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TROPICAL STORM ISAAC DISCUSSION NUMBER   7
NATIONAL WEATHER SERVICE MIAMI FL
11 PM EDT FRI SEP 22 2000
 
ISAAC HAS CONTINUED TO GET BETTER ORGANIZED THIS EVENING WITH A
SMALL CDO FEATURE HAVING DEVELOPED OVER THE SYSTEM CENTER DURING THE
PAST 4 HOURS.  IN FACT...A 2311Z SSMI OVERPASS INDICATED A 90
PERCENT CLOSED...10NM DIAMETER EYE-LIKE FEATURE EMBEDDED IN THE
CENTRAL DEEP CONVECTION.  THE INITIAL INTENSITY WAS INCREASED TO 
55 KT BASED ON THE SSMI POSSIBLE EYE FEATURE AND SATELLITE INTENSITY
ESTIMATES OF 45 TO 50 KT FROM SAB AND TAFB.  THE OUTFLOW CONTINUES
TO IMPROVE AND ALSO EXPAND TO THE WEST.

THE INITIAL MOTION ESTIMATE IS 280/11...WHICH IS GENEROUS WITH THE
FORWARD SPEED.  ISAAC MAY HAVE SLOWED ENOUGH TO FINALLY ALLOW THE
INNER CORE WIND FIELD TO CONSOLIDATE AND BECOME DEEPER IN THE
VERTICAL.  THE NEW MODEL GUIDANCE FOR THIS PACKAGE HAS SHIFTED
FURTHER SOUTH AND MORE WESTWARD THAN THE PREVIOUS PACKAGE.  IN
FACT...THE GFDL MODEL ACTUALLY TAKES ISAAC SOUTH OF DUE WEST AFTER
24 HOURS.  WHILE I AM NOT READY TO BUY OFF ON THAT TYPE OF MOTION...
THE GFDL USUALLY HANDLES DEEP VERTICAL SYSTEMS QUITE WELL...SO THE
OVERALL TREND OF MORE WESTWARD AND LESS POLEWARD MOTION WAS 
STRONGLY CONSIDERED.  THE OFFICIAL TRACK IS A LITTLE SOUTH OF THE
PREVIOUS TRACK AND BETWEEN THE LBAR AND GUNS MODELS.  THIS SCENARIO
IS REASONABLE BASED ON THE SYNOPTIC-SCALE UPPER-LEVEL TROUGH TO THE
WEST OF ISAAC BEGINNING TO SHOW SIGNS OF WEAKENING AND LIFTING OUT
TO THE NORTH.  THIS SHOULD ALLOW THE SUBTROPICAL RIDGE TO THE NORTH
TO REMAIN INTACT AND EVEN STRENGTHEN... WHICH SHOULD ACT TO KEEP
ISAAC ON A MORE WESTERLY COURSE.

AS THE PREVIOUS FORECAST ALLUDED TO...ISAAC APPEARS TO ALREADY HAVE
THE STRUCTURE TO BECOME A HURRICANE...AND POSSIBLY A SIGNIFICANT ONE
AT THAT. HOWEVER...THE INITIAL INTENSITY AND INTENSIFICATION WAS NOT
INCREASED MUCH OVER THE PREVIOUS FORECAST.  THIS IS DUE TO THE SMALL
EYE-LIKE FEATURE BEING SUSCEPTIBLE TO MINOR AMOUNTS OF SHEAR WHICH
CAN EASILY DISRUPT THE SMALL INNER CORE CONVECTION.  HOWEVER...GIVEN
THE ALREADY WELL-ESTABLISHED OUTFLOW PATTERN...THEN THE OFFICIAL
FORECAST MAY BE TOO LOW IF THE SMALL EYE-FEATURE PERSISTS AND
ACTUALLY BECOMES A WELL-ESTABLISHED EMBEDDED EYE DRING THE NEXT 6 TO
12 HOURS.  COOLER SSTS AHEAD WOULD LIKELY PRECLUDE ISAAC FROM
BECOMING A MAJOR HURRICANE DURING THIS FORECAST PERIOD.
 
NOTE: THERE CONTINUES TO BE AN UNNERVING SHIFT IN THE LONGER RANGE
PROGNOSIS WITH THE VARIOUS GFDL MODELS AND THE NOGAPS MODEL TRENDING
MORE TOWARD A WESTWARD TRACK BY 72 HOURS...WHICH IS VERY SIMILAR TO
THE 22/00Z MRFX RUN LAST NIGHT.  SOME EFFECTS BY ISAAC ON THE LESSER
ANTILLES...WHILE UNLIKELY...CANNOT BE RULED OUT AT THIS TIME.
 
FORECASTER STEWART
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     23/0300Z 13.8N  31.7W    55 KTS
12HR VT     23/1200Z 14.1N  33.5W    60 KTS
24HR VT     24/0000Z 14.6N  36.0W    65 KTS
36HR VT     24/1200Z 15.0N  38.5W    70 KTS
48HR VT     25/0000Z 15.5N  41.0W    75 KTS
72HR VT     26/0000Z 16.0N  46.0W    80 KTS
 
 
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