Tropical Storm HELENE

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TROPICAL STORM HELENE DISCUSSION NUMBER   9
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082006
1100 AM EDT THU SEP 14 2006
 
A QUIKSCAT AMBIGUITY ANALYSIS FROM 0802 UTC WAS HELPFUL IN LOCATING
THE CENTER WHICH WAS FURTHER SOUTH THAN THE PREVIOUS ADVISORY. 
VISIBLE SATELLITE IMAGERY SUGGESTS HELENE IS SLOWLY BECOMING BETTER
ORGANIZED WITH INCREASING CURVATURE NOTED IN THE BANDING FEATURES.
SATELLITE INTENSITY ESTIMATES ARE 45 KT FROM TAFB...35 KT FROM SAB
AND AFWA. THE INITIAL INTENSITY IS INCREASED SLIGHTLY TO 40 KT FOR
THIS ADVISORY.  

THE STORM CONTINUES TO RACE WESTWARD AT 275/20 AS IT CURRENTLY
REMAINS SOUTH OF THE SUBTROPICAL RIDGE. THE SPREAD IN THE TRACK
GUIDANCE REMAINS LARGE ESPECIALLY BEYOND 24 TO 36 HOURS. 
AS NOTED IN THE PREVIOUS DISCUSSION...THE MODELS OFFER TWO
SOLUTIONS. THE GFS...ITS ENSEMBLE MEAN...AND THE GFDL ARE TO THE
RIGHT AND FORECAST THE RIDGE TO THE NORTH TO WEAKEN QUITE RAPIDLY
ALLOWING THE STORM TO TURN NORTHWESTWARD IN ABOUT 12 HOURS. THE
UKMET KEEPS THE SUBTROPICAL RIDGE AND MAINTAINS A WEST-NORTHWEST
TRACK LONGER. IN GENERAL ALL OF THE MODELS DECREASE THE FORWARD 
SPEED BY HALF OVER THE CENTRAL ATLANTIC DURING DAYS 3-5. BY THAT
TIME THE TRACK FORECAST IS QUITE UNCERTAIN SINCE THE GUIDANCE
ENVELOPE IS MORE THAN 450 MILES WIDE. THE NEW OFFICIAL TRACK
FORECAST IS MOVED TO THE LEFT OF THE PREVIOUS ADVISORY...AND NOW 
IN AGREEMENT WITH THE MODEL CONSENSUS.
 
THE INTENSIFICATION OF HELENE IS LIKELY TO BE SLOW INITIALLY BECAUSE 
OF THE CURRENTLY BROAD CIRCULATION OF THE CYCLONE. CONVECTION SHOULD
EVENTUALLY CONSOLIDATE NEAR THE CIRCULATION CENTER AND ALLOW FOR
MORE STEADY STRENGTHENING DURING THE NEXT DAY OR TWO. SSTS ALONG
THE FORECAST TRACK ARE FORECAST TO REMAIN 27-28 CELSIUS AND THE
SYSTEM IS EXPECTED TO REMAIN IN A WEAK SHEAR ENVIRONMENT. THUS 
HELENE IS FORECAST TO BECOME A HURRICANE BY 36 HOURS. THE SHIPS AND
GFDL MODELS INDICATE A PEAK INTENSITY OF 90-95 KT WHILE THE FSU
SUPERENSEMBLE INDICATES HELENE COULD BECOME A MAJOR HURRICANE. 
THE OFFICIAL FORECAST IS LOWER THAN ALL THE GUIDANCE FOR THE FIRST
DAY OR TWO GIVEN THE CURRENT SLOW RATE OF INTENSIFICATION 
THEREAFTER...THE FORECAST IS CLOSE TO THE SHIPS AND GFDL GUIDANCE
AND BRINGS HELENE TO A CATEGORY TWO HURRICANE. 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      14/1500Z 13.4N  36.8W    40 KT
 12HR VT     15/0000Z 13.8N  39.1W    45 KT
 24HR VT     15/1200Z 14.8N  42.0W    55 KT
 36HR VT     16/0000Z 16.2N  44.1W    65 KT
 48HR VT     16/1200Z 17.5N  46.0W    75 KT
 72HR VT     17/1200Z 19.5N  49.0W    85 KT
 96HR VT     18/1200Z 21.5N  51.0W    90 KT
120HR VT     19/1200Z 24.5N  53.0W    90 KT
 
$$
FORECASTER COBB/BEVEN
 
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