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TROPICAL STORM ISAAC DISCUSSION NUMBER 6
NATIONAL WEATHER SERVICE MIAMI FL
5 PM EDT FRI SEP 22 2000
THIS AFTERNOON ISAAC HAS BEEN BUILDING A FOUNDATION FOR FUTURE
STRENGTHENING...AS IT HAS CONSOLIDATED A HEALTHY CONVECTIVE
STRUCTURE THROUGH THE DIURNAL MINIMUM. OUTFLOW REMAINS GOOD TO THE
EAST AND NORTH. DVORAK INTENSITY ESTIMATES REMAIN 35 KT FROM KGWC
AND 45 KT FROM TAFB AND SAB...AND SO THE ADVISORY INTENSITY REMAINS
45 KT. WITH THE IMPROVED STRUCTURE STRENGTHENING SHOULD BEGIN SOON
AND ISAAC IS FORECAST TO BE A HURRICANE IN 24 HOURS. BEYOND
THAT...THE INTENSITY GUIDANCE IS NOT AS ENTHUSIASTIC AS EARLIER...
WITH A HINT THAT THE UPPER TROUGH AHEAD OF THE CYCLONE WILL BE
INCREASING THE VERTICAL SHEAR. THE INTENSITY FORECAST...AS
BEFORE...CALLS FOR LITTLE CHANGE IN STRENGTH LATE IN THE FORECAST
PERIOD UNTIL THE FUTURE OF THE UPPER TROUGH BECOMES MORE CLEAR.
THERE HAS BEEN NO CHANGE TO THE SHORT TERM FORECAST PHILOSPHY.
ISAAC IS EMBEDDED IN THE EASTERLIES TO THE SOUTH OF A MID-
TROPOSPHERIC RIDGE. THIS RIDGE IS FORECAST TO MAINTAIN ITSELF AND
MOVE WESTWARD WITH THE CYCLONE OVER THE NEXT THREE DAYS. THE MODEL
GUIDANCE RESPONDS TO THIS WITH A STEADY WEST-NORTHWESTWARD TRACK
THROUGH THE FORECAST PERIOD. THE OFFICIAL FORECAST IS SIMILAR TO
THE PREVIOUS ADVISORY AND IS CLOSE TO THE CONSENSUS OF THE GFDL...
UKMET...AND NOGAPS.
THERE HAS BEEN SOMETHING OF A SHIFT IN THE LONGER RANGE PROGNOSIS...
HOWEVER. THE LATEST GFDL DOES NOT SHOW AS STRONG A NORTHWARD TURN
IN THE FOUR TO FIVE DAY PERIOD AS THE PREVIOUS RUN...AND THE NOGAPS
HAS SHIFTED IN A SIMILAR FASHION. SOME IMPACT ON THE LESSER
ANTILLES...WHILE UNLIKELY...CANNOT BE RULED OUT AT THIS TIME.
FORECASTER FRANKLIN
FORECAST POSITIONS AND MAX WINDS
INITIAL 22/2100Z 13.8N 31.0W 45 KTS
12HR VT 23/0600Z 14.4N 33.4W 55 KTS
24HR VT 23/1800Z 15.0N 36.1W 65 KTS
36HR VT 24/0600Z 15.5N 38.4W 70 KTS
48HR VT 24/1800Z 16.0N 40.5W 75 KTS
72HR VT 25/1800Z 17.0N 45.0W 75 KTS
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