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HURRICANE ISAAC DISCUSSION NUMBER  18
NATIONAL WEATHER SERVICE MIAMI FL
5 PM EDT MON SEP 25 2000

AN ILL-DEFINED EYE HAS BEEN NOTED ON IR IMAGES...ALONG WITH A DIMPLE
FEATURE ON VISIBLE PICTURES.  SATELLITE INTENSITY ESTIMATES ARE
STILL CLOSE TO 90 KNOTS...WHICH IS USED FOR THE CURRENT INTENSITY. 
WATER VAPOR IMAGES CONTINUE TO SHOW SOME WESTERLIES AHEAD OF THE
SYSTEM.  HOWEVER ANALYSES FROM THE UNIVERSITY OF WISCONSIN CIMSS
SHOW DECREASING SHEAR NORTHWEST OF ISAAC.  IT REMAINS TO BE SEEN
WHETHER THE OUTFLOW...WHICH IS CURRENTLY POOR OVER THE SOUTHERN HALF
OF THE TROPICAL CYCLONE...WILL IMPROVE WITH TIME.  THE INTENSITY
FORECAST IS UNCHANGED FROM THE PREVIOUS ADVISORY...AND SHOWS NO
SIGNIFICANT LOSS OF STRENGTH THROUGH THE PERIOD.

MOTION ESTIMATE REMAINS ABOUT THE SAME AS BEFORE...300/9.  GLOBAL
MODELS CONTINUE TO DEPICT ISAAC MOVING ALONG THE SOUTHERN AND
SOUTHWESTERN PERIPHERY OF THE LARGE SUBTROPICAL ANTICYCLONE OVER THE
CENTRAL ATLANTIC.  BY THE END OF THE FORECAST PERIOD...THE HURRICANE
SHOULD BE NEAR THE WESTERN END OF THE ANTICYCLONE...AND ABOUT READY
TO MAKE A SIGNIFICANT TURN TOWARD THE RIGHT.  THE OFFICIAL FORECAST
ONLY HINTS AT THIS...AND IS CLOSE TO THE LATEST GFDL MODEL SOLUTION.

THE U.K. MET OFFICE MODEL MOVES ISAAC SOMEWHAT FASTER IN 2 TO 3 DAYS
BUT I AM RELUCTANT TO PICK UP THE FORWARD SPEED TOO MUCH. 
 
FORECASTER PASCH
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     25/2100Z 17.8N  41.5W    90 KTS
12HR VT     26/0600Z 18.5N  42.8W    90 KTS
24HR VT     26/1800Z 19.5N  44.6W    90 KTS
36HR VT     27/0600Z 20.6N  46.8W    90 KTS
48HR VT     27/1800Z 21.8N  49.0W    85 KTS
72HR VT     28/1800Z 24.5N  53.5W    85 KTS
 
 
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