Hurricane HELENE

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HURRICANE HELENE DISCUSSION NUMBER  36
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082006
500 AM EDT THU SEP 21 2006
 
THE OVERALL CONVECTIVE PATTERN HAS DECREASED IN AREAL COVERAGE AND
THE EYE HAS RECENTLY BECOME CLOUD-OBSCURED. SATELLITE INTENSITY
ESTIMATES ARE 77 KT FROM ALL THREE AGENCIES...SO THE ADVISORY
INTENSITY HAS BEEN DECREASED TO 80 KT. THE UPPER-LEVEL OUTFLOW
PATTERN REMAINS EXCELLENT IN ALL QUADRANTS...BUT WATER VAPOR AND
INFRARED SATELLITE IMAGERY INDICATE SOME MID-LEVEL SHEAR IS NOW
UNDERCUTTING THE FAVORABLE OUTFLOW LAYER.
 
THE INITIAL MOTION ESTIMATE IS 010/11. HELENE IS BEGINNING TO ROUND
THE WESTERN PERIPHERY OF THE SUBTROPICAL RIDGE SITUATED EAST-WEST
ALONG 29N-30N LATITUDE. A DEEP-LAYER TROUGH MOVING OFF THE U.S.
EAST COAST THAT EXTENDS FROM NEW ENGLAND SOUTHWARD TO THE SOUTHERN
FLORIDA PENINSULA IS PROGRESSING EASTWARD AT 20-25 KT AND SHOULD
GRADUALLY TURN HELENE NORTHEASTWARD DURING THE NEXT 24-36 HOURS...
WITH MORE RAPID ACCELERATION EXPECTED AFTER THAT. THE OFFICIAL
FORECAST TRACK HAS BEEN SHIFTED SLIGHTLY EASTWARD...BUT NOT NEARLY
AS FAR EAST AND SOUTH AS THE UKMET AND ECMWF MODELS...WHICH BRING
HELENE VERY CLOSE TO THE NORTHWESTERN AZORES IN 72-96 HOURS. THOSE
TWO MODELS HAD A DISTINCT RIGHT BIAS DURING THE RECENT EVOLUTION OF
FORMER HURRICANE GORDON...SO LESS WEIGHT WAS PLACED ON THOSE 
SOLUTIONS AND THE OFFICIAL TRACK IS SIMILAR TO THE MODEL CONSENSUS. 

AS HELENE BEGINS TO ACCELERATE NORTHEASTWARD...THE MID-LEVEL SHEAR
COULD ABATE ENOUGH TO ALLOW FOR SOME SLIGHT INTENSIFICATION WHILE
THE CYCLONE IS STILL OVER 27C SSTS. BY 72 HOURS...HOWEVER...HELENE
IS EXPECTED TO COME UNDER THE INFLUENCE OF STRONG CYCLONIC MID- TO
UPPER-LEVEL FLOW AND INCREASING BAROCLINIC EFFECTS...CAUSING THE
CYCLONE TO TRANSITION OVER TO A POWERFUL EXTRATROPICAL LOW PRESSURE
SYSTEM SIMILAR TO WHAT FORMER HURRICANE GORDON DID YESTERDAY.
 
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      21/0900Z 28.4N  56.8W    80 KT
 12HR VT     21/1800Z 30.1N  56.5W    85 KT
 24HR VT     22/0600Z 32.8N  55.3W    90 KT
 36HR VT     22/1800Z 35.6N  52.7W    85 KT
 48HR VT     23/0600Z 38.5N  48.7W    75 KT...BECOMING EXTRATROPICAL
 72HR VT     24/0600Z 43.9N  39.1W    65 KT...EXTRATROPICAL
 96HR VT     25/0600Z 48.0N  30.0W    60 KT...EXTRATROPICAL
120HR VT     26/0600Z 53.0N  22.0W    55 KT...EXTRATROPICAL
 
$$
FORECASTER STEWART
 
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