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| #454577 (Received by flhurricane at: 4:52 AM 29.Aug.2011) TCDAT2 TROPICAL DEPRESSION TWELVE DISCUSSION NUMBER 1 NWS NATIONAL HURRICANE CENTER MIAMI FL AL122011 500 AM AST MON AUG 29 2011 THE AREA OF LOW PRESSURE IN THE FAR EASTERN ATLANTIC TO THE SOUTH OF THE CAPE VERDE ISLANDS HAS GAINED SUFFICIENT CONVECTIVE ORGANIZATION TO BE CLASSIFIED AS A TROPICAL DEPRESSION. A CURVED CONVECTIVE BAND WRAPS AROUND THE WESTERN AND SOUTHERN SIDE OF THE SYSTEM AND THE INITIAL INTENSITY IS SET TO 30 KT BASED ON THE 06Z DVORAK CLASSIFICATION FROM TAFB. THE DEPRESSION IS CURRENTLY IN AN EASTERLY SHEAR ENVIRONMENT SITUATED TO THE SOUTH OF AN UPPER-LEVEL RIDGE OVER THE EASTERN ATLANTIC. SOME EASTERLY SHEAR IS EXPECTED TO PERSIST...BUT OTHERWISE THE ENVIRONMENT SHOULD BE CONDUCIVE FOR STRENGTHENING THROUGH THE PERIOD. THE OFFICIAL INTENSITY FORECAST SHOWS SLOW STRENGTHENING IN GOOD AGREEMENT WITH THE SHIPS...GFDL... AND HWRF MODELS THROUGH 48 HOURS. AFTER THAT TIME THE CYCLONE SHOULD STRENGTHEN A LITTLE MORE QUICKLY AS IT MOVES OVER PROGRESSIVELY HIGHER OCEAN HEAT CONTENT. AT DAYS 3 THROUGH 5 THE NHC FORECAST IS CLOSEST TO BUT A LITTLE BELOW THE HWRF MODEL. THE SHIPS FORECAST LATE IN THE PERIOD APPEARS TOO LOW DUE TO STRONG SOUTHERLY SHEAR IMPARTED BY THE TROPICAL CYCLONE VORTEX IN THE GFS MODEL FIELDS... WHICH IS DISPLACED TO THE SOUTH AND EAST OF THE OFFICIAL TRACK. THE INITIAL MOTION ESTIMATE IS TOWARD THE WEST OR 270 DEGREES AT 13 KNOTS. THE DEPRESSION IS CURRENTLY SITUATED TO THE SOUTH OF THE SUBTROPICAL RIDGE...BUT THE WESTERN EDGE OF THE RIDGE WILL WEAKEN OVER THE NEXT DAY OR TWO AS A MID/UPPER-LEVEL LOW CUTS OFF AND RETROGRADES ACROSS THE CENTRAL ATLANTIC. THIS WILL ALLOW THE DEPRESSION TO TURN TOWARD THE WEST-NORTHWEST DURING THE NEXT DAY OR SO. AFTER 48 HOURS...THE MID/UPPER-LEVEL-LOW WEAKENS AND THE RIDGE RE-STRENGTHENS TO THE NORTHEAST OF THE CYCLONE...WHICH SHOULD RESULT IN A CONTINUED WEST-NORTHWESTWARD MOTION THROUGH THE END OF THE PERIOD. THE OFFICIAL NHC TRACK FORECAST IS SOUTH OF AND FASTER THAN MOST OF THE GUIDANCE FOR THE FIRST 48 HOURS...AS MOST OF THE DYNAMICAL MODELS SHOW A MORE POLEWARD AND SLOWER MOTION THAN IS CURRENTLY OBSERVED. DURING THIS TIME THE NHC TRACK IS BETWEEN THE DYNAMICAL MODEL CONSENSUS AND THE LBAR...WHICH TYPICALLY PERFORMS WELL IN THE DEEP TROPICS. AT 72 HOURS AND BEYOND THE NHC FORECAST IS CLOSE TO A BLEND OF THE HWRF...GFDL...AND ECMWF MODELS AND IS MUCH FASTER THAN THE LATEST RUN OF THE GFS. FORECAST POSITIONS AND MAX WINDS INIT 29/0900Z 9.4N 26.3W 30 KT 35 MPH 12H 29/1800Z 9.8N 28.3W 35 KT 40 MPH 24H 30/0600Z 10.7N 30.9W 40 KT 45 MPH 36H 30/1800Z 11.9N 33.5W 45 KT 50 MPH 48H 31/0600Z 13.1N 36.3W 50 KT 60 MPH 72H 01/0600Z 15.0N 42.5W 65 KT 75 MPH 96H 02/0600Z 16.5N 48.0W 75 KT 85 MPH 120H 03/0600Z 18.0N 52.0W 85 KT 100 MPH $$ FORECASTER BRENNAN |
