910 
 WTNT43 KNHC 212036
 TCDAT3
 HURRICANE HELENE DISCUSSION NUMBER  38
 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082006
 500 PM EDT THU SEP 21 2006
 
 A 1531 UTC TRMM MICROWAVE OVERPASS INDICATED THAT BANDING FEATURES
 WERE CONFINED TO THE NORTHEASTERN SEMICIRCLE OF HELENE.  WITHIN THE
 PAST COUPLE OF HOURS...HELENE'S APPEARANCE IN CONVENTIONAL
 SATELLITE IMAGERY HAS IMPROVED A LITTLE WITH AN AREA OF COLD 
 CLOUD TOPS EXPANDING OVER THE NORTHERN PART OF THE CIRCULATION. 
 HOWEVER...DVORAK INTENSITY ESTIMATES HAVE NOT CHANGED AND THE
 INITIAL INTENSITY WILL REMAIN 70 KT FOR THIS ADVISORY.  THE
 OFFICIAL INTENSITY FORECAST DOES NOT ANTICIPATE WEAKENING DURING
 THE NEXT DAY OR SO...AS HELENE IS EXPECTED TO REMAIN IN AN
 ENVIRONMENT OF ONLY MODEST SHEAR AND OVER 26 TO 27C WATER. 
 THEREAFTER...HELENE IS FORECAST TO GRADUALLY WEAKEN AS SHEAR
 INCREASES AND SSTS COOL.  THE NHC FORECAST CALLS FOR HELENE TO
 BECOME EXTRATROPICAL IN 2 TO 3 DAYS.  THE INTENSITY FORECAST DURING
 THE EXTRATROPICAL PHASE IS A LITTLE HIGHER THAN THE SHIPS GUIDANCE
 AND FOLLOWS THE GLOBAL MODELS MORE CLOSELY.
 
 THE INITIAL MOTION ESTIMATE IS 010/12.  HELENE CONTINUES TO MOVE
 NORTHWARD AROUND THE WESTERN PERIPHERY OF THE SUBTROPICAL RIDGE
 CENTERED WELL TO THE EAST OF THE HURRICANE.  THE TRACK GUIDANCE
 SHOWS THAT HELENE SHOULD VERY SOON ROUND THE RIDGE AND TURN
 NORTH-NORTHEASTWARD AHEAD OF A LARGE DEEP-LAYERED TROUGH ALONG
 THE U.S. EAST COAST.  THE DYNAMICAL MODELS ARE IN GOOD AGREEMENT ON
 THIS SCENARIO DURING THE NEXT COUPLE OF DAYS.  AS HELENE BECOMES
 EXTRATROPICAL THE TRACK MODELS DIVERGE SIGNIFICANTLY.  THE UKMET
 TURNS THE CYCLONE EASTWARD...WHILE THE GFS TAKE IT QUICKLY
 NORTHWARD MERGING IT WITH A LARGER EXTRATROPICAL CYCLONE IN ABOUT 5
 DAYS.  THE OFFICIAL FORECAST LIES IN BETWEEN THESE SOLUTIONS AND IS
 CLOSE TO THE GFS ENSEMBLE MEAN AND THE TRACK SHOWN BY THE ECMWF
 TRACK...BUT IS A LITTLE SLOWER THAN THE LATER MODEL.
  
 EVEN THOUGH HELENE IS FORECAST TO RECURVE OVER THE CENTRAL ATLANTIC
 OCEAN...SOME LONG PERIOD SWELLS COULD AFFECT PORTIONS OF THE EAST
 COAST OF NORTH AMERICA AND BERMUDA DURING THE NEXT COUPLE OF DAYS.
  
 FORECAST POSITIONS AND MAX WINDS
  
 INITIAL      21/2100Z 30.7N  56.8W    70 KT
  12HR VT     22/0600Z 32.7N  55.9W    70 KT
  24HR VT     22/1800Z 35.4N  53.2W    70 KT
  36HR VT     23/0600Z 37.9N  49.1W    70 KT
  48HR VT     23/1800Z 40.7N  43.7W    65 KT...EXTRATROPICAL
  72HR VT     24/1800Z 46.5N  32.5W    60 KT...EXTRATROPICAL
  96HR VT     25/1800Z 52.0N  23.0W    60 KT...EXTRATROPICAL
 120HR VT     26/1800Z 58.0N  18.0W    55 KT...EXTRATROPICAL
  
 $$
 FORECASTER BROWN