Tropical Storm ISAAC

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TROPICAL STORM ISAAC DISCUSSION NUMBER  11
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL092006
500 AM AST SAT SEP 30 2006
 
ISAAC LOOKS MUCH MORE TROPICAL THAN IT DID 24 HR AGO...WITH
CONVECTIVE TOPS COLDER THAN -60C NOT FAR NORTHWEST OF THE CENTER AND
THE FIRST SIGNS OF A 30-35 NM WIDE EYE.  THE CONVECTION IS NOT YET
WRAPPED AROUND THE CENTER ENOUGH FOR THE DVORAK SATELLITE INTENSITY
ESTIMATES TO REACH HURRICANE STRENGTH...SO ISAAC REMAINS A 60 KT
TROPICAL STORM FOR THIS ADVISORY.  THE STORM CURRENTLY HAS FAIR TO
GOOD CIRRUS OUTFLOW IN THE NORTHERN SEMICIRCLE.

THE INITIAL MOTION IS 300/6.  WATER VAPOR IMAGERY SHOWS A LARGE
DEEP-LAYER TROUGH OVER THE EASTERN UNITED STATES AND THE ADJACENT
WESTERN ATLANTIC...WITH AN EMBEDDED SHORTWAVE TROUGH MOVING
SOUTHEASTWARD THROUGH THE OHIO VALLEY.  A MID/UPPER-LEVEL
ANTICYCLONE IS CENTERED NEAR 36N56W WITH A SHORTWAVE RIDGE
EXTENDING NORTHWARD TO NEWFOUNDLAND.  THE LARGE-SCALE MODELS
FORECAST THE U. S. TROUGH TO MOVE EASTWARD AND INTENSIFY OVER THE
WESTERN ATLANTIC DURING THE NEXT THREE DAYS AS AN ASSOCIATED
SURFACE LOW FORMS.  THIS SHOULD TURN ISAAC NORTHWARD BY 36 HR AND
NORTH-NORTHEASTWARD BY 72 HR IN AGREEMENT WITH THE TRACK GUIDANCE. 
THE NEW FORECAST TRACK IS SIMILAR TO THE PREVIOUS TRACK DURING THE
FIRST 36 HR...THEN SHIFTED A LITTLE WESTWARD THEREAFTER.  IT IS IN
BEST AGREEMENT WITH THE CONSENSUS MODELS.  DEPENDING ON HOW ISAAC
INTERACTS WITH THE LOW...THERE IS A CHANCE IT COULD MOVE MORE
NORTHWARD AFTER 48 HR THAN CURRENTLY FORECAST.

THE LARGE-SCALE MODELS FORECAST ISAAC TO REMAIN IN A LIGHT TO
MODERATE VERTICAL SHEAR ENVIRONMENT FOR ABOUT 36 HR.  THE SHIPS
MODELS FORECAST A 65 KT INTENSITY IN 36 HR...THE FSU SUPERENSEMBLE
A 70 KT INTENSITY IN 48 HR...AND THE GFDL A 90 KT INTENSITY IN 48
HR.  BASED ON THIS...THE INTENSITY FORECAST IS NUDGED UPWARD A
LITTLE FROM THE PREVIOUS ADVISORY.  AFTER 36-48 HR...INCREASING
SHEAR AND INTERACTION WITH DEEP-LAYER TROUGH SHOULD LEAD TO
WEAKENING AND EXTRATROPICAL TRANSITION.  ONE DISAGREEMENT IN THE
MODELS IS HOW LONG ISAAC WILL MAINTAIN ITS IDENTITY.  THE GFS
ABSORBS THE STORM INTO THE BAROCLINIC LOW IN ABOUT 48 HR...WHILE
THE OTHER MODELS HOLD ON TO IT FOR ABOUT 72 HR.  BASED ON THIS...
THE TIME OF BEING ABSORBED INTO THE LOW WILL BE MOVED UP A DAY FROM
THE PREVIOUS ADVISORY.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      30/0900Z 30.3N  58.0W    60 KT
 12HR VT     30/1800Z 30.9N  59.0W    65 KT
 24HR VT     01/0600Z 32.3N  60.4W    70 KT
 36HR VT     01/1800Z 34.7N  61.0W    70 KT
 48HR VT     02/0600Z 39.0N  60.2W    60 KT
 72HR VT     03/0600Z 47.5N  54.5W    50 KT...INLAND EXTRATROPICAL
 96HR VT     04/0600Z...ABSORBED BY EXTRATROPICAL LOW
 
$$
FORECASTER BEVEN
 
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