000 WTPZ44 KNHC 242037 TCDEP4 TROPICAL STORM FRANK DISCUSSION NUMBER 13 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL EP092010 200 PM PDT TUE AUG 24 2010 FRANK HAS CONTINUED TO GET BETTER ORGANIZED TODAY AS INDICATED BY IMPROVED BANDING FEATURES TO THE WEST AND EAST OF THE CENTER. THE SMALL CENTRAL DENSE OVERCAST FEATURE NOTED EARLIER TODAY HAS ERODED SOMEWHAT DURING THE PAST COUPLE OF HOURS. HOWEVER...THE CENTER REMAINS EMBEDDED IN THE CONVECTION AT LEAST 35 NMI IN FROM THE EASTERN EDGE OF THE CONVECTIVE CLOUD MASS. SATELLITE INTENSITY ESTIMATES WERE T4.0/65 KT FROM TAFB AND T3.5/55 KT FROM SAB. NHC 3-HR AVERAGE ADT VALUES HAVE BEEN RUNNING BETWEEN T3.8/61 KT AND T3.7/59 KT. PEAK ADT VALUES WERE T3.9/63 KT BETWEEN 15-16Z. BASED ON THIS INFORMATION...THE INTENSITY IS NUDGED UPWARD TO 60 KT. A 1554Z ASCAT PASS WAS USED TO DEFINE THE 50- AND 34-KT WIND RADII. THE INITIAL MOTION IS 285/07. THE SUBTROPICAL RIDGE TO THE NORTH OF FRANK IS FORECAST TO KEEP THE CYCLONE MOVING WEST-NORTHWESTWARD THROUGHOUT THE FORECAST PERIOD...WITH A GRADUAL DECREASE IN FORWARD SPEED EXPECTED BY DAYS 4 AND 5 AS A MID-LEVEL TROUGH DIGS SOUTHWARD AND WEAKENS THE WESTERN PERIPHERY OF THE RIDGE. THE GFDL MODEL... WHICH HAD BEEN THE RIGHT-MOST OUTLIER MODEL WITH A TRACK FAIRLY CLOSE TO MEXICO HAS SHIFTED WESTWARD AND IS NOW BACK IN THE FOLD AS ONE OF THE LEFT-MOST MODELS IN THE GUIDANCE SUITE. THE OFFICIAL FORECAST TRACK IS JUST AN UPDATE OF THE PREVIOUS ADVISORY TRACK AND REMAINS BETWEEN THE CONSENSUS MODELS TVCN AND TVCC. A SLIGHT INCREASE IN VERTICAL SHEAR COMBINED WITH SOME DRY AIR ENTRAINMENT IN THE EASTERN SEMICIRCLE APPEARS TO HAVE WEAKENED THE CENTRAL CONVECTION DURING THE PAST FEW HOURS. HOWEVER...MODEST CONVECTION THAT IS DEVELOPING IN THE DRY SLOT SUGGESTS THAT THE EFFECTS OF THE DRY AIR SHOULD BE SHORT-LIVED. FRANK IS ALSO MOVING INTO A REGION OF LOWER VERTICAL SHEAR BASED ON UW-CIMSS SHEAR ANALYSES AND WATER VAPOR-DERIVED WINDS. THE GFS AND ECMWF MODELS CONTINUE TO FORECAST FRANK TO DEVELOP AN UPPER-LEVEL ANTICYCLONE OVER THE SYSTEM BY 36 HOURS...WHICH WOULD EASILY SUPPORT FORMATION OF A HURRICANE...AND THIS IS REFLECTED IN THE OFFICIAL FORECAST. EVEN THE GFDL MODEL BRINGS FRANK UP TO 74 KT BY 36 HOURS. BY DAY 4...FRANK SHOULD BE MOVING OVER SUB-26C SSTS...WHICH SHOULD CAUSE GRADUAL WEAKENING TO ENSUE. THE OFFICIAL INTENSITY FORECAST IS SIMILAR TO THE SHIPS AND LGEM INTENSITY MODELS THROUGH 24 HOURS... AND HIGHER THAN THOSE MODELS AFTER THAT BASED ON THE FAVORABLE UPPER-LEVEL OUTFLOW PATTERN FORECAST TO DEVELOP OVER THE CYCLONE. FORECAST POSITIONS AND MAX WINDS INITIAL 24/2100Z 15.6N 102.1W 60 KT 12HR VT 25/0600Z 16.1N 103.4W 60 KT 24HR VT 25/1800Z 16.8N 105.1W 65 KT 36HR VT 26/0600Z 17.4N 106.7W 70 KT 48HR VT 26/1800Z 18.1N 108.5W 75 KT 72HR VT 27/1800Z 19.3N 111.2W 75 KT 96HR VT 28/1800Z 20.2N 112.6W 65 KT 120HR VT 29/1800Z 21.5N 113.5W 55 KT $$ FORECASTER STEWART