[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 131455
TCDAT4
HURRICANE IVAN DISCUSSION NUMBER  45
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
11 AM EDT MON SEP 13 2004

THE INITIAL MOTION IS 310/07. THE LAST 4 RECON FIXES SUGGEST THAT
IVAN HAS SLOWED A LITTLE AND HAS TURNED NORTHWESTWARD. THIS MAY
JUST BE A TEMPORARY MOTION...BUT WITH INCREASING WEST TO
NORTHWESTERLY 300 MB FLOW IMPINGING ON IVAN...THIS MAY TURN OUT TO
BE MORE OF AN ESTABLISHED TREND. THE NHC MODEL GUIDANCE HAS SHIFTED
SLIGHTLY WESTWARD...AND SO HAS THE OFFICIAL FORECAST TRACK. HOWEVER
...LATEST WATER VAPOR IMAGERY AND 12Z UPPER-AIR DATA INDICATE A 500
MB LOW OVER WESTERN TENNESSEE WITH A SHARP NORTH-SOUTH ORIENTED
TROUGH EXTENDING SOUTHWARD INTO THE NORTH CENTRAL GULF OF
MEXICO...WHICH HAS ERODED THE SUBTROPICAL RIDGE AXIS OVER THE
CENTRAL AND EASTERN GULF. THIS HAS ALLOWED THE RIDGE ACROSS TEXAS
AND LOUISIANA TO BUILD EASTWARD OVER THE WESTERN GULF. THE
MID-LEVEL WIND FLOW ACROSS THE FLORIDA PENINSULA HAS ALSO VEERED
FROM EASTERLY TO SOUTHERLY DURING THE PAST 24 HOURS...SUGGESTING
THAT THE RIDGE HAS ERODED A LITTLE FARTHER EAST THAN THE NOGAPS AND
GFDN MODELS HAVE BEEN FORECASTING. THEREFORE...THE FORECAST TRACK
REMAINS ON THE EASTERN EDGE OF THE MODEL GUIDANCE ENVELOPE.

THE LATEST AIR FORCE RESERVE RECON REPORT INDICATED THE PRESSURE HAD
DROPPED ANOTHER 2 MB IN THE PAST 2 HOURS WHILE THE EYE DIAMETER HAS
STABILIZED AT A RATHER LARGE 28-30 NMI. IT IS POSSIBLE THAT IVAN
COULD STRENGTHEN ANOTHER 5 KT OR SO BEFORE INCREASING NORTHWESTERLY
300 MB SHEAR DISRUPTS THE INNER CORE CONVECTION. IN THE LONGER
TERM...IVAN WILL BE MOVING OVER SSTS NEAR 30C...WITH THE SHEAR
DECREASING AFTER 24 HOURS BEFORE IT PICKS UP AGAIN BY 48-60 HOURS.
WHILE THE GULF WATERS ARE WARM...THE DEPTH OF THE WARM WATER IS
RELATIVELY SHALLOW...EXCEPT FOR A WARM EDDY OVER THE NORTH-CENTRAL
GULF. REGARDLESS...IVAN IS FORECAST TO BE A MAJOR HURRICANE AT U.S.
LANDFALL.

FORECASTER STEWART

FORECAST POSITIONS AND MAX WINDS

INITIAL      13/1500Z 20.6N  84.4W   140 KT
 12HR VT     14/0000Z 21.5N  85.3W   140 KT
 24HR VT     14/1200Z 23.2N  86.5W   135 KT
 36HR VT     15/0000Z 25.2N  87.4W   130 KT
 48HR VT     15/1200Z 27.2N  87.7W   120 KT
 72HR VT     16/1200Z 31.0N  86.5W   100 KT...INLAND
 96HR VT     17/1200Z 34.0N  84.5W    45 KT...INLAND
120HR VT     18/1200Z 36.5N  81.5W    25 KT...INLAND


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