731 
 WTNT43 KNHC 200243
 TCDAT3
 HURRICANE HELENE DISCUSSION NUMBER  31
 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082006
 1100 PM EDT TUE SEP 19 2006
  
 SATELLITE INTENSITY ESTIMATES AVERAGE NEAR 100 KT FROM TAFB...SAB
 AND AFWA. HOWEVER...A 2039 UTC TRMM OVERPASS INDICATED THAT HELENE
 WAS IN THE MIDDLE OF AN EYEWALL REPLACEMENT CYCLE WHICH SUGGESTS
 THE SLIGHT FILLING OF THE CYCLONE IS NOT YET FINISHED. IN ADDITION
 RECENT INFRARED SATELLITE IMAGERY SHOWS SOME ASYMMETRIES IN THE
 DEEP CONVECTION WITH A FAIRLY RAGGED EYE AS WELL. THE INITIAL
 INTENSITY IS KEPT AT 95 KT FOR THIS ADVISORY.
  
 THE INITIAL MOTION IS 275 DEGREES AT 8 KT...HOWEVER A TURN TOWARD
 THE WEST-NORTHWEST AND NORTHWEST IS EXPECTED SOON. THE TRACK MODEL
 GUIDANCE IS TIGHTLY CLUSTERED THROUGH 48-72 HOURS AND VARIES WITH
 FORWARD SPEED THEREAFTER. THE GFDL MODEL AND GUNA CONSENSUS WERE
 FOLLOWED CLOSELY FOR THIS PACKAGE SINCE THESE MODELS HAVE EXHIBITED
 THE BEST PERFORMANCE THUS FAR WITH HELENE. THE OFFICIAL FORECAST IS
 SIMILAR TO THE PREVIOUS ADVISORY AND INDICATES A QUICK RECURVATURE
 TO THE NORTH IN THE NEXT 12-24 HOURS...AND ACCELERATION TO THE
 NORTHEAST AFTER 48 HOURS ON THE SOUTHEAST SIDE OF A LARGE 500 MB 
 TROUGH MOVING OFF THE U.S. EAST COAST. 
  
 HELENE REMAINS IN A LOW SHEAR ENVIRONMENT FOR NOW WITH STRONG UPPER
 TROPOSPHERIC OUTFLOW OVER ALL BUT THE SOUTHWEST PORTION OF THE
 CIRCULATION. ONCE THE EYEWALL REPLACEMENT IS COMPLETED THERE IS THE
 POTENTIAL FOR SOME STRENGTHENING AND HELENE COULD REGAIN MAJOR
 HURRICANE STATUS IN THE NEXT 12-24 HOURS. BEYOND 36 HOURS
 INCREASING SOUTHWESTERLY SHEAR IS LIKELY TO BRING ABOUT GRADUAL
 WEAKENING. BY 96 HOURS...GLOBAL MODELS...SUCH AS THE GFS...SHOW
 CONSIDERABLE COLD AIR ADVECTION OVER THE SOUTHWESTERN SEMICIRCLE OF
 THE CYCLONE...JUSTIFYING A FORECAST OF TRANSITION TO A STRONG
 EXTRATROPICAL STORM AROUND THAT TIME. 
  
 FORECAST POSITIONS AND MAX WINDS
  
 INITIAL      20/0300Z 24.6N  54.8W    95 KT
  12HR VT     20/1200Z 25.5N  55.7W   100 KT
  24HR VT     21/0000Z 27.5N  56.8W   100 KT
  36HR VT     21/1200Z 30.0N  56.8W    95 KT
  48HR VT     22/0000Z 32.7N  56.0W    95 KT
  72HR VT     23/0000Z 39.0N  50.0W    85 KT
  96HR VT     24/0000Z 45.0N  40.0W    75 KT...EXTRATROPICAL
 120HR VT     25/0000Z 49.0N  30.0W    60 KT...EXTRATROPICAL
  
 $$
 FORECASTER COBB/PASCH