Hurricane HELENE

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HURRICANE HELENE DISCUSSION NUMBER  35
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082006
1100 PM EDT WED SEP 20 2006
 
OVERALL...HELENE HAS CHANGED LITTLE IN STRUCTURE DURING THE PAST
SEVERAL HOURS. SEVERAL MICROWAVE IMAGES DURING THE AFTERNOON AND
EVENING SHOWED MINIMAL CONVECTION SOUTH AND SOUTHEAST OF THE
CENTER...POSSIBLY DUE TO ENTRAINMENT OF DRY AIR SEEN IN DATA FROM
NOAA RESEARCH AIRCRAFT.  SINCE THE LAST MICROWAVE IMAGE...THERE HAS
BEEN SOME COOLING OF THE CLOUD TOPS NEAR THE CENTER...ALTHOUGH THE
EYE REMAINS POORLY DEFINED.  THE INITIAL INTENSITY IS REDUCED TO 85
KT BASED ON FURTHER EXAMINATION OF DATA FROM THE NOAA P-3 MISSION
THIS AFTERNOON.  THIS AGREES WITH SATELLITE INTENSITY ESTIMATES OF
90 KT FROM SAB AND AFWA...AND 77 KT FROM TAFB.

HELENE HAS TURNED TO THE RIGHT DURING THE PAST 6-12 HR WITH THE
INITIAL MOTION NOW 360/8.  THE HURRICANE IS ROUNDING THE WESTERN
END OF THE SUBTROPICAL RIDGE AND WILL RECURVE INTO THE WESTERLIES
DURING THE NEXT 24-36 HR.  ALL TRACK GUIDANCE AGREES WITH THIS
SCENARIO...ALTHOUGH THERE REMAINS SOME SPREAD IN BOTH DIRECTION AND
SPEED AFTER 24-36 HR.  THE OVERALL MODEL CONSENSUS IS SLOWER THAN 6
HR AGO.  THE NEW TRACK FORECAST IS SHIFTED A LITTLE TO THE EAST
OF...AND IS SLOWER THAN...THE PREVIOUS FORECAST. IT IS IN BEST
AGREEMENT WITH THE FSU SUPERENSEMBLE AND THE GFS ENSEMBLE MEAN.

IN THE SHORT TERM...ALL GUIDANCE FORECASTS HELENE TO STRENGTHEN A
LITTLE...AND THIS REMAINS IN THE INTENSITY FORECAST.  AFTER
RECURVATURE...INCREASING SHEAR AND COOLING SEA SURFACE TEMPERATURES
SHOULD CAUSE GRADUAL WEAKENING.  THE LATEST LARGE-SCALE MODEL RUNS
SEND MIXED SIGNALS ON EXTRATROPICAL TRANSITION.  THE MODEL SURFACE
FIELDS FORECAST A COLD FRONT TO INTERACT WITH HELENE AT ABOUT 48
HR.  HOWEVER...FSU CYCLONE PHASE DIAGRAMS BASED ON THOSE MODELS
SUGGEST THAT HELENE MAY NOT BECOME A CLASSIC COLD CORE CYCLONE. 
INSTEAD...THEY SUGGEST THAT HELENE WILL BECOME AN ASYMMETRIC WARM
CORE SYSTEM AS TRANSITION BEGINS...THEN REGAIN A SYMMETRIC WARM
CORE.  THIS WILL LIKELY BE DUE TO A NON-TROPICAL SECLUSION PROCESS
RATHER THAN THE NORMAL CONVECTIVE PROCESS OF A TROPICAL CYCLONE. 
GIVEN THE UNCERTAINTY...THE ONSET OF EXTRATROPICAL TRANSITION WILL
BE DELAYED 24 HR FROM THE PREVIOUS FORECASTS.  REGARDLESS OF THE
EXACT STRUCTURAL EVOLUTION...HELENE SHOULD REMAIN A LARGE AND
POWERFUL CYCLONE AS IT CROSSES THE NORTH ATLANTIC.
 
EVEN THOUGH HELENE IS FORECAST TO RECURVE OVER THE OPEN WATERS OF
THE CENTRAL ATLANTIC...SOME LONG PERIOD SWELLS COULD AFFECT PORTIONS
OF THE EAST COAST OF NORTH AMERICA AND BERMUDA.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      21/0300Z 27.1N  57.0W    85 KT
 12HR VT     21/1200Z 28.6N  57.0W    90 KT
 24HR VT     22/0000Z 31.2N  56.3W    90 KT
 36HR VT     22/1200Z 34.0N  54.5W    85 KT
 48HR VT     23/0000Z 37.2N  51.2W    75 KT
 72HR VT     24/0000Z 43.0N  42.0W    70 KT...BECOMING EXTRATROPICAL
 96HR VT     25/0000Z 48.0N  32.0W    65 KT...EXTRATROPICAL
120HR VT     26/0000Z 52.0N  24.0W    55 KT...EXTRATROPICAL
 
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FORECASTER BEVEN
 
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