[Tropical] Tropical Cyclone Discussion

[Nashville EMWIN]

WTNT44 KNHC 092030
TCDAT4
HURRICANE IVAN DISCUSSION NUMBER  30
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
5 PM EDT THU SEP 09 2004

BOTH NOAA AND AIR FORCE RECONNAISSANCE PLANES HAVE BEEN IN THE EYE
OF IVAN DURING THE PAST FEW HOURS. MINIMUM PRESSURE REMAINS AT
ABOUT 921 MB BUT MAXIMUM FLIGHT LEVEL WINDS HAVE DECREASED TO 144
KNOTS.  THIS SUGGESTS THAT INITIAL INTENSITY CAN BE LOWERED TO 130
KNOTS AT THIS TIME.  THERE WERE SOME INDICATIONS FROM MICROWAVE
DATA AND NOW CONFIRMED WITH THE PLANE THAT IVAN HAS A DOUBLE
EYEWALL STRUCTURE. THIS MAY EXPLAIN THE RECENT SLIGHT WEAKENING.
THE HURRICANE WILL PROBABLY FLUCTUATE BETWEEN CAT 4 AND 5 WHILE
MOVING THROUGH AN AREA OF LOW SHEAR AND HIGH OCEANIC HEAT CONTENT
BEFORE REACHING CUBA. THEREAFTER...THE EFFECTS OF LAND AND
INCREASING SHEAR WILL CAUSE A GRADUAL WEAKENING.

IVAN CONTINUES TO MOVE TOWARD THE WEST-NORTHWEST OR 300 DEGREES AT
13 KNOTS.  THIS GENERAL MOTION WITH A GRADUAL DECREASE IN FORWARD
SPEED IS EXPECTED DURING THE NEXT 2 TO 3 DAYS.  AS MENTIONED IN THE
PREVIOUS DISCUSSION...THERE IS HIGH CONFIDENCE IN THIS PORTION OF
THE FORECAST BECAUSE THE HURRICANE WILL BE STEERED BY THE FLOW
SURROUNDING A WELL-ESTABLISHED SUBTROPICAL RIDGE. ALL MODELS ARE
HIGHLY CLUSTERED DURING THIS PERIOD...BRINGING THE HURRICANE
NEAR OR OVER JAMAICA...THE CAYMAN ISLANDS AND WESTERN CUBA WITH A
DECREASE IN FORWARD SPEED. THEREAFTER...THE TRACK CONTINUES TO BE
UNCERTAIN SINCE STEERING CURRENTS ARE FORECAST TO WEAKEN AND MODELS
DIVERGE. HOWEVER...MODELS FROM 12Z ARE IN SLIGHTLY BETTER AGREEMENT
THAN IN EARLIER RUNS...AND IN GENERAL HAVE SHIFTED A LITTLE BIT
WESTWARD. THE OFFICIAL FORECAST ALSO SHOWS A SLIGHT WESTWARD SHIFT.

THE WIND RADII WERE ADJUSTED BASED ON THE STEPPED FREQUENCY
MICROWAVE RADIOMETER DATA ONBOARD THE NOAA P3 PLANE CURRENTLY
INVESTIGATING IVAN.

FORECASTER AVILA

FORECAST POSITIONS AND MAX WINDS

INITIAL      09/2100Z 15.0N  72.5W   130 KT
 12HR VT     10/0600Z 16.1N  74.2W   130 KT
 24HR VT     10/1800Z 17.5N  76.2W   135 KT
 36HR VT     11/0600Z 18.8N  78.0W   130 KT
 48HR VT     11/1800Z 19.7N  79.3W   140 KT
 72HR VT     12/1800Z 21.5N  81.5W   140 KT
 96HR VT     13/1800Z 25.0N  83.0W   115 KT
120HR VT     14/1800Z 28.0N  83.5W   100 KT


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