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Tropical Depression FOUR (Text)

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TROPICAL DEPRESSION FOUR DISCUSSION NUMBER   2
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL042006
1100 PM EDT MON AUG 21 2006
 
TROPICAL DEPRESSION FOUR REMAINS A LARGE AND WELL-ORGANIZED
SYSTEM...DESPITE A DECREASE IN THE INNER CORE CONVECTION. A
21/1914Z QUIKSCAT OVERPASS INDICATED THE DEPRESSION HAD A SURFACE
WIND FIELD ENVELOPE AT LEAST 500 NMI IN DIAMETER...WITH NUMEROUS
25-30 KT UNCONTAMINATED WIND VECTORS LOCATED IN THE DRY SLOTS.
BASED ON THIS INFORMATION...AND THE LACK OF INNER CORE CONVECTION
...THE INITIAL INTENSITY IS BEING MAINTAINED AT 30 KT.
 
THE INITIAL MOTION ESTIMATE IS 285/13...BASED ON A 10-HOUR AVERAGE
MOTION USING PASSIVE AND QUIKSCAT MICROWAVE SATELLITE DATA. THE
AFOREMENTIONED QUIKSCAT OVERPASS INDICATED THE CENTER OF TD-4 WAS
SOUTH OF 12N LATITUDE AT THAT TIME...PROBABLY DUE TO REFORMATION OF
THE BROAD LOW-LEVEL CENTER CLOSER TO THE BAND OF DEEP CONVECTION IN
THE SOUTHERN SEMICIRCLE. THE GLOBAL MODELS...AS WELL AS THE GFDL
AND THE BAM MODELS...REMAIN TIGHTLY CLUSTERED AND IN EXCELLENT
AGREEMENT ON AN IMMEDIATE NORTHWESTWARD MOTION. UNFORTUNATELY...
THE MODELS HAVE ALSO BEEN EXCELLENTLY WRONG THUS FAR. TD-4 IS
CURRENTLY AT LEAST 60 NMI SOUTHWEST OF THE 18Z GFS FORECAST
POSITION FOR 22/00Z...AND THE OTHER MODELS ARE SIMILARLY TOO FAR TO
THE NORTHEAST OF THE CURRENT POSITION. AS A RESULT...I AM NOT TOO
ENTHRALLED WITH THE MODEL SOLUTIONS...AND WE MAY SEE THE CYCLONE
TURN OUT TO BE MORE OF A WEST-RUNNER THAN A RECURVER THROUGH THE
STRONG SUBTROPICAL RIDGE LOCATED TO THE NORTH. WATER VAPOR IMAGERY
AS WELL AS WATER VAPOR-DERIVED WINDS AND UPPER-AIR DATA FROM THE
CAPE VERDE ISLANDS SUGGEST THAT THE MID-LEVEL EASTERLY FLOW ON THE
SOUTH SIDE OF THE RIDGE IS STRONGER THAN THE MODELS HAVE BEEN
FORECASTING...AND THE RIDGE TO THE NORTH ALSO APPEARS TO BE
BUILDING WESTWARD. THE FORECAST TRACK WAS SHIFTED TO THE WEST OF
THE PREVIOUS ADVISORY...BUT NOT AS FAR WEST AS THE CLIPER MODEL AND
STATISTICAL-DYNAMICAL MODELS OUT OF RESPECT FOR THE SKILL OF THE
GLOBAL MODELS. HOWEVER...IF TD-4 DOES NOT MAKE IT AS FAR NORTH AS
20N LATITUDE WITHIN 4 DAYS...THEN THE CYCLONE WILL LIKELY MISS THE
WEAKNESS IN THE RIDGE THAT ALL THE MODELS FORECAST TO DEVELOP.

THE INTENSITY FORECAST WAS LOWERED SLIGHTLY IN THE FIRST 48 HOURS
DUE TO THE LACK OF INNER-CORE CONVECTION AND WEAK TO MODERATE
UPPER-LEVEL NORTHEASTERLY SHEAR FORECAST TO UNDERCUT THE OTHERWISE
IMPRESSIVE AND EXPANDING OUTFLOW PATTERN. AFTER THAT...GRADUAL
STRENGTHENING IS EXPECTED AS THE SHEAR DECREASES TO 5-10 KT AND THE
CYCLONE IS FORECAST TO BE OVER WARMER WATERS. 
 
DUE TO THE LARGE WIND FIELD DEPICTED IN THE EARLIER QUIKSCAT
OVERPASS...12-FT SEA HEIGHT RADII WERE ADDED TO THE FORECAST.
  
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      22/0300Z 12.2N  23.2W    30 KT
 12HR VT     22/1200Z 12.9N  25.1W    35 KT
 24HR VT     23/0000Z 14.1N  27.7W    40 KT
 36HR VT     23/1200Z 15.9N  30.6W    45 KT
 48HR VT     24/0000Z 17.5N  33.4W    50 KT
 72HR VT     25/0000Z 20.5N  38.4W    60 KT
 96HR VT     26/0000Z 24.0N  44.5W    65 KT
120HR VT     27/0000Z 27.0N  51.0W    65 KT
 
$$
FORECASTER STEWART
 
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