Hurricane HELENE

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HURRICANE HELENE DISCUSSION NUMBER  34
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082006
500 PM EDT WED SEP 20 2006
 
A NOAA P-3 EQUIPPED WITH THE SFMR INSTRUMENT HAS BEEN SAMPLING
HELENE THIS AFTERNOON.  THUS FAR...THE DATA CONFIRMS THAT THE
EYEWALL IS POORLY DEFINED AND THE SYSTEM HAS A LARGE WIND FIELD.
AS A RESULT...THE INTENSITY SHOULD BE SLOW TO RESPOND TO CHANGES IN
THE ENVIRONMENT. WHILE THE DATA FROM THE SFMR AND THE LARGE WIND
FIELD ARGUE AGAINST A 90 KT HURRICANE...AN EYEWALL DROP REPORTED AN
IMPRESSIVE 960 MB.  IN LIGHT OF THE PRESSURE AND THE FACT THAT THE
PLANE HAS NOT YET SAMPLED THE ENTIRE STORM...THE INITIAL INTENSITY
IS HELD AT 90 KT.  IF THE PLANE DOES NOT FIND HIGHER WINDS IN
SUBSEQUENT PASSES...THE INTENSITY WILL NEED TO BE LOWERED LATER.

THE INTENSITY FORECAST REMAINS HIGHLY PROBLEMATIC DUE TO THE
CYCLONE INTERACTING WITH A STRONG MIDDLE TO UPPER-LEVEL TROUGH OVER
THE NEXT TWO DAYS FOLLOWED BY EXTRATROPICAL TRANSITION.  WHILE IT
IS DIFFICULT TO ANTICIPATE TROUGH INTERACTIONS...GLOBAL MODELS
INDICATE THAT HELENE MAY ENCOUNTER WEAKER SHEAR IN THE SHORT-TERM
PRIOR TO THE ONSET OF STRONG SOUTHWESTERLY FLOW AHEAD OF THE
TROUGH.  ALL OF THE AVAILABLE INTENSITY GUIDANCE INDICATES HELENE
MAY STRENGTHEN AS A RESULT OF THIS MORE FAVORABLE ENVIRONMENT.  BY
DAY 2...HELENE STARTS TO GET TANGLED UP WITH A BAROCLINIC ZONE
ULTIMATELY RESULTING IN EXTRATROPICAL TRANSITION.  SINCE BAROCLINIC
FORCING IS EXPECTED DURING THE EXTRATROPICAL TRANSITION...ONLY SLOW
WEAKENING IS FORECAST.  INDEED HELENE MAY BE A LARGE AND POWERFUL
EXTRATROPICAL STORM OVER THE NORTH CENTRAL ATLANTIC.  THE
OFFICIAL INTENSITY FORECAST IS LITTLE CHANGED FROM THE PREVIOUS
ADVISORY.
 
HELENE IS IN THE PROCESS OF A NORTHWARD TURN AND THE INITIAL MOTION
ESTIMATE IS 315/08.  GLOBAL MODELS CONTINUE TO BE TIGHTLY CLUSTERED
THROUGH 72 HOURS AND ONLY MINOR CHANGES ARE REQUIRED TO THE
PREVIOUS FORECAST. BEYOND 72 HOURS...MOST OF THE GLOBAL MODELS HAVE
SHIFTED CONSIDERABLY NORTH WITH THE GFS AND GFDL ALONG THE NORTH
SIDE OF THE MODEL SUITE AND THE NOGAPS NEAR THE SOUTH SIDE OF THE
GUIDANCE. SINCE THE NOGAPS PERFORMED WELL DURING GORDON...THE
OFFICIAL FORECAST IS ONLY SHIFTED A LITTLE TO THE NORTH AND
CURRENTLY LIES JUST SOUTH OF THE DYNAMICAL MODEL CONSENSUS. 
 
EVEN THOUGH HELENE IS FORECAST TO RECURVE OVER THE OPEN WATERS OF
THE CENTRAL ATLANTIC...LONG PERIOD SWELLS ARE ALREADY REGISTERING
AT SEVERAL EAST COAST BUOYS.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      20/2100Z 26.6N  57.1W    90 KT
 12HR VT     21/0600Z 27.9N  57.5W    90 KT
 24HR VT     21/1800Z 30.6N  57.2W    95 KT
 36HR VT     22/0600Z 33.9N  55.5W    90 KT
 48HR VT     22/1800Z 37.5N  52.3W    85 KT...BECOMING EXTRATROPICAL
 72HR VT     23/1800Z 43.5N  43.0W    75 KT...EXTRATROPICAL
 96HR VT     24/1800Z 48.5N  35.0W    65 KT...EXTRATROPICAL
120HR VT     25/1800Z 52.0N  28.0W    55 KT...EXTRATROPICAL
 
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FORECASTER RHOME/AVILA
 
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