Hurricane ISAAC

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HURRICANE ISAAC DISCUSSION NUMBER  13
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL092006
500 PM EDT SAT SEP 30 2006
 
CONVECTION HAS CONTINUED TO INCREASE IN THE EYEWALL OF ISSAC THIS
AFTERNOON. IN ADDITION THE EYE HAS BECOME A LITTLE BETTER-DEFINED
WITH AN EXPANDING OUTFLOW PATTERN IN ALL QUADRANTS EXCEPT TO THE
SOUTHWEST. DVORAK T-NUMBERS FROM SAB/AFWA ARE INCREASING...
AND THE LOCAL 3 HOUR OBJECTIVE T-NUMBERS ARE NEAR T4.2... EQUATING
TO 70 KT. THE INITIAL INTENSITY WILL BE SET AT 70 KT FOR THIS
ADVISORY IN AGREEMENT WITH THE ABOVE ESTIMATES. ISAAC STILL HAS A
LITTLE MORE TIME TO INTENSIFY BEFORE SOUTHWESTERLY SHEAR ASSOCIATED
WITH AN UPPER TROUGH IMPACTS THE HURRICANE. STEADY WEAKENING
SHOULD BEGIN BY LATE TOMORROW AS THE SHEAR BECOMES STRONG AND
WATERS COOL BELOW 26C. THE OFFICIAL FORECAST IS SIMILAR TO THE
PREVIOUS ONE AND IS CLOSE TO ICON... THE INTENSITY CONSENSUS OF
GFDL AND SHIPS. GFDL CONTINUES TO INSIST ON STRENGTHENING AFTER
RECURVATURE BUT THIS MAY BE OVERDONE DUE TO VERY STRONG SHEAR
FORECAST TO OCCUR BY ALL GLOBAL MODELS. THESE GLOBAL MODELS ALSO
EXPECT ISAAC TO BE A POWERFUL EXTRATROPICAL LOW UNTIL IT IS
EVENTUALLY ABSORBED BY A LARGER MIDDLE-LATITUDE CYCLONE.
 
THE CYCLONE HAS SPED UP AND IS NOW MOVING ABOUT 310/9... THOUGH A
SHORTER-TERM MOTION IS FASTER. THE FORECAST PHILOSOPHY IS THE SAME
AS BEFORE. ISAAC IS EXPECTED TO MOVE IN BETWEEN A RIDGE TO THE EAST
OVER THE CENTRAL ATLANTIC AND A LARGE TROUGH OVER THE WESTERN
ATLANTIC. AT THIS POINT THE BIGGEST QUESTION IS HOW CLOSE WILL
EXTRATROPICAL ISAAC GET TO NEWFOUNDLAND. THE ECMWF...GFS AND GFS
ENSEMBLE MEAN MODELS WHIP EXTRATROPICAL ISAAC AROUND A LARGER
EXTRATROPICAL LOW AND ARE... CONSEQUENTLY... FARTHER TO THE LEFT
THAN THE OTHER MODELS... WHICH KEEP ISAAC MORE SEPARATE FROM THE
LOW. GLOBAL MODELS DO AGREE THAT ITS REMNANTS WILL FORM A POWERFUL
STORM OVER THE NORTH ATLANTIC AND MIGHT BE SIMILAR TO THE STRENGTH
OF EXTRATROPICAL FLORENCE A FEW WEEKS AGO. THE OFFICIAL FORECAST IS
CLOSE TO THE PREVIOUS FORECAST TRACK WITH THE MODELS NOW HONED IN ON
RECURVATURE ALONG ABOUT 61W... AND SHOWS SOME THREAT TO
NEWFOUNDLAND IN ABOUT 2-3 DAYS AS A STRONG EXTRATROPICAL SYSTEM. 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      30/2100Z 31.6N  59.5W    70 KT
 12HR VT     01/0600Z 32.6N  60.3W    75 KT
 24HR VT     01/1800Z 35.0N  60.8W    75 KT
 36HR VT     02/0600Z 39.0N  60.0W    70 KT...BECOMING EXTRATROPICAL
 48HR VT     02/1800Z 44.0N  56.8W    60 KT...EXTRATROPICAL
 72HR VT     03/1800Z 52.0N  46.0W    45 KT...EXTRATROPICAL
 96HR VT     04/1800Z...ABSORBED
 
$$
FORECASTER BLAKE/STEWART
 
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