054 
 WTNT43 KNHC 222043
 TCDAT3
 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
  
 $$
 FORECASTER RHOME/KNABB