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| #105410 (Received by flhurricane at: 10:31 PM 21.Aug.2006) TCDAT4 TROPICAL DEPRESSION FOUR DISCUSSION NUMBER 2 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL042006 1100 PM EDT MON AUG 21 2006 TROPICAL DEPRESSION FOUR REMAINS A LARGE AND WELL-ORGANIZED SYSTEM...DESPITE A DECREASE IN THE INNER CORE CONVECTION. A 21/1914Z QUIKSCAT OVERPASS INDICATED THE DEPRESSION HAD A SURFACE WIND FIELD ENVELOPE AT LEAST 500 NMI IN DIAMETER...WITH NUMEROUS 25-30 KT UNCONTAMINATED WIND VECTORS LOCATED IN THE DRY SLOTS. BASED ON THIS INFORMATION...AND THE LACK OF INNER CORE CONVECTION ...THE INITIAL INTENSITY IS BEING MAINTAINED AT 30 KT. THE INITIAL MOTION ESTIMATE IS 285/13...BASED ON A 10-HOUR AVERAGE MOTION USING PASSIVE AND QUIKSCAT MICROWAVE SATELLITE DATA. THE AFOREMENTIONED QUIKSCAT OVERPASS INDICATED THE CENTER OF TD-4 WAS SOUTH OF 12N LATITUDE AT THAT TIME...PROBABLY DUE TO REFORMATION OF THE BROAD LOW-LEVEL CENTER CLOSER TO THE BAND OF DEEP CONVECTION IN THE SOUTHERN SEMICIRCLE. THE GLOBAL MODELS...AS WELL AS THE GFDL AND THE BAM MODELS...REMAIN TIGHTLY CLUSTERED AND IN EXCELLENT AGREEMENT ON AN IMMEDIATE NORTHWESTWARD MOTION. UNFORTUNATELY... THE MODELS HAVE ALSO BEEN EXCELLENTLY WRONG THUS FAR. TD-4 IS CURRENTLY AT LEAST 60 NMI SOUTHWEST OF THE 18Z GFS FORECAST POSITION FOR 22/00Z...AND THE OTHER MODELS ARE SIMILARLY TOO FAR TO THE NORTHEAST OF THE CURRENT POSITION. AS A RESULT...I AM NOT TOO ENTHRALLED WITH THE MODEL SOLUTIONS...AND WE MAY SEE THE CYCLONE TURN OUT TO BE MORE OF A WEST-RUNNER THAN A RECURVER THROUGH THE STRONG SUBTROPICAL RIDGE LOCATED TO THE NORTH. WATER VAPOR IMAGERY AS WELL AS WATER VAPOR-DERIVED WINDS AND UPPER-AIR DATA FROM THE CAPE VERDE ISLANDS SUGGEST THAT THE MID-LEVEL EASTERLY FLOW ON THE SOUTH SIDE OF THE RIDGE IS STRONGER THAN THE MODELS HAVE BEEN FORECASTING...AND THE RIDGE TO THE NORTH ALSO APPEARS TO BE BUILDING WESTWARD. THE FORECAST TRACK WAS SHIFTED TO THE WEST OF THE PREVIOUS ADVISORY...BUT NOT AS FAR WEST AS THE CLIPER MODEL AND STATISTICAL-DYNAMICAL MODELS OUT OF RESPECT FOR THE SKILL OF THE GLOBAL MODELS. HOWEVER...IF TD-4 DOES NOT MAKE IT AS FAR NORTH AS 20N LATITUDE WITHIN 4 DAYS...THEN THE CYCLONE WILL LIKELY MISS THE WEAKNESS IN THE RIDGE THAT ALL THE MODELS FORECAST TO DEVELOP. THE INTENSITY FORECAST WAS LOWERED SLIGHTLY IN THE FIRST 48 HOURS DUE TO THE LACK OF INNER-CORE CONVECTION AND WEAK TO MODERATE UPPER-LEVEL NORTHEASTERLY SHEAR FORECAST TO UNDERCUT THE OTHERWISE IMPRESSIVE AND EXPANDING OUTFLOW PATTERN. AFTER THAT...GRADUAL STRENGTHENING IS EXPECTED AS THE SHEAR DECREASES TO 5-10 KT AND THE CYCLONE IS FORECAST TO BE OVER WARMER WATERS. DUE TO THE LARGE WIND FIELD DEPICTED IN THE EARLIER QUIKSCAT OVERPASS...12-FT SEA HEIGHT RADII WERE ADDED TO THE FORECAST. FORECAST POSITIONS AND MAX WINDS INITIAL 22/0300Z 12.2N 23.2W 30 KT 12HR VT 22/1200Z 12.9N 25.1W 35 KT 24HR VT 23/0000Z 14.1N 27.7W 40 KT 36HR VT 23/1200Z 15.9N 30.6W 45 KT 48HR VT 24/0000Z 17.5N 33.4W 50 KT 72HR VT 25/0000Z 20.5N 38.4W 60 KT 96HR VT 26/0000Z 24.0N 44.5W 65 KT 120HR VT 27/0000Z 27.0N 51.0W 65 KT |
