[Tropical] Tropical Cyclone Discussion

[Nashville EMWIN]

WTNT44 KNHC 100232
TCDAT4
HURRICANE IVAN DISCUSSION NUMBER  31
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
11 PM EDT THU SEP 09 2004

THE LAST REPORT FROM AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT
INDICATED THAT THE CENTRAL PRESSURE OF IVAN WAS STILL 923 MB...BUT
THAT THE MAXIMUM FLIGHT-LEVEL WINDS HAD DECREASED TO NEAR 130 KT.
BASED MAINLY ON THE PRESSURE...THE INITIAL INTENSITY REMAINS A
POSSIBLY GENEROUS 130 KT FOR THIS PACKAGE.  THE AIRCRAFT DATA
INDICATE THAT THE OUTER WIND MAXIMA MENTIONED IN THE PREVIOUS
ADVISORY WAS CONTRACTING...AND THE LATEST SATELLITE IMAGERY
SUGGESTS THAT IVAN MAY BE COMPLETING THE CURRENT CONCENTRIC EYEWALL
CYCLE.  HOWEVER...THE AIRCRAFT DATA ALSO INDICATES ANOTHER WIND
MAXIMA FORMING OVER THE NORTHERN SEMICIRCLE...WHICH MAY BE THE
PRECURSOR OF THE NEXT CONCENTRIC EYEWALL CYCLE.

THE INITIAL MOTION IS NOW 300/11.  THERE IS HIGH CONFIDENCE IN THE
FIRST 36-48 HR OF THE FORECAST TRACK...AS THE SUBTROPICAL RIDGE TO
THE NORTHEAST OF IVAN SHOULD STEER IT GENERALLY WEST-NORTHWESTWARD.
CONFIDENCE DIMINISHES BEYOND THAT TIME AS THE RIDGE WEAKENS DUE TO
AN UPPER-LEVEL TROUGH OVER THE EASTERN UNITED STATES AND AN
UPPER-LEVEL LOW OVER THE ATLANTIC CURRENTLY NEAR 31N52W.  ALL
GUIDANCE AGREES ON A TURN TOWARD THE NORTH...WITH DIFFERENCES ON
HOW FAR EAST THAT TURN WILL OCCUR.  THE GFS WEAKENS THE RIDGE THE
MOST AND BRINGS IVAN NORTHWARD THROUGH THE FLORIDA PENINSULA.  THE
UKMET AND NOGAPS KEEP MORE RIDGING NORTH OF IVAN AND THUS KEEP A
MORE NORTH-NORTHWESTWARD MOTION.  THE NOGAPS AND GFS HAVE BOTH
SHIFTED A LITTLE TO THE LEFT...WHILE THE GFDL HAS SHIFTED TO THE
RIGHT.  THE OFFICIAL TRACK FORECAST IS BASICALLY AN UPDATE ON THE
PREVIOUS PACKAGE...SHIFTING SLIGHTLY TO THE RIGHT AFTER 72 HR TO
BRING IT IN BEST AGREEMENT WITH THE GUNS AND CONU CONSENSUS MODELS.

THE INTENSITY FORECAST IS PROBLEMATIC.  THE BIGGEST CONTROLLING
FACTOR THROUGH THE FIRST 72 HR IS LIKELY TO BE CONCENTRIC EYEWALL
CYCLES.  THE INTENSITY FORECAST ASSUMES THAT IVAN WILL COMPLETE THE
CURRENT CYCLE BEFORE IT REACHES JAMAICA AND THUS INTENSIFY BEFORE
HITTING THE ISLAND.  FURTHER INTENSIFICATION IS LIKELY OVER THE
WARM WATER BETWEEN JAMAICA AND CUBA IF THE STORM STRUCTURE REMAINS
GOOD.  ON THE OTHER HAND...THE GFS...NOGAPS...AND SHIPS ALL
FORECAST SIGNIFICANT SHEAR TO DEVELOP ACROSS IVAN STARTING IN ABOUT
48 HR...WHICH COULD SLOW OR STOP DEVELOPMENT IN SPITE OF THE WARM
WATER.  GIVEN THE CURRENT IMPRESSIVE OUTFLOW PATTERN...48 HR MIGHT
BE TOO SOON.  THE INTENSITY FORECAST WILL DELAY THE ONSET OF
SIGNIFICANT SHEAR UNTIL 72 HR AND HOLDS THE INTENSITY UP
ACCORDINGLY.

FORECASTER BEVEN

FORECAST POSITIONS AND MAX WINDS

INITIAL      10/0300Z 15.5N  73.3W   130 KT
 12HR VT     10/1200Z 16.5N  74.8W   130 KT
 24HR VT     11/0000Z 17.7N  76.6W   135 KT
 36HR VT     11/1200Z 18.9N  78.3W   130 KT
 48HR VT     12/0000Z 19.9N  79.6W   140 KT
 72HR VT     13/0000Z 22.0N  81.5W   140 KT
 96HR VT     14/0000Z 25.5N  82.5W   120 KT
120HR VT     15/0000Z 29.5N  83.0W    95 KT...INLAND


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