Hurricane ISAAC

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HURRICANE ISAAC DISCUSSION NUMBER  15
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL092006
500 AM EDT SUN OCT 01 2006
 
METEOSAT IMAGERY DURING THE GOES ECLIPSE SHOWED THAT THE RAGGED EYE
OF ISAAC CONTINUED TO BE PRESENT OVERNIGHT.  DVORAK INTENSITY
ESTIMATES ARE BOTH T4.5...77 KT...FROM TAFB AND SAB...WHICH IS
CONSISTENT WITH EARLIER AMSU INTENSITY VALUES AS WELL.  A
WELL-DEFINED EYE WAS ALSO OBSERVED AT 0013 UTC IN SSMIS MICROWAVE
IMAGERY.  THEREFORE...THE INTENSITY IS ANALYZED TO BE 75 KT.

CURRENT MOTION OF ISAAC IS 335/8...A BIT TO THE RIGHT OF SIX HOURS
EARLIER.  THE HURRICANE IS SITUATED BETWEEN THE SOUTHWESTERN FLANK
OF A RATHER SMALL DEEP LAYERED SUBTROPICAL RIDGE AND A VERY LARGE
TROUGH ALONG THE UNITED STATES ATLANTIC SEABOARD.  ISAAC IS LIKELY
TO MAKE THE TURN TO THE NORTH IN THE NEXT 12 HOURS AND WILL BE
ACCELERATING UNDER THE INFLUENCE OF STRONG SOUTWESTERLIES.  ALL
RELIABLE GUIDANCE ARE TIGHTLY CLUSTERED THROUGH 72 HOURS WITH THIS
RECURVING SCENARIO.  THE FORECAST TRACK IS CLOSE TO THE MODEL
CONSENSUS AND THE PREVIOUS ADVISORY THROUGH 48 HOURS.  AT 72 HOURS
THE TRACK IS SOMEWHAT SLOWER THE THE LAST FORECAST.

ISAAC LIKELY IS NEAR PEAK INTENSITY...AS INCREASING VERTICAL SHEAR
SHOULD SOON BE IMPINGING ON THE CYCLONE.  ADDITIONALLY...IT SHOULD
ENCOUNTER VERY COLD SSTS BEGINNING IN 24-36 HOURS AS IT MOVES NORTH
OF THE GULF STREAM.  ALL GLOBAL MODELS SUGGEST A QUICK
TRANSFORMATION TO AN EXTRATROPICAL CYCLONE MAY OCCUR IN ABOUT 24-36
HOURS AS ISAAC INTERACTS WITH A STRONG BAROCLINIC ZONE.  ONLY THE
GFDL HURRICANE MODEL RETAINS ISAAC AS A TROPICAL CYCLONE FOR MORE
THAN 36 HOURS.  ALSO THE GLOBAL MODELS HAVE BACKED OFF SOMEWHAT AS
TO THE INTENSITY OF THE EXTRATROPICALLY-TRANSITIONED ISAAC AT DAYS
4 AND 5.  AFTER ABOUT THREE DAYS...HOWEVER...IT IS DIFFICULT TO
DISTINGUISH ISAAC'S VORTEX FROM A LARGER EXTRATROPICAL STORM.  THE
INTENSITY FORECAST IS FOR GRADUAL WEAKENING DURING THE NEXT 24
HOURS...THEN SOMEWHAT QUICKER WEAKENING AFTERWARD AS AN
EXTRATROPICAL CYCLONE.  THIS PREDICTION IS SIMILAR TO THE PREVIOUS
ADVISORY AND IS A BLEND OF THE SHIPS...GFDL...AND GLOBAL MODELS.

DUE TO THE UNCERTAINTY IN THE EXTRATROPICAL TRANSITION TIMING...THE
CANADIAN WEATHER SERVICE HAS ISSUED A TROPICAL STORM WATCH FOR THE
AVALON PENINSULA.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      01/0900Z 33.1N  60.1W    75 KT
 12HR VT     01/1800Z 35.5N  60.4W    75 KT
 24HR VT     02/0600Z 39.8N  58.9W    65 KT
 36HR VT     02/1800Z 44.6N  55.4W    55 KT...EXTRATROPICAL
 48HR VT     03/0600Z 48.5N  50.9W    50 KT...EXTRATROPICAL
 72HR VT     04/0600Z 52.5N  42.0W    45 KT...EXTRATROPICAL
 
$$
FORECASTER LANDSEA/STEWART
 
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