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TROPICAL STORM ISAAC DISCUSSION NUMBER   3
NATIONAL WEATHER SERVICE MIAMI FL
11 PM EDT THU SEP 21 2000
 
TROPICAL DEPRESSION THIRTEEN HAS BECOME BETTER ORGANIZED THIS
EVENING AND HAS STRENGTHENED INTO TROPICAL STORM ISAAC.  BANDING
FEATURES HAVE IMPROVED SIGNIFCANTLY DURING THE PAST 6 HOURS...AND
THE OUTFLOW PATTERN HAS BECOME BETTER DEVELOPED AND MORE CIRCULAR. 
A PRONOUNCED POLEWARD OUTFLOW CHANNEL HAS ALSO DEVELOPED.  SATELLITE
INTENSITY ESTIMATES FROM TAFB AND SAB SUPPPORT AT LEAST A 35 KT
INITIAL INTENSITY.

INITIAL MOTION IS 285/13.  NOW THAT ISAAC HAS DEVELOPED INTO A
DEEPER VERTICAL SYSTEM...IT SHOULD BE STEERED MORE BY THE DEEP LAYER
FLOW WHICH TAKES THE SYSTEM OFF TO THE WEST-NORTHWEST THROUGH THE
PERIOD.  THE UKMET MODEL IS SIMILAR TO THE PREVIOUS FORECAST
TRACKS...ONLY SLOWER...AND THE GFDL IS TO THE RIGHT OF THE OFFICIAL
TRACK.  BY 72 HOURS...THE GLOBAL MODELS DIFFER IN THE POSITION OF
THE UPPER-LEVEL LOW CURRENTLY NEAR 55W LONGITUDE...AND ALSO THE
DEVELOPMENT OF THE UPPER-LEVEL RIDGE TO THE NORTH OF ISAAC.  ALL
THREE MODELS AGREE ON MOVING THE UPPER-LEVEL LOW TO THE WEST  AND
GRADUALLY WEAKENING IT.  THE UKMET KEEPS THE UPPER-LEVEL HIGH TO THE
NORTHEAST OF ISAAC...WHILE THE AVN MODEL DEVELOPS THE HIGH TO THE
NORTHWEST AND BRINGS NORTHWESTLY FLOW ON THE SYSTEM BY 72 HOURS. THE
NOGAPS MODEL IS IN BETWEEN THE AVN AND UKMET SOLUTIONS.  IF THE AVN
MODEL VERIFIES...THEN THE NORTHERLY FLOW WOULD TEND TO TURN ISAAC
MORE WESTERLY AFTER 60 TO 72 HOURS.  THE OFFICIAL FORECAST IS IN THE
SOUTHERN PORTION OF THE FORECAST GUIDANCE ENVELOPE AND ALMOST ON TOP
OF THE PREVIOUS FORECAST.
 
ISSAC MAY BE THE ONLY TRUE DEEP-TROPICS TROPICAL CYCLONE SO FAR THIS
YEAR WITHOUT ENCOUNTERING ADVERSE SHEAR ACROSS THE MID-ATLANTIC.  AS
PREVIOUSLY MENTIONED...ALL OF THE GLOBAL MODELS AGREE ON MOVING THE
UPPER-LEVEL LOW/TROUGH ALONG 55W WESTWARD AND WEAKENING IT. 
ALSO...THE DISTURBANCE WEST OF ISSAC CURRENTLY ALONG 40W IS ACTING
TO DEFLECT SOME OF THE WESTERLY SHEAR WELL NORTH OF ISAAC...WHICH
MAY HAVE ALLOWED THE POLEWARD OUTFLOW CHANNEL TO DEVELOP.  THE
INTENSITY FORECAST IS HIGHER AND THE INTENSIFICATION RATE FASTER
THAN THE PREVIOUS FORECAST...BUT THE OFFICIAL FORECAST IS STILL 
BELOW THE SHIPS INTENSITY MDOEL.  HOWEVER...WITH SUCH A WELL-
ORGANIZED CONVECTIVE PATTERN AND POLEWARD OUTFLOW CHANNEL...RAPID
INTENSIFICATION AND HIGHER INTENSITIES MAY HAVE TO BE CONSIDERED ON
THE NEXT ADVISORY...IF THE DEEP CONVECTION PERSISTS NEAR THE CENTER.

FORECASTER STEWART
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     22/0300Z 12.5N  26.2W    35 KTS
12HR VT     22/1200Z 13.0N  28.2W    45 KTS
24HR VT     23/0000Z 13.6N  30.7W    55 KTS
36HR VT     23/1200Z 14.1N  33.3W    65 KTS
48HR VT     24/0000Z 14.5N  36.0W    70 KTS
72HR VT     25/0000Z 15.5N  41.0W    75 KTS
 
 
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