Hurricane HELENE

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HURRICANE HELENE DISCUSSION NUMBER  20
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082006
500 AM EDT SUN SEP 17 2006
 
SHOWER AND THUNDERSTORM ACTIVITY CONTINUES TO ORGANIZE AROUND 
THE CENTER OF HELENE...WITH A BROAD BANDING TYPE EYE IS SEEN IN
INFRARED SATELLITE IMAGERY.  GIVEN THE SATELLITE APPEARANCE...A
SOMEWHAT SURPRISING...VERY LARGE 45 TO 60 NM WIDE EYE WAS DETECTED
ON A RECENT 0502 UTC AQUA MICROWAVE OVERPASS.  BASED ON THE
MICROWAVE DATA AND SATELLITE INTENSITY ESTIMATES OF 77 KT FROM 
SAB AND TAFB...THE INITIAL INTENSITY IS INCREASED TO 75 KT.  

THE FUTURE TRACK OF HELENE APPEARS TO BE HIGHLY DEPENDENT ON HOW 
THE VARIOUS MODELS ARE HANDLING A COUPLE OF KEY FEATURES. THE FIRST  
IS THE AMOUNT OF RIDGING THAT BUILDS BETWEEN HELENE AND THE
DEPARTING HURRICANE GORDON TO THE NORTH...AND THE SECOND IS THE
STRENGTH OF THE TROUGH ALONG THE U.S. EAST COAST IN A COUPLE OF
DAYS. IN ANY CASE...THE DYNAMICAL MODELS REMAIN IN GOOD AGREEMENT
ON A GENERAL WEST-NORTHWEST TO NORTHWEST HEADING DURING THE NEXT
COUPLE OF DAYS.  THEREAFTER...THE MODEL SPREAD REMAINS EXTREMELY
LARGE. THE GFS REMAINS ON THE LEFT SIDE OF THE GUIDANCE...AS IT
WEAKENS THE TROUGH BETWEEN THE U.S. AND BERMUDA AND BUILDS A NARROW
MID- TO UPPER-LEVEL RIDGE NORTH OF THE HURRICANE...ALLOWING HELENE
TO MOVE MORE WESTWARD.  THE REMAINDER OF THE DYNAMICAL MODELS KEEP
ENOUGH OF A TROUGH NEAR BERMUDA TO IMPART A NORTHWESTWARD MOTION AT
DAYS 4 AND 5.  IT IS INTERESTING TO NOTE THAT THE GFDL...WHICH USES
THE INITIAL CONDITIONS OF THE GFS MODEL...IS ON THE FAR RIGHT SIDE
OF THE MODEL GUIDANCE.  UNTIL IT BECOMES MORE CLEAR AS TO WHICH
MODEL SOLUTION MAY BE CORRECT...THE OFFICIAL TRACK REMAINS NEAR THE
MODEL CONSENSUS...WHICH IS A LITTLE WEST OF THE PREVIOUS FORECAST.
 
WITH LOW SHEAR AND WARMER SSTS ALONG THE PREDICTED PATH OF
HELENE...THE ENVIRONMENT APPEARS FAVORABLE FOR ADDITIONAL
STRENGTHENING DURING THE NEXT FEW DAYS.  THE OFFICIAL INTENSITY
FORECAST FOLLOWS THE SLIGHTLY HIGHER GFDL AND FSU SUPERENSEMBLE
THROUGH 72 HOURS.  THEREAFTER...THE GLOBAL MODELS PREDICT THAT
HELENE WILL BE APPROACHING AN AREA OF INCREASING SOUTHWESTERLY
SHEAR...AND THE INTENSITY FORECAST SHOWS SOME WEAKENING AT DAYS 4 
AND 5...HOWEVER NOT AS MUCH INDICATED BY THE SHIPS MODEL.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      17/0900Z 20.0N  48.2W    75 KT
 12HR VT     17/1800Z 20.7N  49.3W    80 KT
 24HR VT     18/0600Z 21.7N  50.5W    85 KT
 36HR VT     18/1800Z 22.6N  51.7W    90 KT
 48HR VT     19/0600Z 23.3N  53.2W    95 KT
 72HR VT     20/0600Z 24.0N  56.0W    95 KT
 96HR VT     21/0600Z 26.0N  58.0W    85 KT
120HR VT     22/0600Z 29.0N  59.0W    80 KT
 
$$
FORECASTER BROWN/KNABB
 
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