451 
 WTNT43 KNHC 240237
 TCDAT3
 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