Tropical Depression THIRTEEN

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TROPICAL DEPRESSION THIRTEEN SPECIAL DISCUSSION NUMBER   1
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL132007
615 PM EDT TUE SEP 25 2007
 
DATA FROM AN AIR FORCE RECONNAISSANCE AIRCRAFT SHOW THAT THE AREA OF
LOW PRESSURE IN THE SOUTHWESTERN GULF OF MEXICO HAS A WELL-DEFINED
CIRCULATION.  CONVECTION ASSOCIATED WITH THE CENTER HAS COME IN
PULSES...WITH SUFFICIENT REGULARITY FOR THE LOW TO QUALIFY AS A
TROPICAL DEPRESSION.  THE INITIAL INTENSITY OF 25 KT IS BASED ON
MAXIMUM FLIGHT-LEVEL WINDS OF 29 KT IN THE SOUTHWEST QUADRANT.

THE AIRCRAFT FIXES HAVE BEEN JUMPING AROUND A BIT...BUT A
LONGER-TERM MOTION ESTIMATE IS 220/3.  THE DEPRESSION IS IN A WEAK
STEERING ENVIRONMENT...AND LITTLE OVERALL MOTION IS EXPECTED FOR
THE NEXT DAY OR TWO.  AFTER THAT...GLOBAL MODELS SUGGEST THAT
MID-LEVEL HIGH PRESSURE WILL BUILD OVER THE CENTRAL GULF...AND THIS
PATTERN SHOULD BEGIN TO NUDGE THE CYCLONE TO THE SOUTHWEST.

THERE IS STILL SOME SOUTHWESTERLY SHEAR OVER THE CYCLONE ASSOCIATED
WITH AN UPPER LOW ALONG THE MEXICAN COAST NEAR TAMPICO.  THIS SHEAR
IS EXPECTED TO RELAX AND BE REPLACED BY AN UPPER ANTICYCLONE OVER
THE NEXT COUPLE OF DAYS...WHICH WOULD ALLOW THE SYSTEM AN
OPPORTUNITY TO STRENGTHEN.  NONE OF THE DYNAMICAL MODELS...
INCLUDING THE HWRF AND GFDL...SHOW MUCH DEVELOPMENT...ALTHOUGH THE
STATISTICAL SHIPS MODEL TAKES THE SYSTEM TO NEAR HURRICANE STRENGTH
IN THREE DAYS.  THE OFFICIAL FORECAST BLENDS THIS GUIDANCE BUT IS
CLOSER TO THE SHIPS SOLUTION. 
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      25/2215Z 21.7N  95.0W    25 KT
 12HR VT     26/0600Z 21.4N  95.0W    30 KT
 24HR VT     26/1800Z 21.2N  95.0W    35 KT
 36HR VT     27/0600Z 21.0N  95.0W    40 KT
 48HR VT     27/1800Z 20.8N  95.1W    45 KT
 72HR VT     28/1800Z 20.5N  95.5W    55 KT
 96HR VT     29/1800Z 20.0N  97.0W    50 KT...INLAND
120HR VT     30/1800Z...DISSIPATED
 
$$
FORECASTER FRANKLIN
 
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