033 
 WTNT43 KNHC 170900
 TCDAT3
 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