Hurricane HELENE

ZCZC MIATCDAT3 ALL
TTAA00 KNHC DDHHMM
HURRICANE HELENE DISCUSSION NUMBER  47
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082006
500 PM EDT SAT SEP 23 2006
 
HELENE CURRENTLY DISPLAYS BOTH TROPICAL AND EXTRATROPICAL
CHARACTERISTICS.  THE CYCLONE HAS A FRONTAL-LIKE AND ASYMMETRIC
APPEARANCE IN SATELLITE IMAGERY WITH LIMITED DEEP CONVECTION.
HOWEVER...FSU CYCLONE PHASE SPACE DIAGRAMS DIAGNOSE HELENE TO STILL
HAVE A DEEP WARM CORE WHICH IS CONSISTENT WITH ITS ABILITY TO
STILL PRODUCE CONVECTION...ALBEIT LIMITED...NEAR THE CENTER.  HELENE
IS SOMEWHERE IN PROCESS OF EXTRATROPICAL TRANSITION AND IT IS NOT
CLEAR AT WHAT POINT THE TRANSFORMATION WILL BE COMPLETE.  SINCE THE
OVERALL STRUCTURE APPEARS UNCHANGED SINCE THE EARLIER 0915 UTC
QUIKSCAT DATA...WHICH SHOWED 80 KT MAXIMUM WINDS...THE INITIAL
INTENSITY WILL REMAIN AT 80 KT FOR THIS ADVISORY.  WATER VAPOR
IMAGERY SHOWS A MIDDLE TO UPPER-LEVEL SHORTWAVE TROUGH QUICKLY
APPROACHING HELENE FROM THE WEST RESULTING IN THE UPPER-LEVEL
OUTFLOW BECOMING INCREASINGLY RESTRICTED.  IF THIS TREND
CONTINUES...AN INCREASINGLY SHEARED HELENE COULD LOSE WHAT REMAINS
OF ITS INNER CORE CONVECTION AND THEREFORE LOSE WHAT TROPICAL
CHARACTERISTICS IT HAS LEFT.  ASSUMING EXTRATROPICAL TRANSITION
COMPLETES IN ABOUT 12 HOURS...AS CURRENTLY FORECAST...THE CYCLONE
SHOULD GRADUALLY WEAKEN OVER THE NEXT DAY OR SO. MORE RAPID
WEAKENING IS FORECAST THEREAFTER AS THE CYCLONE LOSES SOME OF ITS
BAROCLINIC SUPPORT.
 
HELENE HAS BEEN MOVING A LITTLE SOUTH OF THE PREVIOUS FORECAST TRACK
WITH AN INITIAL MOTION ESTIMATE OF 070/19.  THERE HAS OTHERWISE
BEEN LITTLE CHANGE IN THE TRACK FORECAST PHILOSOPHY IN THE
SHORT-TERM...AND GUIDANCE REMAINS TIGHTLY CLUSTERED THROUGH 48
HOURS.  THEREAFTER...THE TRACK GUIDANCE BEGINS TO DIVERGE BUT NOT
NEARLY AS MUCH AS BEFORE.  THE NOGAPS MODEL...WHICH WAS PREVIOUSLY
THE NORTHERN OUTLIER...HAS SHIFTED SIGNIFICANTLY SOUTH AND IS NOW
ALONG THE SOUTHERN SIDE OF THE GUIDANCE SUITE.  THE UKMET
MODEL...WHICH PREVIOUSLY SHOWED A TURN DUE SOUTH WITH THE
ECMWF...HAS SHIFTED NORTH AND IS NOW IN GOOD AGREEMENT WITH THE
OFFICIAL FORECAST.  THE ECMWF CONTINUES TO SHOW THE SOUTHWARD
TURN...BUT THIS IS CONSIDERED A LESS LIKELY SCENARIO GIVEN THE NOW
BETTER AGREEMENT AMONG THE OTHER DYNAMICAL MODELS. THE OFFICIAL
FORECAST IS SHIFTED JUST A LITTLE SOUTH OF THE PREVIOUS ONE. 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      23/2100Z 38.0N  44.4W    80 KT
 12HR VT     24/0600Z 39.1N  40.9W    75 KT...EXTRATROPICAL
 24HR VT     24/1800Z 41.3N  36.4W    70 KT...EXTRATROPICAL
 36HR VT     25/0600Z 43.2N  32.4W    65 KT...EXTRATROPICAL
 48HR VT     25/1800Z 44.6N  28.8W    55 KT...EXTRATROPICAL
 72HR VT     26/1800Z 47.0N  21.0W    45 KT...EXTRATROPICAL
 96HR VT     27/1800Z 49.0N  15.0W    35 KT...EXTRATROPICAL
120HR VT     28/1800Z 51.0N  11.0W    30 KT...EXTRATROPICAL
 
$$
FORECASTER RHOME/KNABB
 
NNNN