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Hurricane HELENE (Text)

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HURRICANE HELENE DISCUSSION NUMBER  48
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082006
1100 PM EDT SAT SEP 23 2006
 
HELENE CONTINUES TO HANG ON TO TROPICAL CHARACTERISTICS.  THE
OVERALL APPEARANCE IS VERY SIMILAR TO SIX HOURS AGO...AND
CONVECTION HAS REMAINED CLOSE TO CENTER ACROSS THE NORTHWESTERN
SEMICIRCLE.   AN AMSU OVERPASS AT 2025 UTC...STILL SHOWED THE
SYSTEM MAINTAINING A WELL-DEFINED MID- TO UPPER-LEVEL WARM CORE
WHICH SUPPORTS THE ANALYSIS OF THE FSU CYCLONE PHASE SPACE
DIAGRAMS.  THUS...WE ARE KEEPING HELENE A 75 KT HURRICANE FOR THIS
ADVISORY.  ANALYSIS OF A 2132 UTC QUIKSCAT PASS REVEALED THAT THE
WIND RADII HAVE CHANGED IN ORIENTATION SINCE THE PREVIOUS PASS...
AND THE INITIAL WIND RADII REFLECT THESE WINDS.

THE BIGGEST CHALLENGE WITH THIS FORECAST HAS BEEN DETERMINING WHEN
EXTRATROPICAL TRANSITION WILL COMPLETE.  BASED UPON THE CURRENT
APPEARANCE...THIS SHOULD OCCUR WITHIN 6 TO 18 HOURS...WITH THE HELP
OF AN APPROACHING SHORT-WAVE TROUGH.  AFTER WHICH...SLOW WEAKENING
IS ANTICIPATED IN THE FIRST 36 HOURS FOLLOWED BY A FASTER WEAKENING
RATE DUE TO INCREASING SHEAR AND DECREASED BAROCLINIC ENERGY.

THE INITIAL MOTION IS ESTIMATED AT 070/20...BUT A SHORTER 6-HOUR
MOTION COULD BE CLOSER TO 25 KT.  THE OVERALL FORECAST TRACK
PHILOSOPHY HAS NOT CHANGED.  HELENE SHOULD MAINTAIN AN EAST-
NORTHEAST TO NORTHEAST MOTION THROUGH 4 DAYS.  THE MODEL GUIDANCE
IS WELL CLUSTERED FOR THE FIRST 36 HOURS...THEN THE GFS AND ECMWF
TAKE A SOUTHWARD TURN.  SINCE THE GFS MODEL LACKS RUN-TO-RUN
CONSISTENCY...IT HAS BEEN GIVEN VERY LITTLE WEIGHT.  THE OFFICIAL
FORECAST IS CLOSE TO THE PREVIOUS FORECAST BUT SLIGHTLY SLOWER IN
THE LATER PERIODS AND JUST SOUTH OF THE GFDL. 

FORECAST POSITIONS AND MAX WINDS
 
INITIAL      24/0300Z 39.1N  41.7W    75 KT
 12HR VT     24/1200Z 40.5N  38.1W    70 KT...EXTRATROPICAL
 24HR VT     25/0000Z 42.5N  34.2W    70 KT...EXTRATROPICAL
 36HR VT     25/1200Z 43.8N  31.0W    65 KT...EXTRATROPICAL
 48HR VT     26/0000Z 45.0N  27.5W    55 KT...EXTRATROPICAL
 72HR VT     27/0000Z 47.0N  20.0W    45 KT...EXTRATROPICAL
 96HR VT     28/0000Z 48.5N  15.0W    35 KT...EXTRATROPICAL
120HR VT     29/0000Z...DISSIPATED
 
$$
FORECASTER MAINELLI/BEVEN
 
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