[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 130841
TCDAT4
HURRICANE IVAN DISCUSSION NUMBER  44
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
5 AM EDT MON SEP 13 2004

A GPS DROPSONDE IN THE NORTHEAST QUADRANT OF THE EYEWALL AT 0510Z
MEASURED AN AVERAGE WIND SPEED OF 176 KT THROUGH THE LOWEST 500
METERS OF THE DROP.  THIS CONVERTS TO A SURFACE WIND OF 141 KT.  SO
THE INITIAL WIND SPEED IS KEPT AT 140 KT.  THE SHIPS MODEL SHOWS
CLOSE TO 20 KT OF WESTERLY SHEAR AFFECTING IVAN BUT RELAXES THIS
SHEAR A BIT AT 24 AND 36 HOURS BEFORE INCREASING IT SIGNIFICANTLY.
THE OFFICIAL INTENSITY FORECAST SHOWS GRADUAL WEAKENING AFTER 12
HOURS BUT IVAN COULD STILL BE A FORMIDABLE HURRICANE WHEN IT
REACHES THE NORTHERN GULF OF MEXICO COAST.

THE TRACK SCENARIO REMAINS THE SAME.  THE HURRICANE IS FORECAST TO
GRADUALLY TURN NORTHWARD OVER THE NEXT 72 HOURS INTO A WEAKNESS IN
THE SUBTROPICAL RIDGE...AND THEN NORTHEASTWARD AS IT APPROACHES THE
WESTERLIES.  THE RIDGE AND ITS WEAKNESS ARE NOT VERY PRONOUNCED AND
THE FORWARD MOTION IS EXPECTED TO BE SLOW THROUGHOUT THE FORECAST
PERIOD...10 KT OR LESS.  WHEN THE FORWARD MOTION IS SLOW ...THE
DIRECTION OF MOTION IS LESS CERTAIN.  THIS IS SEEN IN THE SPREAD OF
THE GLOBAL MODELS.  THE GFS SHOWS A LANDFALL IN THE FLORIDA
PANHANDLE WHILE THE UKMET AND NOGAPS SHOW A LANDFALL IN
SOUTHEASTERN LOUISIANA...ALL IN ABOUT 72 HOURS.  THE CONSENSUS OF
THE MODELS IS NEAR THE MISSISSIPPI/ALABAMA BORDER.  THE INITIAL
MOTION ESTIMATE IS 300/08.  MOST OF THE GLOBAL TRACK GUIDANCE
MODELS HAVE SHIFTED TO THE LEFT FROM THE PREVIOUS MODEL RUNS...
EXCEPT THE GFDL WHICH HAS SHIFTED A LITTLE TO THE RIGHT.  THE
OFFICIAL FORECAST TRACK IS SHIFTED ABOUT 60 N MI LEFT OF THE
PREVIOUS ADVISORY AT 72 HOURS AND IS STILL TO THE RIGHT OF ALL
GUIDANCE EXCEPT FOR THE GFS.  IF I DID NOT HAVE A PREVIOUS FORECAST
TO MAINTAIN SOME CONTINUITY WITH...I WOULD HAVE SHIFTED THE TRACK
EVEN FURTHER TO THE LEFT.  MEANWHILE...SINCE THE MOTION IS STILL
WEST-NORTHWESTWARD...THE CENTER COULD MISS THE WESTERN TIP OF CUBA
AND MOVE THROUGH THE YUCATAN CHANNEL AND COULD EVEN APPROACH THE
YUCATAN PENINSULA IF A 300 DEGREE HEADING IS MAINTAINED FOR ANOTHER
24 HOURS.

FORECASTER LAWRENCE

FORECAST POSITIONS AND MAX WINDS

INITIAL      13/0900Z 20.2N  83.9W   140 KT
 12HR VT     13/1800Z 21.2N  84.8W   140 KT
 24HR VT     14/0600Z 22.5N  86.0W   135 KT
 36HR VT     14/1800Z 24.2N  87.1W   130 KT
 48HR VT     15/0600Z 26.0N  87.5W   120 KT
 72HR VT     16/0600Z 30.0N  87.5W   105 KT
 96HR VT     17/0600Z 33.5N  85.5W    45 KT...INLAND
120HR VT     18/0600Z 36.0N  83.0W    25 KT...INLAND


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