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| #8165 (Received by flhurricane at: 5:24 PM 13.Sep.2004) 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 |
