WTNT42 KNHC 290850
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
 

WTNT42 KNHC 291438
TCDAT2
 
TROPICAL DEPRESSION TWELVE DISCUSSION NUMBER   2
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL122011
1100 AM AST MON AUG 29 2011
 
SATELLITE IMAGES INDICATE THAT THE CLOUD PATTERN OF THE DEPRESSION
HAS CHANGED LITTLE IN ORGANIZATION FROM EARLIER THIS MORNING. 
THERE ARE SOME BROKEN BANDS OVER THE WESTERN AND SOUTHERN PORTION
OF THE CIRCULATION...AND THE CENTER OF THE CYCLONE IS NEAR THE
NORTHEASTERN EDGE OF THE MAIN AREA OF DEEP CONVECTION.  DVORAK
INTENSITY ESTIMATES REMAIN AT 25 AND 30 KT FROM SAB AND TAFB...
RESPECTIVELY...AND THE INTIAL INTENSITY IS KEPT AT 30 KT.  CURRENTLY
THERE IS MODERATE TO STRONG EAST-NORTHEASTERLY SHEAR OVER THE
SYSTEM AS EVIDENCED BY ITS APPEARANCE AND GLOBAL MODEL OUTPUT.  THE
SHIPS GUIDANCE INDICATES THAT THIS SHEAR WILL SOON LESSEN...WHICH
SHOULD ALLOW THE DEPRESSION TO STRENGTHEN.  THE OFFICIAL INTENSITY
FORECAST IS A LITTLE BELOW THE LATEST LGEM GUIDANCE.

THE INITIAL MOTION IS WESTWARD OR 275/13.  THERE IS A MID-LEVEL
RIDGE TO THE NORTH OF THE DEPRESSION AT THIS TIME...BUT  A MID- TO
UPPER-LEVEL LOW IS FORECAST TO CREATE A WEAKNESS IN THE RIDGE NEAR
40-50W OVER THE NEXT COUPLE OF DAYS.  THIS FEATURE SHOULD CAUSE THE
TROPICAL CYCLONE TO TURN TOWARD THE WEST-NORTHWEST WITHIN THE NEXT
DAY OR SO.  LATER IN THE FORECAST PERIOD...GLOBAL MODELS SHOW THE
WEAKNESS IN THE RIDGE SHIFTING WESTWARD AND THIS SHOULD PREVENT THE
CYCLONE FROM MOVING SIGNIFICANTLY POLEWARD DURING THE FORECAST
PERIOD.  THE OFFICIAL TRACK FORECAST IS SIMILAR TO THE PREVIOUS ONE
AND ON THE SOUTHERN SIDE OF THE GUIDANCE ENVELOPE.   
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  29/1500Z  9.8N  27.5W   30 KT  35 MPH
 12H  30/0000Z 10.3N  29.2W   35 KT  40 MPH
 24H  30/1200Z 11.2N  31.4W   45 KT  50 MPH
 36H  31/0000Z 12.3N  34.1W   50 KT  60 MPH
 48H  31/1200Z 13.4N  37.2W   55 KT  65 MPH
 72H  01/1200Z 15.0N  43.0W   70 KT  80 MPH
 96H  02/1200Z 16.5N  48.5W   80 KT  90 MPH
120H  03/1200Z 18.5N  53.0W   90 KT 105 MPH
 
$$
FORECASTER PASCH
 

WTNT42 KNHC 292033
TCDAT2
 
TROPICAL DEPRESSION TWELVE DISCUSSION NUMBER   3
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL122011
500 PM AST MON AUG 29 2011
 
THE DEPRESSION HAS NOT BECOME BETTER ORGANIZED DURING THE DAY. 
BANDING FEATURES HAVE NOT BECOME MORE DEFINED AND THE LOW-LEVEL
CENTER REMAINS NEAR THE NORTHEASTERN EDGE OF THE MAIN AREA OF DEEP
CONVECTION.  THE CURRENT INTENSITY IS HELD AT 30 KT IN AGREEMENT
WITH DVORAK ESTIMATES FROM BOTH TAFB AND SAB.  THE SHIPS GUIDANCE
INDICATES THAT THE STRONG EAST-NORTHEASTERLY VERTICAL WIND SHEAR
CURRENTLY AFFECTING THE SYSTEM WILL LESSEN WITHIN A DAY OR
SO...WHICH SHOULD ALLOW STRENGTHENING TO COMMENCE SOON.  THE
OFFICIAL WIND SPEED FORECAST IS NOT FAR FROM THE
STATISTICAL-DYNAMICAL INTENSITY GUIDANCE.

INITIAL MOTION IS JUST NORTH OF DUE WEST OR ABOUT 280/12. THERE HAS
BEEN LITTLE CHANGE TO THE TRACK FORECAST OR FORECAST REASONING FROM
THIS MORNING. A MID-LEVEL SUBTROPICAL RIDGE IS CURRENTLY NORTH OF
THE DEPRESSION. OVER THE NEXT COUPLE OF DAYS...THE TROPICAL CYCLONE
IS EXPECTED TO TURN TOWARD THE WEST-NORTHWEST IN RESPONSE TO A
WEAKNESS IN THE RIDGE NEAR 50W LONGITUDE. GLOBAL MODELS PREDICT
THAT THE WEAKNESS WILL GRADUALLY SHIFT WESTWARD OVER TIME. AS A
RESULT...A CONTINUED WEST-NORTHWESTWARD TRACK IS FORECAST THROUGH
THE REMAINDER OF THE PERIOD. THE OFFICIAL FORECAST TRACK IS BETWEEN
THE LATEST ECMWF AND GFS SOLUTION...BUT LEANS MORE TOWARD THE ECMWF
TRACK AS THE GFS SEEMS TO BE TOO SLOW IN THE LATTER PART OF THE
FORECAST PERIOD.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  29/2100Z 10.2N  28.4W   30 KT  35 MPH
 12H  30/0600Z 10.9N  30.1W   35 KT  40 MPH
 24H  30/1800Z 11.9N  32.5W   45 KT  50 MPH
 36H  31/0600Z 13.0N  35.5W   50 KT  60 MPH
 48H  31/1800Z 14.0N  38.6W   55 KT  65 MPH
 72H  01/1800Z 15.5N  45.0W   70 KT  80 MPH
 96H  02/1800Z 17.0N  50.0W   80 KT  90 MPH
120H  03/1800Z 19.0N  55.0W   90 KT 105 MPH
 
$$
FORECASTER PASCH
 

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
 

