431 
 WTNT42 KNHC 300240
 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