936 
 WTNT43 KNHC 232031
 TCDAT3
 HURRICANE HELENE DISCUSSION NUMBER  47
 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082006
 500 PM EDT SAT SEP 23 2006
  
 HELENE CURRENTLY DISPLAYS BOTH TROPICAL AND EXTRATROPICAL
 CHARACTERISTICS.  THE CYCLONE HAS A FRONTAL-LIKE AND ASYMMETRIC
 APPEARANCE IN SATELLITE IMAGERY WITH LIMITED DEEP CONVECTION.
 HOWEVER...FSU CYCLONE PHASE SPACE DIAGRAMS DIAGNOSE HELENE TO STILL
 HAVE A DEEP WARM CORE WHICH IS CONSISTENT WITH ITS ABILITY TO
 STILL PRODUCE CONVECTION...ALBEIT LIMITED...NEAR THE CENTER.  HELENE
 IS SOMEWHERE IN PROCESS OF EXTRATROPICAL TRANSITION AND IT IS NOT
 CLEAR AT WHAT POINT THE TRANSFORMATION WILL BE COMPLETE.  SINCE THE
 OVERALL STRUCTURE APPEARS UNCHANGED SINCE THE EARLIER 0915 UTC
 QUIKSCAT DATA...WHICH SHOWED 80 KT MAXIMUM WINDS...THE INITIAL
 INTENSITY WILL REMAIN AT 80 KT FOR THIS ADVISORY.  WATER VAPOR
 IMAGERY SHOWS A MIDDLE TO UPPER-LEVEL SHORTWAVE TROUGH QUICKLY
 APPROACHING HELENE FROM THE WEST RESULTING IN THE UPPER-LEVEL
 OUTFLOW BECOMING INCREASINGLY RESTRICTED.  IF THIS TREND
 CONTINUES...AN INCREASINGLY SHEARED HELENE COULD LOSE WHAT REMAINS
 OF ITS INNER CORE CONVECTION AND THEREFORE LOSE WHAT TROPICAL
 CHARACTERISTICS IT HAS LEFT.  ASSUMING EXTRATROPICAL TRANSITION
 COMPLETES IN ABOUT 12 HOURS...AS CURRENTLY FORECAST...THE CYCLONE
 SHOULD GRADUALLY WEAKEN OVER THE NEXT DAY OR SO. MORE RAPID
 WEAKENING IS FORECAST THEREAFTER AS THE CYCLONE LOSES SOME OF ITS
 BAROCLINIC SUPPORT.
  
 HELENE HAS BEEN MOVING A LITTLE SOUTH OF THE PREVIOUS FORECAST TRACK
 WITH AN INITIAL MOTION ESTIMATE OF 070/19.  THERE HAS OTHERWISE
 BEEN LITTLE CHANGE IN THE TRACK FORECAST PHILOSOPHY IN THE
 SHORT-TERM...AND GUIDANCE REMAINS TIGHTLY CLUSTERED THROUGH 48
 HOURS.  THEREAFTER...THE TRACK GUIDANCE BEGINS TO DIVERGE BUT NOT
 NEARLY AS MUCH AS BEFORE.  THE NOGAPS MODEL...WHICH WAS PREVIOUSLY
 THE NORTHERN OUTLIER...HAS SHIFTED SIGNIFICANTLY SOUTH AND IS NOW
 ALONG THE SOUTHERN SIDE OF THE GUIDANCE SUITE.  THE UKMET
 MODEL...WHICH PREVIOUSLY SHOWED A TURN DUE SOUTH WITH THE
 ECMWF...HAS SHIFTED NORTH AND IS NOW IN GOOD AGREEMENT WITH THE
 OFFICIAL FORECAST.  THE ECMWF CONTINUES TO SHOW THE SOUTHWARD
 TURN...BUT THIS IS CONSIDERED A LESS LIKELY SCENARIO GIVEN THE NOW
 BETTER AGREEMENT AMONG THE OTHER DYNAMICAL MODELS. THE OFFICIAL
 FORECAST IS SHIFTED JUST A LITTLE SOUTH OF THE PREVIOUS ONE. 
  
 FORECAST POSITIONS AND MAX WINDS
  
 INITIAL      23/2100Z 38.0N  44.4W    80 KT
  12HR VT     24/0600Z 39.1N  40.9W    75 KT...EXTRATROPICAL
  24HR VT     24/1800Z 41.3N  36.4W    70 KT...EXTRATROPICAL
  36HR VT     25/0600Z 43.2N  32.4W    65 KT...EXTRATROPICAL
  48HR VT     25/1800Z 44.6N  28.8W    55 KT...EXTRATROPICAL
  72HR VT     26/1800Z 47.0N  21.0W    45 KT...EXTRATROPICAL
  96HR VT     27/1800Z 49.0N  15.0W    35 KT...EXTRATROPICAL
 120HR VT     28/1800Z 51.0N  11.0W    30 KT...EXTRATROPICAL
  
 $$
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