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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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