[Tropical] Tropical Cyclone Discussion

[Nashville EMWIN]

WTNT41 KNHC 300248
TCDAT1
HURRICANE FRANCES DISCUSSION NUMBER  21
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
11 PM EDT SUN AUG 29 2004

THE FIRST RECONNAISSANCE MISSION WAS IN THE HURRICANE FOR SEVERAL
HOURS ENDING AT 23Z.  THE LAST CENTRAL PRESSURE WAS 954 MB...UP
FROM 949 MB A FEW HOURS EARLIER.  954 MB CORRESPONDS TO ABOUT 108
KT.  THE HIGHEST 700 MB FLIGHT LEVEL WIND REPORTED WAS 118 KT AT A
POSITION ABOUT 9 N MI NORTHWEST OF THE CENTER...90 PERCENT OF 118
KT REDUCES TO A SURFACE WIND OF 106 KT.  THERE WERE SEVERAL GPS
DROPSONDES NEAR THE EYEWALL.  THE HIGHEST SURFACE WIND DETERMINED
FROM THESE DROPS WAS 107 KT USING APPROPRIATE REDUCTION TECHNIQUES.
THEREFORE THE MAXIMUM WIND IN FRANCES IS REDUCED FROM 115 TO 110 KT.

THE SHIPS MODEL FORECASTS THE WIND NEAR 110 KT THROUGH 72 HOURS AND
THEN REDUCES IT TO UNDER 100 KT AFTER 72 HOURS.  THE GFDL IS
SIMILAR.  THE GFS AND UKMET ARE MORE AGGRESSIVE BUT THESE MODELS
ARE NOT AS SKILLFUL AS THE SHIPS AND GFDL.  THE OFFICIAL WIND SPEED
FORECAST IS 110 KT THROUGH 120 HOURS AS A COMPROMISE OF THE ABOVE
GUIDANCE...REALIZING THAT OUR SKILL AT INTENSITY FORECASTING LEAVES
SOMETHING TO BE DESIRED...ESPECIALLY AT LONGER FORECAST PERIODS.

THE INITIAL MOTION ESTIMATE IS 275/07.  THE GLOBAL TRACK GUIDANCE
MODELS SHOW A MOSTLY WESTWARD TRACK FOR 24 TO 36 HOURS FOLLOWED BY
A WEST-NORTHWESTWARD TRACK THROUGH 120 HOURS WITH THE SUBTROPICAL
RIDGE HOLDING TO THE NORTH OF THE HURRICANE.  THE MODELS ARE IN
FAIRLY GOOD AGREEMENT ABOUT THIS SCENARIO ALTHOUGH THE GFS IS 50 TO
100 N MI SOUTH OF THE OTHER MODELS THROUGH 72 HOURS BUT THEN COMES
INTO BETTER AGREEMENT BY 120 HOURS WHEN ALL OF THE MODELS TEND TO
CONVERGE ON THE NORTHERN BAHAMAS.  THE OFFICIAL FORECAST TRACK IS
ADJUSTED SLIGHTLY SOUTH OF THE PREVIOUS ADVISORY THROUGH 72 HOURS
AND IS SIMILAR TO IT AT 96 AND 120 HOURS.

NOAA HAS FLOWN A SYNOPTIC SCALE MISSION TONIGHT AND DATA FROM THIS
MISSION IS BEING USED TO INITIALIZE THE 00Z MODEL RUNS.  THIS
HOPEFULLY IMPROVED GUIDANCE WILL BE AVAILABLE FOR THE NEXT ADVISORY
PACKAGE.

FORECASTER LAWRENCE

FORECAST POSITIONS AND MAX WINDS

INITIAL      30/0300Z 18.9N  56.2W   110 KT
 12HR VT     30/1200Z 19.1N  58.2W   110 KT
 24HR VT     31/0000Z 19.5N  60.8W   110 KT
 36HR VT     31/1200Z 20.0N  63.4W   110 KT
 48HR VT     01/0000Z 20.8N  66.2W   110 KT
 72HR VT     02/0000Z 22.5N  71.0W   110 KT
 96HR VT     03/0000Z 24.5N  74.5W   110 KT
120HR VT     04/0000Z 26.5N  77.0W   110 KT


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