Hurricane HELENE

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HURRICANE HELENE DISCUSSION NUMBER  23
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082006
1100 PM EDT SUN SEP 17 2006
 
THE HURRICANE IS LOOKING STRONGER THIS EVENING WITH THE RING OF
EYEWALL CONVECTION COOLING AROUND THE WARM EYE. IN ADDITION... THE
INNER CORE IS MORE SYMMETRIC THAN A FEW HOURS AGO WITH CIRRUS
OUTFLOW EXPANDING IN ALL DIRECTIONS... ESPECIALLY TO THE EAST.
SUBJECTIVE AND OBJECTIVE DVORAK ESTIMATES SUGGEST THAT HELENE IS A
MAJOR HURRICANE AND THE INITIAL INTENSITY HAS BEEN CONSERVATIVELY
INCREASED TO 100 KT... MAKING HELENE THE SECOND MAJOR HURRICANE OF
THE SEASON. THERE ARE NO OBVIOUS HINDRANCES TO FUTURE
INTENSIFICATION OVER THE NEXT COUPLE OF DAYS AS HELENE MOVES OVER
PROGRESSIVELY WARMER WATERS AND STAYS IN A RELATIVELY LIGHT SHEAR
ENVIRONMENT. CURIOUSLY... THE SHIPS/GFDL DO NOT SHOW MUCH FURTHER
STRENGTHENING IN AN ENVIRONMENT THAT IS SEEMINGLY FAVORABLE FOR
MORE INTENSIFICATION. THE OFFICIAL WIND SPEED FORECAST IS A LITTLE
HIGHER THAN SHIPS/GFDL AND CLOSE TO THE FSU SUPERENSEMBLE. A SLOW
WEAKENING COULD COMMENCE IN A COUPLE OF DAYS AS SOME MODELS HINT AT
AN INCREASE IN SHEAR. HOWEVER THE LARGE SPREAD OF THE TRACK MODELS
IN THE EXTENDED PERIOD MAKES FOR A MORE UNCERTAIN INTENSITY
FORECAST THAN USUAL AT 3-5 DAYS.
 
THE INITIAL MOTION IS A LITTLE SLOWER THAN PREVIOUSLY ESTIMATED...
ABOUT 315/7. GLOBAL MODELS ARE IN GOOD AGREEMENT ON THIS GENERAL
MOTION CONTINUING FOR ABOUT 24 HOURS FOLLOWED BY A TURN TOWARD THE
WEST OR WEST-NORTHWEST AS RIDGING REBUILDS IN BETWEEN GORDON AND
HELENE. THEREAFTER... THE BIG PROBLEM IS IF A TROUGH... NOW
CURRENTLY JUST OFFSHORE OF THE UNITED STATES... WEAKENS THE RIDGE
ENOUGH TO ALLOW THE HURRICANE TO BE PICKED UP BY A SECOND TROUGH IN
A FEW DAYS. MODEL GUIDANCE IS GENERALLY SPLIT INTO TWO CLUSTERS.
THE GFS/UKMET/NOGAPS SUGGEST THAT THE TROUGH WILL LEAVE THE
HURRICANE BEHIND WHILE THE ECMWF/GFDL BELIEVE THE TROUGH WILL
ACCELERATE THE HURRICANE OUT TO SEA. MODEL GUIDANCE HAS BEEN
TRENDING WESTWARD WITH TIME... AND THE OFFICIAL FORECAST LEANS
TOWARD THE WESTWARD CLUSTER OF GFS/UKMET/NOGAPS.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      18/0300Z 21.6N  49.4W   100 KT
 12HR VT     18/1200Z 22.3N  50.2W   105 KT
 24HR VT     19/0000Z 23.0N  51.5W   110 KT
 36HR VT     19/1200Z 23.3N  53.1W   110 KT
 48HR VT     20/0000Z 23.6N  54.8W   110 KT
 72HR VT     21/0000Z 25.0N  58.0W   100 KT
 96HR VT     22/0000Z 27.5N  59.5W    95 KT
120HR VT     23/0000Z 29.5N  61.5W    90 KT
 
$$
FORECASTER BLAKE/PASCH
 
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