[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 132114
TCDAT4
HURRICANE IVAN DISCUSSION NUMBER  46
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
5 PM EDT MON SEP 13 2004

THE INITIAL MOTION IS 330/08. THE LAST FEW RECON FIXES INDICATE THAT
IVAN HAS MADE A SLIGHT TURN TO RIGHT...OR TO THE NORTH-NORTHWEST.
THIS MOTION HAS LIKELY BEEN INDUCED BY A COMBINATION OF IMPINGING
UPPER-LEVEL NORTHWESTERLY FLOW AND THE MID-LEVEL TROUGH OVER THE
NORTH-CENTRAL AND NORTHEASTERN GULF OF MEXICO CAUSING THE RIDGE TO
THE NORTH OF IVAN TO WEAKEN.  HOWEVER...THE TROUGH IS FORECAST TO
SLOWLY LIFT OUT NORTHEASTWARD...ALLOWING THE RIDGE TO BUILD BACK
WESTWARD IN ABOUT 12HR. THIS IS EXPECTED TO NUDGE IVAN BACK TO A
MORE NORTHWESTWARD MOTION. AFTERWARDS...A SECOND TROUGH OVER THE
U.S. WEST COAST IS FORECAST TO MOVE QUICKLY EASTWARD AND ERODE THE
RIDGE AGAIN OVER THE CENTRAL GULF. THE EROSION OF THE RIDGE...
COMBINED WITH INCREASING WEST TO SOUTHWESTERLY MID- TO UPPER-LEVEL
FLOW OVER THE NORTHWESTERN GULF OF MEXICO BY 48 HOURS SHOULD TURN
IVAN NORTHWARD AND THEN NORTHEASTWARD AS THE HURRICANE APPROACHES
THE NORTHERN GULF COAST. THE UKMET AND CANADIAN MODELS HAVE MADE A
LARGE SWING TO THE EAST FROM THE 13/00Z MODEL RUN. ONLY THE NOGAPS
MODEL TAKES IVAN OVER NEW ORLEANS AND THIS APPEARS TO BE DUE TO THE
MODEL CREATING TOO LARGE OF A MID-LEVEL CIRCULATION...WHICH IN TURN
INDUCES A STRONGER RIDGE TO THE NORTH THAT HELPS TO KEEP IVAN
MOVING NORTH-NORTHWESTWARD THROUGHOUT THE FORECAST PERIOD. THE
NOGAPS SOLUTION IS NOT BEING CONSIDERED AT THIS TIME. THE OFFICIAL
FORECAST TRACK IS CLOSE TO THE PREVIOUS TRACK AND REMAINS NEAR THE
EXTREME EASTERN PORTION OF THE NHC MODEL GUIDANCE ENVELOPE...AND A
LITTLE TO THE RIGHT OF GFS TRACK WHICH IS ALREADY TO THE LEFT OF
THE CURRENT POSITION AND RECENT MOTION OF IVAN.

THE PLETHORA OF RECON REPORTS TODAY INDICATE THAT IVAN HAS
MAINTAINED CATEGORY 5 INTENSITY OF ABOUT 140 KT. DESPITE THE LARGE
EYE OF 25-30 NMI...THE PRESSURE HAS CONTINUED TO FALL. SINCE THE
PUBLIC ADVISORY WAS ISSUED...THE PRESSURE HAS DROPPED TO 910 MB AS
MEASURED BY A DROPSONDE. SINCE THE EYE REMAINS QUITE DISTINCT AS
NOTED IN BOTH VISIBLE AND INFRARED SATELLITE IMAGERY...SOME
ADDITIONAL SLIGHT STRENGTHENING OF 5 KT OR SO COULD OCCUR LATER
TONIGHT. BY 24 HOURS OR SO...INCREASING NORTHWESTERLY 300 MB SHEAR
MAY DISRUPT THE INNER CORE CONVECTION AND BRING ABOUT SOME SLOW
WEAKENING. IN THE LONGER TERM...MORE SIGNIFICANT WEST TO
SOUTHWESTERLY VERTICAL SHEAR IS EXPECTED TO AFFECT IVAN AS IT NEARS
LANDFALL. REGARDLESS...IVAN IS FORECAST TO BE A MAJOR HURRICANE OF
AT LEAST 100 KT AT ITS U.S. LANDFALL.

FORECASTER STEWART

FORECAST POSITIONS AND MAX WINDS

INITIAL      13/2100Z 21.3N  84.9W   140 KT
 12HR VT     14/0600Z 22.7N  85.7W   140 KT
 24HR VT     14/1800Z 24.2N  86.7W   135 KT
 36HR VT     15/0600Z 26.1N  87.6W   130 KT
 48HR VT     15/1800Z 28.2N  87.7W   120 KT
 72HR VT     16/1800Z 31.5N  86.7W    90 KT...INLAND
 96HR VT     17/1800Z 34.0N  84.5W    40 KT...INLAND
120HR VT     18/1800Z 36.5N  81.5W    25 KT...INLAND


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