Tropical Storm IVAN

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TROPICAL STORM IVAN DISCUSSION NUMBER   7
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
5 AM EDT SAT SEP 04 2004
 
THE SATELLITE SIGNATURE ASSOCIATED WITH IVAN HAS IMPROVED OVERNIGHT
AND DVORAK INTENSITY ESTIMATES ARE 45 KT AND 55 KT FROM TAFB AND
SAB RESPECTIVELY. THE OFFICIAL INTENSITY SPLITS THE DIFFERENCE AT
50 KT. UNIVERSITY OF WISCONSIN-CIMSS SATELLITE ANALYSIS CONTINUES
TO SHOW MODERATE VERTICAL SHEAR OVER IVAN. HOWEVER...SATELLITE
IMAGERY INDICATES THAT THE SHEAR IS NOT DISRUPTING UPPER-LEVEL
OUTFLOW WHICH IS UNOBSTRUCTED IN ALL QUADRANTS INCLUDING TO THE
EAST.  IVAN HAS BEEN STEADILY STRENGTHENING AND THE SHEAR IS
FORECAST TO DECREASE. THUS...THERE ARE NO APPARENT REASONS WHY IVAN
SHOULD NOT CONTINUE STRENGTHENING DURING THE FORECAST PERIOD. THE
SHIPS MODEL BRINGS THE WIND SPEED TO NEAR 100 KT IN 120 HOURS WHILE
THE GFDL CONTINUES TO FORECAST MORE VIGOROUS STRENGTHENING TO 135
KT. THUS FAR...THE SHIPS HAS BEEN THE BETTER PERFORMER OF THE TWO
MODELS AND THE OFFICIAL FORECAST FOLLOWS IT CLOSELY.
 
IVAN APPEARS TO BE MOVING ALONG AS EXPECTED AND THE INITIAL MOTION
IS 270/16. THERE HAS BEEN LITTLE CHANGE IN THE OVERALL FORECAST
PHILOSOPHY AS THE GLOBAL MODELS CONTINUE TO SHOW A SUBTROPICAL RIDGE
BUILDING WESTWARD WITH IVAN. THIS WOULD TEND TO STEER IVAN ON A
GENERAL WEST-NORTHWEST MOTION BETWEEN 15-17 KNOTS. BEYOND 72
HOURS...GLOBAL MODELS SHOW IVAN APPROACHING A WEAKNESS IN THE
SUBTROPICAL RIDGE WHICH WOULD ALLOW A MORE POLEWARD MOTION AND
SLIGHTLY SLOWER SPEED. IT IS INTERESTING TO NOTE THAT THE NOGAPS IS
NOW IN MUCH BETTER AGREEMENT WITH THE OTHER DYNAMICAL MODELS AND
THE NHC MODEL SUITE IS MORE TIGHTLY CLUSTERED.  THE GFS IS SOMEWHAT
OF AN OUTLIER AS IT WEAKENS IVAN EAST OF THE LESSER ANTILLES AND
THEN MOVES IT MORE WESTWARD. GIVEN THE CURRENT TREND OF
INTENSIFICATION...THIS SOLUTION IS HARD TO ACCEPT. ONLY MINOR
CHANGES ARE MADE TO THE OFFICIAL FORECAST WHICH ESSENTIALLY FOLLOWS
THE MODEL CONSENSUS.  ON THIS TRACK..IVAN WOULD AFFECT THE LESSER
ANTILLES IN 4 TO 5 DAYS.
 
FORECASTER RHOME/JARVINEN
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      04/0900Z  9.0N  37.4W    50 KT
 12HR VT     04/1800Z  9.1N  39.8W    55 KT
 24HR VT     05/0600Z  9.7N  43.0W    60 KT
 36HR VT     05/1800Z 10.5N  46.2W    65 KT
 48HR VT     06/0600Z 11.4N  49.4W    75 KT
 72HR VT     07/0600Z 13.0N  55.5W    85 KT
 96HR VT     08/0600Z 14.5N  61.0W    95 KT
120HR VT     09/0600Z 17.0N  65.6W   100 KT
 
 
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