOVES INTO THE CENTRAL CARIBBEAN...THIS TROUGH WILL BE PROVIDING BOTH WESTERLY SHEAR AND AND A NORTHWARD STEERING COMPONENT. THE GFDL RESPONDS TO THIS BY GRADUALLY LIFTING THE TRACK AROUND THE UPPER LOW AND TOWARD THE GREATER ANTILLES. THE GLOBAL MODELS ARE FOR THE MOST PART FARTHER SOUTH AND KEEP THE SYSTEM WEAKER IN RESPONSE TO THE SHEAR. I AM REMINDED THAT THE GLOBAL MODELS CORRECTLY FORECAST THE DISSIPATION OF CHRIS A WHILE BACK...AND THE GFDL DOESN'T ALWAYS HANDLE SHEAR WELL. THE OFFICIAL FORECAST LEANS TOWARD THE SOUTHERN OPTION. WATER VAPOR IMAGERY SHOWS BOTH A DRY ENVIRONMENT AS WELL AS SOME WESTERLY SHEAR AHEAD OF THE CYCLONE IN THE CARIBBEAN. HOWEVER...THE GFS RETROGRADES THE UPPER LOW AND IF THIS OCCURS WOULD RESULT IN A MORE FAVORABLE ENVIRONMENT FOR STRENGTHENING. THE SHIPS MODEL...WHICH IS BASED ON THE GFS FIELDS...RESPONDS TO THIS AND TAKES THE CYCLONE UP TO HURRICANE STRENGTH IN 60 HOURS. THE GFDL INTENSITIES ARE LOWER...BUT THIS RESULTS FROM A GFDL TRACK WHICH IS OVER LAND. THE OFFICIAL FORECAST TEMPERS THE SHIPS GUIDANCE WITH THE MINIMAL DEVELOPMENT SHOWN BY THE GLOBAL MODELS. FORECAST POSITIONS AND MAX WINDS INITIAL 24/2100Z 12.9N 62.4W 30 KT 12HR VT 25/0600Z 13.2N 65.5W 35 KT 24HR VT 25/1800Z 13.8N 68.0W 40 KT 36HR VT 26/0600Z 14.6N 70.2W 45 KT 48HR VT 26/1800Z 15.5N 72.5W 50 KT 72HR VT 27/1800Z 17.0N 76.5W 55 KT 96HR VT 28/1800Z 18.5N 80.5W 60 KT 120HR VT 29/1800Z 20.5N 85.0W 60 KT |
