Hurricane HELENE

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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
 
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