Hurricane HELENE

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HURRICANE HELENE DISCUSSION NUMBER  41
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082006
1100 AM EDT FRI SEP 22 2006
 
THE SATELLITE PRESENTATION OF HELENE HAS DEGRADED SLIGHTLY OVER THE
LAST SEVERAL HOURS...WITH INNER CORE CONVECTION STARTING TO SPUTTER
AND FAVOR THE NORTHEASTERN QUADRANT. ACCORDINGLY...DVORAK DATA T
NUMBERS HAVE DECREASED...BUT THE INITIAL INTENSITY IS MAINTAINED AT
75 KT FOR NOW IN ACCORDANCE WITH THE DVORAK CI NUMBERS. INCREASING
VERTICAL SHEAR AND DECREASING SSTS APPEAR TO BE TAKING THEIR TOLL
ON HELENE...AND THE CYCLONE SHOULD GRADUALLY WEAKEN AS IT SLOWLY
LOSES TROPICAL CHARACTERISTICS DURING THE NEXT COUPLE OF DAYS. ON
DAYS 2 AND 3...STRONG BAROCLINIC ENERGY IS EXPECTED TO LIMIT
FURTHER WEAKENING...WITH HELENE FORECAST BY ALL THE GLOBAL MODELS
TO BE A LARGE AND POWERFUL EXTRATROPICAL CYCLONE OVER THE OPEN
WATERS OF THE CENTRAL NORTH ATLANTIC. IT SHOULD BE NOTED THAT THE
PRECISE TIMING OF EXTRATROPICAL TRANSITION IS DIFFICULT TO DISCERN
GIVEN SOME DIFFERENCES IN STRUCTURAL EVOLUTION AMONG THE
MODELS...AS SHOWN BY THE FSU PHASE SPACE DIAGRAMS. TOWARD THE END
OF THE FORECAST...GLOBAL MODELS SUGGEST THAT HELENE WILL LOSE SOME
OF ITS BAROCLINIC SUPPORT WHICH WOULD INDUCE A WEAKENING TREND. THE
OFFICIAL FORECAST SHOWS A LITTLE MORE WEAKENING DURING THE FIRST 24
HOURS GIVEN THE RECENT TRENDS IN THE CONVECTIVE PATTERN. BEYOND 24
HOURS...THE OFFICIAL INTENSITY FORECAST IS ESSENTIALLY AN UPDATE OF
THE PREVIOUS ONE.
 
HELENE IS STARTING TO ACCELERATE NORTHEASTWARD AS IT ROUNDS THE
WESTERN PERIPHERY OF A MID-TROPOSPHERIC RIDGE...AND THE INITIAL
MOTION ESTIMATE IS NOW 040/18. THE HURRICANE IS EMBEDDED WITHIN
STRONG SOUTHWESTERLY FLOW AHEAD OF A MIDDLE TO UPPER-LEVEL TROUGH
OVER THE WESTERN ATLANTIC. THE GUIDANCE REMAINS TIGHTLY CLUSTERED
THROUGH 72 HOURS...AND MODELS ARE NOW IN BETTER AGREEMENT AT DAYS 4
AND 5. THE OFFICIAL FORECAST IS IN BEST AGREEMENT WITH THE NOGAPS
MODEL AND IS A LITTLE SLOWER AND TO THE SOUTH OF THE PREVIOUS
ADVISORY.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      22/1500Z 34.4N  54.2W    75 KT
 12HR VT     23/0000Z 36.2N  51.6W    70 KT
 24HR VT     23/1200Z 37.9N  47.4W    65 KT
 36HR VT     24/0000Z 40.0N  42.8W    65 KT
 48HR VT     24/1200Z 42.4N  37.5W    65 KT...EXTRATROPICAL
 72HR VT     25/1200Z 47.5N  29.0W    65 KT...EXTRATROPICAL
 96HR VT     26/1200Z 52.5N  20.0W    55 KT...EXTRATROPICAL
120HR VT     27/1200Z 56.5N  15.0W    50 KT...EXTRATROPICAL
 
$$
FORECASTER RHOME/KNABB
 
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