Hurricane HELENE

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HURRICANE HELENE DISCUSSION NUMBER  18
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082006
500 PM EDT SAT SEP 16 2006
 
DVORAK ESTIMATES FROM TAFB AND SAB HAVE NOT CHANGED AND A RAGGED EYE
HAS BEEN OBSERVED THIS AFTERNOON IN THE VISIBLE IMAGERY.  CIMSS
EXPERIMENTAL ADVANCED DVORAK TECHNIQUE ALSO SUGGESTS MINIMAL
HURRICANE INTENSITY.  THUS...A 65 KT INITIAL INTENSITY IS RETAINED. 
TODAY...A NOAA SALEX...SAHARAN AIR LAYER EXPERIMENT...MISSION ON
THE G-IV AIRCRAFT HAS BEEN TAKING OBSERVATIONS IN THE PERIPHERY OF
HELENE.  WHILE THESE DROPWINDSONDES ARE SHOWING SUBSTANTIAL DRY
AIR...CIRA PRECIPITABLE WATER ANALYSES SUGGEST THAT SUFFICIENT
MOISTURE IS FOUND IN THE SYSTEM'S INNER CORE.  BECAUSE OF 27.5 C
SSTS...LIGHT/MODERATE VERTICAL SHEAR...AND A MOIST ENVIRONMENT...
CONTINUED SLOW INTENSIFICATION IS ANTICIPATED.  AFTER TWO DAYS...
BOTH THE SHEAR AND SSTS INCREASE.  BECAUSE OF THESE CONFLICTING
ENVIRONMENTAL FACTORS...HELENE IS PROJECTED TO PEAK IN INTENSITY
NEAR 85 KT AT DAYS 2 AND 3 WITH SLIGHT WEAKENING THEREAFTER.  IT IS
NOTED THAT THE GFDL MODEL FIELDS RETAIN WEAKER SHEAR INTO LONGER
LEAD TIMES THAN THE GLOBAL MODELS.  THE OFFICIAL FORECAST IS
SLIGHTLY ABOVE THE STANDARD SHIPS MODEL DUE TO THE INFLUENCE OF AN
ENHANCING MICROWAVE PREDICTOR IN THE EXPERIMENTAL SHIPS-MICROWAVE
MODEL AND IS SLIGHTLY BELOW THE PREVIOUS PREDICTION.
 
CURRENT MOTION IS ESTIMATED TO BE 310/11.  HELENE IS MOVING TOWARD A
BREAK IN THE WEST-SOUTHWEST TO EAST-NORTHEAST ORIENTED SUBTROPICAL
RIDGE AXIS.  AFTER DAY 2...THERE IS A LARGE MODEL DIVERGENCE IN THE
TRACK OF HELENE.  THE GFDL AND UK QUICKLY ACCELERATE THE SYSTEM TO
THE NORTH IN ADVANCE OF A STRONG SHORT WAVE MOVING OFF OF THE US
ATLANTIC SEABOARD.  IN CONTRAST...THE GFS...THE GFS ENSEMBLE AND
THE ECMWF MODELS TURN HELENE TO THE WEST-NORTHWEST AND DO NOT HAVE
IT PICKED UP BY THE SHORT WAVE TROUGH.  THE NOGAPS AND GFDN
MODELS...AS WELL AS CONSENSUS APPROACHES...SPLIT THE DIFFERENCE. 
AS IT IS DIFFICULT TO KNOW WHICH GROUP WILL BE CORRECT...THE TRACK
FORECAST DEPICTED HERE IS CLOSE TO THE CONU CONSENSUS AND IS
SLIGHTLY SLOWER AND JUST TO THE RIGHT OF THE PREVIOUS FORECAST. 
PERHAPS WITH THE MODEL ASSIMILATION OF THE SALEX DROPWINDSONDES...
THE LARGE MODEL DIFFERENCES WILL BE REDUCED IN THE NEXT FORECAST
CYCLE.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      16/2100Z 19.3N  46.3W    65 KT
 12HR VT     17/0600Z 20.0N  47.4W    70 KT
 24HR VT     17/1800Z 20.9N  48.7W    80 KT
 36HR VT     18/0600Z 21.8N  49.8W    85 KT
 48HR VT     18/1800Z 22.6N  51.0W    85 KT
 72HR VT     19/1800Z 24.0N  53.5W    85 KT
 96HR VT     20/1800Z 26.0N  56.0W    80 KT
120HR VT     21/1800Z 29.0N  57.5W    75 KT
 
$$
FORECASTER BEVEN/LANDSEA
 
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