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TROPICAL STORM ISAAC DISCUSSION NUMBER   4
NATIONAL WEATHER SERVICE MIAMI FL
5 AM EDT FRI SEP 22 2000
 
TROPICAL STORM ISAAC HAS CONTINUED TO GET BETTER ORGANIZED AND
STRENGTHEN THIS MORNING. THE INITIAL INTENSITY OF 45 KT IS BASED ON
A SATELLITE INTENSITY ESTIMATE OF 45 KT FROM BOTH TAFB AND SAB. 
BANDING FEATURES AND OVERALL CONVECTIVE ORGANIZATION CONTINUES TO
IMPROVE...AND THE OUTFLOW PATTERN IS GOOD TO EXCELLENT TO THE NORTH
AND FAIR ELSEWHERE.  THE POLEWARD OUTFLOW CHANNEL ALSO REMAINS
INTACT AND IS EXPANDING TO THE WEST OF ISAAC.
 
INITIAL MOTION IS 280/17.  ISAAC HAS BECOME A MUCH MORE VERTICALLY
DEEP SYSTEM THAN THE GLOBAL MODELS SEEM TO BE INDICATING.  THE
FORWARD MOTION HAS INCREASED THE PAST 6 HOURS POSSIBLY DUE TO THE
INCREASED DEEP LAYER STEERING FLOW AS INDICATED IN THE UW-CIMMS
WATER VAPOR WIND ANALYSES.  HOWEVER...THE MODEL GUIDANCE IS MUCH
MORE TIGHTLY CLUSTERED THAN THE PREVIOUS THREE RUNS AND GENERALLY
AGREE ON A WEST OR SLIGHTLY WEST-NORTHWEST DIRECTION THROUGHOUT THE
PERIOD.  THE OFFICIAL TRACK FORECAST IS A LITTLE SOUTH OF THE
PREVIOUS TRACK AND FASTER THROUGH 24 HOURS WITH A GRADUAL DECREASE
IN FORWARD SPEED AFTERWARDS.  THE NEW 22/00Z AVN MODEL SEEMS TO HAVE
A BETTER HANDLE ON ISAAC THIS MODEL RUN AND WAS FOLLOWED CLOSELY
ALONG WITH LBAR.
 
THERE IS NO APPARENT SHEAR FORECAST TO AFFECT ISAAC...UNLESS IT
DEVELOPS FROM THE EAST BASED ON THE RECENT INCREASE IN FORWARD
SPEED.  THE OUTFLOW PATTERN REMAINS IMPRESSIVE AND IMPROVING...WHICH
FURTHER SUPPORTS STRENGTHENING.  THE ONLY POSSIBLE INHIBITING FACTOR
IS THE DRY AIR THAT LIES AHEAD OF THE CYCLONE.  IF ISAAC OUTRUNS THE
CONVECTIVE DEVELOPMENT ON THE WEST SIDE OF THE CIRCULATION...THEN
THE STORM-RELATIVE INFLOW WILL ENTRAIN DRY AIR INTO THE SYSTEM AND
CAUSE ISAAC TO WEAKEN. DUE TO THIS UNCERTAINTY...THE INTENSIFICATION
RATE WAS SLOWED AFTER 12 HOURS AND KEPT BELOW THE SHIPS INTENSITY
MODEL WHICH BRINGS ISAAC TO 91 KT IN 72 HOURS.
 
FORECASTER STEWART
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     22/0900Z 12.7N  28.2W    45 KTS
12HR VT     22/1800Z 13.2N  30.5W    55 KTS
24HR VT     23/0600Z 13.6N  33.2W    60 KTS
36HR VT     23/1800Z 14.1N  36.2W    65 KTS
48HR VT     24/0600Z 14.4N  38.5W    75 KTS
72HR VT     25/0600Z 15.0N  43.5W    85 KTS
 
 
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