Tropical Depression FOUR

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TROPICAL DEPRESSION FOUR DISCUSSION NUMBER   1
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL042007
1100 AM EDT MON AUG 13 2007
 
METEOSAT-9 VISIBLE AND INFRARED IMAGERY DURING THE PAST SEVERAL
HOURS REVEALS THAT THE LOW PRESSURE AREA OVER THE FAR EASTERN
TROPICAL ATLANTIC HAS BECOME A TROPICAL DEPRESSION.  THE DEEP
CONVECTION HAS BEEN PERSISTENT FOR MORE THAN 12 HOURS...BUT IT IS
MOSTLY CONFINED WEST OF THE CIRCULATION CENTER DUE TO MODERATE
EASTERLY WIND SHEAR.  A QUIKSCAT OVERPASS AT 0723Z THIS MORNING
ONLY CAPTURED THE EASTERN SEMICIRCLE BUT AN AMBIGUITY ANALYSIS
INDICATES SOME BELIEVABLE 30-KT WINDS.  DVORAK INTENSITY ESTIMATES
BASED ON THE CURVED BAND PATTERN AT 12Z WERE 25-30 KT...ALTHOUGH THE
DEPTH AND COVERAGE OF CONVECTION HAVE DECREASED SINCE 12Z.  THE
ADVISORY INTENSITY IS SET TO 30 KT.

THE LOW-LEVEL CENTER HAS BEEN MOVING SLIGHTLY SOUTH OF DUE WEST THIS
MORNING...AND THE INITIAL MOTION ESTIMATE IS 265/18...BUT MORE
RECENT VISIBLE AND MICROWAVE IMAGES SUGGEST THE CENTER MIGHT BE
EVEN A LITTLE FARTHER SOUTH THAN THE ADVISORY POSITION.  WITH THE
DEEP-LAYER RIDGE NORTH OF THE CYCLONE FORECAST TO STRENGTHEN AND
BUILD WESTWARD...A SLIGHT SOUTHWARD COMPONENT OF MOTION MIGHT
PERSIST FOR A COUPLE OF DAYS...BUT THE TRACK SHOULD BE GENERALLY
WESTWARD AND FAIRLY BRISK.  A VERY GRADUAL BEND TO THE RIGHT AND A
SLIGHTLY SLOWER FORWARD SPEED ARE FORECAST TO BEGIN ON DAY THREE AS
THE RIDGE WEAKENS...BUT ONLY SLIGHTLY.  THE OFFICIAL FORECAST TRACK
IS A BLEND OF THE GFDL...UKMET...AND GFS.

THE EASTERLY WIND SHEAR WILL PROBABLY LIMIT INTENSIFICATION DURING
THE NEXT COUPLE OF DAYS.  THEREAFTER...THE GLOBAL MODELS...EVEN
THOSE THAT DO NOT HAVE A STRONG REPRESENTATION OF THE TROPICAL
CYCLONE ITSELF...FORECAST AN UPPER-LEVEL RIDGE TO BUILD OVER AND
NEAR THE LESSER ANTILLES.  COMBINED WITH WARM SEA-SURFACE
TEMPERATURES ALONG THE TRACK...CONDITIONS APPEAR CONDUCIVE FOR
STEADY STRENGTHENING LATER IN THE FORECAST PERIOD.  THE SHIPS
GUIDANCE FORECASTS NEARLY HURRICANE STRENGTH BY 72 HOURS...AND THE
GFDL FORECAST INTENSITY EXCEEDS 100 KT AT FIVE DAYS.  THE OFFICIAL
FORECAST IS CLOSE TO THE SHIPS GUIDANCE FOR THE FIRST THREE
DAYS...THEN INDICATES A STRENGTHENING HURRICANE...BUT IS NOT AS
AGGRESSIVE AS THE GFDL.

IT IS IMPORTANT TO REMEMBER THAT SIGNIFICANT UNCERTAINTIES EXIST IN
BOTH THE TRACK AND INTENSITY FORECASTS...ESPECIALLY AT LONG RANGES.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      13/1500Z 12.0N  31.6W    30 KT
 12HR VT     14/0000Z 12.0N  34.3W    35 KT
 24HR VT     14/1200Z 11.9N  38.0W    40 KT
 36HR VT     15/0000Z 11.9N  41.4W    45 KT
 48HR VT     15/1200Z 11.9N  44.7W    50 KT
 72HR VT     16/1200Z 13.0N  51.0W    60 KT
 96HR VT     17/1200Z 14.5N  57.5W    75 KT
120HR VT     18/1200Z 16.0N  63.5W    90 KT
 
$$
FORECASTER KNABB
 
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