Tropical Storm ISAAC

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TROPICAL STORM ISAAC DISCUSSION NUMBER   8
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL092006
1100 AM EDT FRI SEP 29 2006
 
SATELLITE IMAGES INDICATE THAT ISAAC IS MAINTAINING ITS SMALL CORE
WITH MODERATE CONVECTION IN BANDS AROUND THE CENTER. A QUIKSCAT
PASS AT 0958 UTC THIS MORNING SHOWED THAT THE MAXIMUM WINDS OF
ISAAC REMAIN AROUND 40 KT. QUIKSCAT ALSO INDICATES THE STORM HAS
DECREASED SOMEWHAT IN SIZE AND THE WIND RADII ARE ADJUSTED INWARD.
THERE ARE NO CHANGES TO THE INTENSITY FORECAST PHILOSOPHY THIS
MORNING. LATER TODAY THE TROPICAL CYCLONE WILL BE MOVING OVER
WARMER WATER AND EXPERIENCING ONLY MODEST VERTICAL WIND SHEAR.
THESE FACTORS... PLUS INCREASING MOISTURE ALOFT BASED ON THE SHIPS
MODEL... SHOULD HELP INITIATE A SLOW INTENSIFICATION TREND. BY 36
TO 48 HOURS... SHEAR IS FORECAST TO INCREASE AND LIMIT
STRENGTHENING. THE INTENSITY FORECAST IS BASICALLY THE SAME AS
BEFORE. HOWEVER... IT IS WORTH NOTING THAT SHIPS AND GFDL INTENSIFY
THE CYCLONE MORE THAN THE OFFICIAL FORECAST AFTER RECURVATURE. THIS
SCENARIO SEEMS UNLIKELY GIVEN THE STRONG SHEAR FORECAST AND SHARPLY
DECREASING SSTS.
 
ISAAC HAS CONTINUED TO TURN MORE WESTWARD AND IS NOW MOVING 290/5.
THE BIGGEST QUESTION IN THE TRACK FORECAST IS HOW LONG DOES THE
WEST-NORTHWESTWARD MOTION CONTINUE BEFORE A TURN BACK TOWARD THE
NORTHWEST BEGINS. THE ECMWF AND NOGAPS MODELS SUGGEST RIDGING WILL
HOLD NORTH OF THE STORM FOR ABOUT 48 HOURS BEFORE STARTING A MORE
NORTHWARD MOTION. THE UKMET CONTINUES TO BE ON THE RIGHT-HAND SIDE
OF THE GUIDANCE BY SUGGESTING A NORTH-NORTHWEST MOTION IN AS LITTLE
AS 24 HOURS. IN GENERAL... GLOBAL MODELS HAVE BEEN TRENDING A
LITTLE TO THE LEFT AND THE OFFICIAL FORECAST IS NUDGED IN THAT
DIRECTION... A TOUCH TO THE WEST OF THE PREVIOUS FORECAST. THE
FORECAST TRACK DURING THE EXTRATROPICAL STAGE REMAINS UNCERTAIN DUE
TO SOME SPREAD IN THE TRACK GUIDANCE. THE COMPUTER MODELS THAT TAKE
THE STORM FARTHEST TO THE WEST ALSO MAKE ISAAC EXTRATROPICAL SOONER
AS IT INTERACTS WITH A DEEP TROUGH IN THE WEST ATLANTIC OCEAN. IN
ADDITION THESE WESTWARD MODELS AND THE GFS ALSO ABSORB ISAAC INTO A
LARGER MID-LATITUDE LOW IN ABOUT 3 DAYS. FOR NOW ISSAC IS FORECAST
TO REMAIN SEPARATE FROM THE OTHER EXTRATROPICAL LOW. HOWEVER IF THE
WESTWARD TREND CONTINUES... THEN THE REMNANTS OF ISAAC COULD BECOME
INDISTINCT FROM THE MID-LATITUDE LOW BY 96 HOURS.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      29/1500Z 29.4N  56.2W    40 KT
 12HR VT     30/0000Z 29.9N  57.1W    45 KT
 24HR VT     30/1200Z 30.7N  58.3W    45 KT
 36HR VT     01/0000Z 31.9N  59.6W    50 KT
 48HR VT     01/1200Z 33.8N  60.5W    50 KT
 72HR VT     02/1200Z 41.6N  58.4W    50 KT...EXTRATROPICAL
 96HR VT     03/1200Z 47.0N  49.0W    50 KT...EXTRATROPICAL
120HR VT     04/1200Z 49.0N  37.0W    50 KT...EXTRATROPICAL
 
$$
FORECASTER BLAKE/STEWART
 
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