Tropical Depression FOUR

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TROPICAL DEPRESSION FOUR DISCUSSION NUMBER   2
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL042007
500 PM EDT MON AUG 13 2007
 
THE DEPRESSION IS MAINTAINING ITSELF IN THE FACE OF EASTERLY WIND
SHEAR OF ABOUT 15 KT. DEEP CONVECTION HAS BEEN PULSATING TODAY AND
HAS REMAINED DISPLACED TO THE WEST OF THE LOW-LEVEL CENTER.
METEOSAT-9 HIGH-RESOLUTION VISIBLE IMAGERY CLEARLY REVEALED THE
EXPOSED LOW-LEVEL CENTER EARLIER THIS AFTERNOON...BUT MORE RECENTLY
THE CONVECTION HAS INCREASED AND PARTIALLY OBSCURED THE CENTER.
DVORAK INTENSITY ESTIMATES ARE UNCHANGED AND THE ADVISORY INTENSITY
REMAINS 30 KT.

TD FOUR IS STILL RACING ALMOST DUE WESTWARD...WITH AN INITIAL MOTION
ESTIMATE OF 265/17. A CONTINUED BRISK AND GENERALLY WESTWARD MOTION
SHOULD CONTINUE DURING THE NEXT COUPLE OF DAYS WHILE THE CYCLONE
MOVES ALONG THE SOUTHERN PERIPHERY OF A DEEP LAYER RIDGE. THE
OFFICIAL FORECAST IS AHEAD OF NEARLY ALL OF THE GUIDANCE DURING
THAT PERIOD...AS THE MODELS DO NOT SEEM TO HAVE A GOOD HANDLE ON
THE FAST INITIAL SPEED. BEYOND 48 HOURS...A TURN TOWARD THE
WEST-NORTHWEST IS FORECAST BY ALL OF THE MODELS. SINCE THIS
MORNING...ALL OF THE TRACK MODEL GUIDANCE HAS SLOWED DOWN AND
SHIFTED TO THE RIGHT...IN RESPONSE TO FORECASTING A SLIGHTLY WEAKER
SUBTROPICAL RIDGE OVER THE WESTERN ATLANTIC LATER IN THE FORECAST
PERIOD. THE NEW OFFICIAL TRACK HAS BEEN ADJUSTED TOWARD THE
CONSENSUS OF THE LATEST MODELS...BUT REMAINS A LITTLE FASTER AND TO
THE WEST OF THE GUIDANCE ENVELOPE...SINCE THE GUIDANCE COULD SHIFT
BACK THE OTHER WAY LATER.

THE SHEAR IS FORECAST TO ABATE WHILE THE SEA-SURFACE TEMPERATURES
WILL INCREASE ALONG THE OFFICIAL TRACK DURING THE NEXT 2-3 DAYS.
THEREAFTER...MOST OF THE GLOBAL MODELS FORECAST THE TROPICAL
CYCLONE TO FIND ITSELF BENEATH AN UPPER-LEVEL RIDGE...AN
ENVIRONMENT THAT WOULD FAVOR STRENGTHENING. A NOTABLE EXCEPTION IS
THE ECMWF...WHICH FORECASTS A STRONGER MID-OCEANIC TROUGH OVER THE
CENTRAL ATLANTIC IN 4-5 DAYS WHICH COULD CAUSE AN INCREASE IN THE
SHEAR.  THAT MODEL...HOWEVER...DOES NOT REPRESENT THE TROPICAL
CYCLONE WELL...AND IT MIGHT NOT HAVE GOOD HANDLE ON HOW THE CYCLONE
COULD MODIFY THE UPPER-LEVEL FLOW. THE SHIPS GUIDANCE FORECASTS 63
KT AT 72 HOURS AND 75 KT BY FIVE DAYS...WHILE THE LGEM REACHES 84
KT AT 120 HOURS. MEANWHILE THE GFDL AND HWRF FORECAST 90 AND 84
KT...RESPECTIVELY...AT FIVE DAYS. CONSIDERING THIS GUIDANCE...THE
NEW OFFICIAL INTENSITY FORECAST IN UNCHANGED FROM THIS MORNING BUT
RETAINS A SIGNIFICANT AMOUNT OF UNCERTAINTY...ESPECIALLY AT THE
LONGER RANGES.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      13/2100Z 11.9N  33.1W    30 KT
 12HR VT     14/0600Z 11.8N  35.5W    35 KT
 24HR VT     14/1800Z 11.8N  38.5W    40 KT
 36HR VT     15/0600Z 11.8N  41.6W    45 KT
 48HR VT     15/1800Z 12.1N  44.7W    50 KT
 72HR VT     16/1800Z 13.0N  50.5W    60 KT
 96HR VT     17/1800Z 15.0N  56.0W    75 KT
120HR VT     18/1800Z 17.0N  61.0W    90 KT
 
$$
FORECASTER KNABB
 
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