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| #145749 (Received by flhurricane at: 5:14 PM 13.Aug.2007) TCDAT4 TROPICAL DEPRESSION FOUR DISCUSSION NUMBER 2 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL042007 500 PM EDT MON AUG 13 2007 THE DEPRESSION IS MAINTAINING ITSELF IN THE FACE OF EASTERLY WIND SHEAR OF ABOUT 15 KT. DEEP CONVECTION HAS BEEN PULSATING TODAY AND HAS REMAINED DISPLACED TO THE WEST OF THE LOW-LEVEL CENTER. METEOSAT-9 HIGH-RESOLUTION VISIBLE IMAGERY CLEARLY REVEALED THE EXPOSED LOW-LEVEL CENTER EARLIER THIS AFTERNOON...BUT MORE RECENTLY THE CONVECTION HAS INCREASED AND PARTIALLY OBSCURED THE CENTER. DVORAK INTENSITY ESTIMATES ARE UNCHANGED AND THE ADVISORY INTENSITY REMAINS 30 KT. TD FOUR IS STILL RACING ALMOST DUE WESTWARD...WITH AN INITIAL MOTION ESTIMATE OF 265/17. A CONTINUED BRISK AND GENERALLY WESTWARD MOTION SHOULD CONTINUE DURING THE NEXT COUPLE OF DAYS WHILE THE CYCLONE MOVES ALONG THE SOUTHERN PERIPHERY OF A DEEP LAYER RIDGE. THE OFFICIAL FORECAST IS AHEAD OF NEARLY ALL OF THE GUIDANCE DURING THAT PERIOD...AS THE MODELS DO NOT SEEM TO HAVE A GOOD HANDLE ON THE FAST INITIAL SPEED. BEYOND 48 HOURS...A TURN TOWARD THE WEST-NORTHWEST IS FORECAST BY ALL OF THE MODELS. SINCE THIS MORNING...ALL OF THE TRACK MODEL GUIDANCE HAS SLOWED DOWN AND SHIFTED TO THE RIGHT...IN RESPONSE TO FORECASTING A SLIGHTLY WEAKER SUBTROPICAL RIDGE OVER THE WESTERN ATLANTIC LATER IN THE FORECAST PERIOD. THE NEW OFFICIAL TRACK HAS BEEN ADJUSTED TOWARD THE CONSENSUS OF THE LATEST MODELS...BUT REMAINS A LITTLE FASTER AND TO THE WEST OF THE GUIDANCE ENVELOPE...SINCE THE GUIDANCE COULD SHIFT BACK THE OTHER WAY LATER. THE SHEAR IS FORECAST TO ABATE WHILE THE SEA-SURFACE TEMPERATURES WILL INCREASE ALONG THE OFFICIAL TRACK DURING THE NEXT 2-3 DAYS. THEREAFTER...MOST OF THE GLOBAL MODELS FORECAST THE TROPICAL CYCLONE TO FIND ITSELF BENEATH AN UPPER-LEVEL RIDGE...AN ENVIRONMENT THAT WOULD FAVOR STRENGTHENING. A NOTABLE EXCEPTION IS THE ECMWF...WHICH FORECASTS A STRONGER MID-OCEANIC TROUGH OVER THE CENTRAL ATLANTIC IN 4-5 DAYS WHICH COULD CAUSE AN INCREASE IN THE SHEAR. THAT MODEL...HOWEVER...DOES NOT REPRESENT THE TROPICAL CYCLONE WELL...AND IT MIGHT NOT HAVE GOOD HANDLE ON HOW THE CYCLONE COULD MODIFY THE UPPER-LEVEL FLOW. THE SHIPS GUIDANCE FORECASTS 63 KT AT 72 HOURS AND 75 KT BY FIVE DAYS...WHILE THE LGEM REACHES 84 KT AT 120 HOURS. MEANWHILE THE GFDL AND HWRF FORECAST 90 AND 84 KT...RESPECTIVELY...AT FIVE DAYS. CONSIDERING THIS GUIDANCE...THE NEW OFFICIAL INTENSITY FORECAST IN UNCHANGED FROM THIS MORNING BUT RETAINS A SIGNIFICANT AMOUNT OF UNCERTAINTY...ESPECIALLY AT THE LONGER RANGES. FORECAST POSITIONS AND MAX WINDS INITIAL 13/2100Z 11.9N 33.1W 30 KT 12HR VT 14/0600Z 11.8N 35.5W 35 KT 24HR VT 14/1800Z 11.8N 38.5W 40 KT 36HR VT 15/0600Z 11.8N 41.6W 45 KT 48HR VT 15/1800Z 12.1N 44.7W 50 KT 72HR VT 16/1800Z 13.0N 50.5W 60 KT 96HR VT 17/1800Z 15.0N 56.0W 75 KT 120HR VT 18/1800Z 17.0N 61.0W 90 KT $$ FORECASTER KNABB |
