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TROPICAL DEPRESSION THIRTEEN DISCUSSION NUMBER   8
NATIONAL WEATHER SERVICE MIAMI FL
11 AM EDT MON SEP 23 2002
 
TROPICAL DEPRESSION THIRTEEN LOOKS IMPRESSIVE ON SATELLITE IMAGERY 
AS FAR AS THE CONVECTION IS CONCERNED. HOWEVER...SURFACE 
OBSERVATIONS SUGGEST THAT THE CENTER IS ELONGATED WEST TO EAST AND 
THE ADVISORY POSITION IS A FULL DEGREE OR MORE WEST OF THE SATELLITE 
POSITONS...WHICH ARE BACK INTO THE DEEP CONVECTION. DURING THE PAST 
HOUR...BARBADOS HAS REPORTED WINDS OF 36 KT WITH GUSTS TO 48 KT IN A 
LIGHT SHOWER. THESE WINDS MAY BE CAUSED BY STRONG DOWNDRAFTS 
BRINGING THE 40 KT GRADIENT-LEVEL WINDS DOWN TO THE SURFACE AS SEEN 
IN THE BARBADOS 12Z SOUNDING. RECON IS CURRENTLY INVESTIGATING THE 
SYSTEM AND MAY FIND SUSTAINED TROPICAL STORM FORCE WINDS FARTHER 
EAST IN THE DEEP CONVECTION...AT WHICH TIME A SPECIAL ADVISORY WILL 
BE ISSUED UPGRADING THE SYSTEM TO TROPICAL STORM STATUS.
 
THE INITIAL MOTION ESTIMATE IS 280/17. EVEN THOUGH THE SURFACE 
CENTER IS BELIEVED TO BE WEST OF THE DEEP CONVECTION...THE 
DEPRESSION STILL APPEARS TO HAVE SLOWED DOWN SLIGHTLY. THE MAIN 
STEERING FEATURE IS THE SUBTROPICAL RIDGE TO THE NORTH AND THIS 
PATTERN IS FORECAST TO HOLD THROUGH AT LEAST THE NEXT 36 TO 48 
HOURS. AFTER THAT...THE VARIOUS GLOBAL MODELS WEAKEN THE RIDGE AND 
TRY TO BRING THE CYCLONE MORE POLEWARD. THE AVN/GFS AND AVN/GFS 
ENSEMBLE MODELS ARE THE RIGHTMOST OF THE NHC GUIDANCE SUITE AND THEY 
BRING THE CYCLONE ACROSS PUERTO RICO IN 48 TO 72 HOURS. THIS MAY BE 
PREMATURE SINCE THE MODELS KEEP TD13 AS A VERY WEAK CYCLONE. THE 
REST OF THE GUIDANCE AGREES ON A GENERAL WEST-NORTHWEST TRACK WITH A 
GRADUAL DECREASE IN THE FORWARD SPEED. THE OFFICIAL FORECAST IS IN 
THE MIDDLE OF THE GUIDANCE ENVELOPE AND IS ALSO JUST A LITTLE EAST 
OF AND SLOWER THAN THE PREVIOUS FORECAST TRACK.
 
THERE IS NO SIGNIFICANT CHANGE TO THE PREVIOUS INTENSITY FORECAST. 
AS THE SYSTEM GRADUALLY SLOWS DOWN...THE CONVECTION SHOULD BECOME 
BETTER ORGANIZED AROUND THE LOW-LEVEL CENTER. UPPER-LEVEL WINDS ALSO 
APPEAR FAVORABLE FOR CONTINUED STRENGTHENING...AND THE SHIPS MODEL 
BRINGS THE CYCLONE TO NEAR 70 KT IN 60 HOURS. RAPID DEVELOPMENT DOES 
NOT APPEAR LIKELY AS THE CYCLONE MOVES THROUGH THE LESSER ANTILLES 
AND THE EASTERN CARIBBEAN...BUT CONDITIONS BECOME MUCH MORE 
FAVORABLE FOR RAPID STRENGTHENING AFTER 48 HOURS WHEN THE SYSTEM IS 
SOUTH OF HISPANIOLA. HOWEVER...POSSIBLE LAND INTERACTION IN THE 
LATER PERIODS DICTATES THAT THE INTENSITY FORECAST REMAIN 
CONSERVATIVE AT THIS TIME.
 
FORECASTER STEWART
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     23/1500Z 12.8N  59.5W    30 KTS
12HR VT     24/0000Z 13.6N  62.1W    35 KTS
24HR VT     24/1200Z 14.3N  64.8W    40 KTS
36HR VT     25/0000Z 15.3N  67.4W    50 KTS
48HR VT     25/1200Z 16.2N  69.4W    60 KTS
72HR VT     26/1200Z 18.0N  73.0W    70 KTS
 
 
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