Hurricane HELENE

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HURRICANE HELENE DISCUSSION NUMBER  42
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082006
500 PM EDT FRI SEP 22 2006
 
THE SATELLITE PRESENTATION OF HELENE CONTINUES TO DEGRADE AS THE
INNER CORE CONVECTION HAS WEAKENED CONSIDERABLY DURING THE LAST 6
HOURS AND THE LOW-LEVEL CIRCULATION IS NOW PARTIALLY EXPOSED.   
THE INITIAL INTENSITY OF 65 KT IS BASED ON A BLEND OF DVORAK
INTENSITY ESTIMATES OF 77 KT...65 KT...AND 55 KT FROM
TAFB...AFWA...AND SAB RESPECTIVELY. 
 
THE OVERALL SATELLITE EVOLUTION DURING THE DAY INDICATES THAT HELENE
IS ENTERING THE EARLY STAGES OF EXTRATROPICAL TRANSITION. IN
ADDITION TO DRY MIDDLE TO UPPER-LEVEL AIR WRAPPING AROUND THE
SOUTHERN SIDE OF THE CYCLONE...THE OVERALL CLOUD SHIELD IS BECOMING
INCREASINGLY DISPLACED NORTH OF THE CENTER...AND VISIBLE IMAGERY
SUGGESTS THAT HELENE IS NOW STARTING TO INTERACT WITH A BAROCLINIC
ZONE JUST TO ITS NORTHWEST. HOWEVER...DETERMINING WHEN
EXTRATROPICAL TRANSITION WILL BE COMPLETE CONTINUES TO BE
PROBLEMATIC. DESPITE THE RAPIDLY CHANGING STRUCTURE SEEN IN
SATELLITE IMAGERY...MOST OF THE GLOBAL MODELS MAINTAIN A SHALLOW
WARM CORE CYCLONE...ALBEIT ASYMMETRIC...FOR ANOTHER COUPLE OF DAYS.
GIVING MORE WEIGHT TO THE RECENT TRENDS...THE OFFICIAL FORECAST NOW
CALLS FOR EXTRATROPICAL TRANSITION BY 24 HOURS...A LITTLE SOONER
THAN BEFORE. ONCE HELENE TRANSFORMS INTO AN EXTRATROPICAL
CYCLONE...BAROCLINIC ENERGY COULD PRODUCE A LITTLE
RESTRENGTHENING...AND HELENE IS FORECAST BY ALL THE GLOBAL MODELS
TO BE A LARGE AND POWERFUL EXTRATROPICAL CYCLONE OVER THE OPEN
WATERS OF THE CENTRAL NORTH ATLANTIC. BY DAYS 4 AND 5...HELENE IS
EXPECTED TO LOSE SOME OF ITS BAROCLINIC SUPPORT AND GRADUALLY
WEAKEN.

HELENE CONTINUES TO ROUND THE WESTERN PERIPHERY OF A MID-LEVEL RIDGE
INTO THE MIDDLE LATITUDE WESTERLIES...RESULTING IN ACCELERATION
NORTHEASTWARD. THE INITIAL MOTION ESTIMATE IS NOW 050/18. TRACK
GUIDANCE HAS SHIFTED NOTICEABLY TO THE SOUTH.  HOWEVER...THE
GUIDANCE ANTICIPATED A MORE NORTHERLY TRACK A COUPLE OF DAYS AGO
THAT NEVER MATERIALIZED.  THE OFFICIAL FORECAST IS MORE
CONSERVATIVELY SHIFTED A LITTLE SOUTH...NOT AS FAR SOUTH AS THE
DYNAMICAL MODEL CONSENSUS.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      22/2100Z 35.4N  53.0W    65 KT
 12HR VT     23/0600Z 36.8N  50.1W    60 KT
 24HR VT     23/1800Z 38.4N  45.3W    65 KT...EXTRATROPICAL
 36HR VT     24/0600Z 40.3N  40.5W    65 KT...EXTRATROPICAL
 48HR VT     24/1800Z 42.2N  36.1W    60 KT...EXTRATROPICAL
 72HR VT     25/1800Z 47.0N  28.0W    55 KT...EXTRATROPICAL
 96HR VT     26/1800Z 51.0N  20.0W    45 KT...EXTRATROPICAL
120HR VT     27/1800Z 55.0N  14.0W    40 KT...EXTRATROPICAL
 
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FORECASTER RHOME/KNABB
 
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