Hurricane ISAAC

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HURRICANE ISAAC DISCUSSION NUMBER  14
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL092006
1100 PM EDT SAT SEP 30 2006
 
LATEST GOES IMAGERY SHOWS THAT THE EYE OF ISAAC HAS INTERMITTENTLY
APPEARING AND DISAPPEARING THROUGHOUT THE EVENING. GIVEN A LACK OF
SIGNIFICANT CHANGE IN THE SATELLITE PRESENTATION...THE INITIAL
INTENSITY IS HELD AT 70 KT FOR THIS ADVISORY...BASED ON A BLEND OF
DVORAK INTENSITY ESTIMATES BETWEEN 65 AND 77 KT. THE INTENSITY
FORECAST CONTINUES TO BE PROBLEMATIC GIVEN OFFSETTING FACTORS IN
THE SHORT-TERM...AND A COMPLEX INTERACTION WITH A MID-LATITUDE
TROUGH THEREAFTER. ISAAC HAS ABOUT 12-24 HOURS OVER WATERS OF AT
LEAST 26C. HOWEVER...WATER VAPOR IMAGERY SHOWS THAT ISAAC IS
EXPERIENCING INCREASING SOUTHERLY SHEAR. BEYOND 24 HOURS...THE
HURRICANE WILL BEGIN TO INTERACT WITH A LARGE MID-LATITUDE TROUGH
MOVING OFF THE EAST COAST OF THE UNITED STATES. THE GLOBAL MODELS
DISAGREE ON WHETHER ISAAC WILL BE ABLE TO UTILIZE THE AVAILABLE
BAROCLINIC ENERGY TO STRENGTHEN DURING EXTRATROPICAL TRANSITION...
OR INSTEAD BE ABSORBED BY A MORE DOMINANT EXTRATROPICAL LOW WHICH
COULD FORM TO ITS WEST. SHOULD ISAAC MERGE WITH A SECOND LOW...IT
WOULD PROBABLY PRODUCE A BROADER AND WEAKER PRESSURE GRADIENT.
CONVERSELY...IF ISAAC REMAINS THE DOMINANT SYSTEM...IT COULD
RE-INTENSIFY AS AN EXTRATROPICAL CYCLONE...IN A SIMILAR MANNER TO
HELENE. THE OFFICIAL FORECAST ESSENTIALLY SPLITS THE DIFFERENCE
BETWEEN THESE TWO SCENARIOS BY SHOWING SLOW WEAKENING THROUGH THE
FIRST 48 HOURS.  THE NEW OFFICIAL FORECAST NOW SHOWS SLIGHT
WEAKENING IN THE SHORT-TERM AND LITTLE CHANGE DURING EXTRATROPICAL
TRANSITION. 
 
ISAAC IS MOVING JUST A LITTLE TO THE RIGHT...OR EAST...OF THE
PREVIOUS ADVISORY WITH AN INITIAL MOTION ESTIMATE OF 330/09.  
GLOBAL MODELS REMAIN IN GENERAL AGREEMENT ON A GRADUAL NORTHWARD
TURN THROUGH 24 HOURS...FOLLOWED BY ACCELERATION TO THE NORTHEAST
AS THE CYCLONE GETS CAUGHT UP IN SOUTHWESTERLY FLOW AHEAD OF THE
MID-LATITUDE TROUGH. THE ECMWF...GFS...AND GFS ENSEMBLE MEAN
ARE ALONG THE LEFT SIDE OF THE MODEL SUITE AND SHOW THE CYCLONE
MOVING OVER OR NEAR NEWFOUNDLAND. CONVERSELY...THE GFDL AND GFDN ARE
MUCH FARTHER EAST. REMAINING IN BETWEEN THOSE EXTREMES AND CLOSE TO
THE MODEL CONSENSUS...THE OFFICIAL FORECAST IS NUDGED A LITTLE TO
THE RIGHT DURING THE FIRST 36 HOURS...AND IS ESSENTIALLY AN UPDATE
OF THE PREVIOUS FORECAST THEREAFTER.  
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      01/0300Z 32.5N  59.9W    70 KT
 12HR VT     01/1200Z 34.0N  60.5W    70 KT
 24HR VT     02/0000Z 37.2N  60.2W    65 KT
 36HR VT     02/1200Z 42.0N  57.9W    60 KT...BECOMING EXTRATROPICAL
 48HR VT     03/0000Z 46.8N  53.5W    60 KT...EXTRATROPICAL
 72HR VT     04/0000Z 53.0N  43.0W    50 KT...EXTRATROPICAL
 96HR VT     05/0000Z 56.0N  30.0W    45 KT...EXTRATROPICAL
120HR VT     06/0000Z...ABSORBED
 
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FORECASTER RHOME/KNABB
 
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