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| #454821 (Received by flhurricane at: 10:43 PM 29.Aug.2011) TCDAT2 TROPICAL DEPRESSION TWELVE DISCUSSION NUMBER 4 NWS NATIONAL HURRICANE CENTER MIAMI FL AL122011 1100 PM AST MON AUG 29 2011 AT THE TIME OF THE SATELLITE FIXES AT 00Z...A LARGE CURVED CONVECTIVE BAND WRAPPED ABOUT HALF WAY AROUND THE CIRCULATION... WHICH RESULTED IN SATELLITE INTENSITY ESTIMATES OF T2.5/35 KT FROM SAB AND T2.0/30 KT WITH A DATA-T NUMBER OF 2.5/35 KT FROM TAFB. HOWEVER...SINCE THAT TIME THE CURVED BAND HAS DISSIPATED...AND THE CONVECTIVE PATTERN HAS DEVOLVED INTO MORE OF A SHEAR PATTERN. A SMALL BURST OF DEEP CONVECTION HAS DEVELOPED OVER AND JUST WEST OF THE CENTER...BUT ONE THUNDERSTORM DOES NOT MAKE A TROPICAL STORM. THEREFORE...THE INTENSITY IS BEING HELD AT 30 KT FOR THIS ADVISORY. THE INITIAL MOTION IS NOW 290/13 KT BASED ON A BLEND OF VISIBLE AND MICROWAVE SATELLITE FIXES. THE GLOBAL AND REGIONAL MODELS ARE IN EXCELLENT AGREEMENT ON A GENERAL WEST-NORTHWESTWARD MOTION OVER THE NEXT 5 DAYS...BUT THERE ARE SPEED DIFFERENCES IN THE MODELS. THE GFDL...UKMET...CANADIAN AND ECMWF MOVE THE DEPRESSION THE FASTEST...WHEREAS THE GFS...HWRF...AND NOGAPS ARE SLOWER. HOWEVER...THE TREND IN THE MOST RECENT MODEL RUNS HAS BEEN TO WEAKEN THE MID/UPPER-LEVEL LOW NEAR 30N55W AND LIFT IT OUT TO THE NORTHEAST SOONER THAN IN PREVIOUS RUNS. THIS RESULTS IN LESS OF A WEAKNESS IN THE SUBTROPICAL RIDGE...WHICH IS ALSO FORECAST TO CHANGE ITS CURRENT NORTH-SOUTH CONFIGURATION TO AN EAST-WEST ORIENTATION BY 72 HOURS. THIS CHANGE IN THE STEERING FLOW SHOULD KEEP THE CYCLONE MOVING ON A WEST-NORTHWESTWARD TRACK FOR THE NEXT 4 DAYS...AND THEN SLOW DOWN AND POSSIBLY TURN MORE WESTWARD BY DAY 5 AS A NARROW RIDGE DEVELOPS TO THE NORTH AND WEST OF THE SYSTEM. SINCE THE DEPRESSION IS BASICALLY ON TRACK...THE NEW NHC FORECAST IS JUST AN UPDATE AND EXTENSION OF THE PREVIOUS ADVISORY TRACK. THE EROSION OF CONVECTIVE BANDING FEATURES HAS LIKELY BEEN DUE A NARROW INTRUSION OF DRY AIR COMING IN THE FROM THE NORTHEAST. HOWEVER...ALL AVAILABLE DIAGNOSTIC AND MODEL FORECAST DATA INDICATE THAT THE CYCLONE IS SURROUNDED BY VERY MOIST MID-LEVEL AIR WITH HUMIDITY VALUES OF 70 PERCENT OR HIGHER...WHICH SHOULD ALLOW FOR DEEP CONVECTION TO RECOVER LATER TONIGHT AND FOR THE SYSTEM TO REACH TROPICAL STORM STATUS WITHIN THE NEXT 6-12 HOURS. MODERATE NORTHEASTERLY SHEAR IS ALSO FORECAST TO AFFECT THE CYCLONE FOR THE NEXT 24 HOURS OR SO...WHICH SHOULD INHIBIT ANY RAPID DEVELOPMENT. AFTER THAT...HOWEVER...THE SHEAR IS EXPECTED TO SHARPLY DECREASE... AND AN UPPER-LEVEL ANTICYCLONE IS FORECAST TO DEVELOP OVER THE SURFACE CIRCULATION. THIS IMPROVEMENT IN THE VENTILATION PROCESS SHOULD ALLOW FOR THE CYCLONE TO AT LEAST STEADILY INTENSIFY AND BECOME A HURRICANE IN 48-72 HOURS AS SUGGESTED BY THE SHIPS... LGEM...HWRF...AND GFDL MODELS. FORECAST POSITIONS AND MAX WINDS INIT 30/0300Z 11.0N 30.0W 30 KT 35 MPH 12H 30/1200Z 11.8N 32.0W 35 KT 40 MPH 24H 31/0000Z 12.8N 34.7W 45 KT 50 MPH 36H 31/1200Z 13.9N 37.8W 50 KT 60 MPH 48H 01/0000Z 14.8N 41.5W 60 KT 70 MPH 72H 02/0000Z 16.4N 47.5W 70 KT 80 MPH 96H 03/0000Z 18.3N 52.9W 80 KT 90 MPH 120H 04/0000Z 20.0N 57.0W 90 KT 105 MPH $$ FORECASTER STEWART |
