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| #287745 (Received by flhurricane at: 4:37 PM 12.Aug.2009) TCDAT2 TROPICAL DEPRESSION TWO DISCUSSION NUMBER 7 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL022009 500 PM AST WED AUG 12 2009 THE DEPRESSION APPEARED TO BE ON THE VERGE OF BECOMING A TROPICAL STORM A FEW HOURS AGO...WITH SATELLITE INTENSITY ESTIMATES OF 35 KT FROM TAFB AND SAB. HOWEVER...SINCE ABOUT 18Z THE CONVECTION HAS DECREASED SIGNIFICANTLY AND HAS MOVED FARTHER AWAY FROM THE EXPOSED LOW-LEVEL CENTER. BASED ON THIS...THE CYCLONE WILL REMAIN A 30 KT DEPRESSION FOR THIS ADVISORY. THE CURRENT DOWNWARD TREND IN THE CONVECTION MAY BE DUE TO AN INCREASE IN EASTERLY VERTICAL WIND SHEAR...AND TO A DRY SLOT WRAPPING AROUND THE CIRCULATION NOTED IN AMSR-E VAPOR IMAGERY NEAR 1534Z. ALSO...AS SEEN EARLIER...MOST OF THE LOW-LEVEL INFLOW INTO THE DEPRESSION SEEMS TO BE COMING FROM THE NORTH...REPRESENTING POSSIBLY MORE STABLE AIR PASSING OVER COOLER WATERS. THE FLOW ON THE SOUTH SIDE OF THE DEPRESSION IS FEEDING INTO A LARGE DISTURBANCE BETWEEN THE CAPE VERDE ISLANDS AND AFRICA. THE INITIAL MOTION IS 265/12. THE DEPRESSION IS ON THE SOUTH SIDE OF A LARGE LOW/MID-LEVEL RIDGE...AND THE LARGE-SCALE MODELS FORECAST THIS TO CONTINUE FOR THE NEXT TWO DAYS OR SO. AFTER THAT TIME...THE MODELS FORECAST A WEAKNESS IN THE RIDGE THAT WOULD STEER THE CYCLONE WEST-NORTHWESTWARD. THE TRACK MODEL GUIDANCE GENERALLY AGREES WITH THIS SCENARIO...AND WHILE THE MODEL CLUSTERS SEEN EARLIER HAVE MERGED THERE IS STILL A SIGNIFICANT SPREAD BY 120 HR. THE GFS AND UKMET ARE ON THE SOUTH SIDE OF THE GUIDANCE ENVELOPE SHOWING A MORE WESTERLY MOTION...WHILE THE HWRF AND BAMD ARE ON THE NORTH SIDE SHOWING A NORTHWESTWARD MOTION BY 120 HR. OVERALL...THE GUIDANCE ENVELOPE HAS AGAIN SHIFTED A LITTLE SOUTHWARD...AND SO HAS THE NEW OFFICIAL FORECAST. THE NEW TRACK IS IN THE CENTER OF THE GUIDANCE ENVELOPE IN BEST AGREEMENT WITH THE TVCN AND TVCC CONSENSUS MODELS. THE DEPRESSION REMAINS OVER SEA SURFACE TEMPERATURES NEAR 27C...WITH SSTS ALONG THE FORECAST TRACK INCREASING TO 28C AFTER 72 HR. THE LARGE-SCALE MODELS FORECAST THE DEPRESSION TO ENCOUNTER MODERATE VERTICAL WIND SHEAR DURING THE FORECAST PERIOD...FIRST FROM THE EAST AND LATER FROM THE SOUTHWEST. ALSO...WATER VAPOR IMAGERY SHOWS VERY DRY AIR AT THE MID/UPPER-LEVELS TO THE NORTHWEST OF THE DEPRESSION. THE LATTER TWO FACTORS SUGGEST THE SYSTEM MAY BE SLOW TO STRENGTHEN...AND NONE OF THE INTENSITY GUIDANCE CURRENTLY FORECASTS AN INTENSITY GREATER THAN 55 KT DURING THE FORECAST PERIOD. BASED ON THIS...THE NEW INTENSITY FORECAST IS AN UPDATE OF THE PREVIOUS FORECAST IN BEST AGREEMENT WITH THE SHIPS MODEL. IT IS POSSIBLE THAT THE SYSTEM COULD WEAKEN TO A TROPICAL WAVE DURING THE FORECAST PERIOD...AS FORECAST BY THE ECMWF...UKMET...AND GFS MODELS. FORECAST POSITIONS AND MAX WINDS INITIAL 12/2100Z 14.3N 35.0W 30 KT 12HR VT 13/0600Z 14.1N 36.7W 35 KT 24HR VT 13/1800Z 14.1N 38.9W 35 KT 36HR VT 14/0600Z 14.2N 41.3W 40 KT 48HR VT 14/1800Z 14.6N 43.7W 45 KT 72HR VT 15/1800Z 16.0N 49.5W 50 KT 96HR VT 16/1800Z 19.0N 56.0W 50 KT 120HR VT 17/1800Z 23.0N 62.0W 45 KT $$ FORECASTER BEVEN |
