Hurricane HELENE

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HURRICANE HELENE DISCUSSION NUMBER  24
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082006
500 AM EDT MON SEP 18 2006
 
THE INITIAL MOTION IS 340/6.  WATER VAPOR IMAGERY SHOWS A SMALL
MID- TO UPPER-LEVEL LOW LOCATED A FEW HUNDRED MILES TO THE
NORTH-NORTHWEST OF THE HURRICANE...AND THIS UPPER LOW MAY BE
RESPONSIBLE FOR THE MORE NORTHWARD CURRENT MOTION.  AS THIS
LOW...ALONG WITH HURRICANE GORDON...LIFT OUT OVER THE NEXT DAY OR
SO...RIDGING SHOULD DEVELOP TO THE NORTH OF HELENE.  THIS RIDGING
IS EXPECTED TO FORCE HELENE ON MORE OF A WESTWARD TRACK OVER THE
NEXT DAY OR TWO...ALTHOUGH IT IS WORTH NOTING THAT THE GUIDANCE HAS
HAD A DECIDED SHORT-TERM WESTWARD BIAS OVER THE LAST COUPLE OF
DAYS.
 
THE LONGER RANGE PROGNOSIS IS LESS CLEAR. A DEEP-LAYER LOW CURRENTLY
EXTENDS SOUTHWARD FROM THE WESTERN ATLANTIC INTO THE BAHAMAS...AND
A SECOND LARGE TROUGH IS MOVING THROUGH THE NORTHERN PLAINS. MODELS
ARE IN GENERAL AGREEMENT ON WEAKENING THE WESTERN ATLANTIC LOW AND
BRINGING THE SECOND TROUGH THROUGH NEW ENGLAND IN 4 TO 5 DAYS...
ALLOWING SOME RIDGING TO DEVELOP IN THE WESTERN ATLANTIC BY DAY 5.
HOWEVER...THE SPECIFIC INTERACTIONS BETWEEN HELENE AND THESE
VARIOUS FEATURES VARY WIDELY AMONG THE MODELS. ON THE RIGHT SIDE OF
THE GUIDANCE SUITE...THE GFDL AND CANADIAN MODELS HAVE HELENE
LIFTED NORTH BY THE WESTERN ATLANTIC LOW JUST ENOUGH TO THEN BE
PICKED UP BY THE PLAINS TROUGH. ON THE LEFT ARE THE UKMET AND
NOGAPS...WHICH KEEP HELENE FARTHER SOUTH INITIALLY AND END UP WITH
IT CAPTURED OR SLOWED BY THE WESTERN ATLANTIC RIDGE ON DAY 5. THE
GFS...WHICH HAD BEEN LEANING LEFT...HAS NOW SHIFTED TO THE CENTER
OF THE GUIDANCE ENVELOPE.  THE 00Z RUN OF THE GFS FORECASTS THE
WESTERN ATLANTIC RIDGE TO BE WEAKER AND DISPLACED TO THE SOUTH OF
THE PREVIOUS RUN...AND THIS IS ENOUGH TO RESULT IN A RATHER LARGE
NORTHWARD SHIFT IN THE TRACK OF HELENE IN THE MODEL. CONVERSELY THE
ECMWF...WHICH HAD BEEN SHOWING A RAPID RECURVATURE...HAS SHIFTED
LEFT AND IS NOW PRETTY CLOSE TO THE GFS. THE BOTTOM LINE OF ALL
THIS IS THAT WHAT HAPPENS IN THE NEXT 24 TO 48 HOURS...THAT
IS...HOW MUCH LATITUDE HELENE CAN GAIN...COULD MAKE ALL THE
DIFFERENCE BETWEEN A QUICK RECURVATURE AND AN AGONIZINGLY SLOW PATH
THROUGH THE WESTERN ATLANTIC.  THE OFFICIAL FORECAST HAS BEEN
SHIFTED A LITTLE TO THE NORTHEAST OF THE PREVIOUS TRACK...BUT IS
STILL TO THE LEFT OF THE GFS/ECMWF AND THE DYNAMICAL MODEL
CONSENSUS.  IT IS PERHAPS OBVIOUS BY NOW THAT I CONSIDER THE TRACK
FORECAST TO HAVE MORE THAN THE NORMAL AMOUNT OF UNCERTAINTY.
 
EYEWALL CONVECTION IS A LITTLE COLDER AND MORE SYMMETRIC THAN IT WAS
6 HOURS AGO. SUBJECTIVE DVORAK INTENSITY ESTIMATES FROM TAFB AND
SAB ARE 102 KT...WHILE THE CIMSS/ADT ESTIMATES ARE A LITTLE HIGHER
AND HAVE BEEN INCREASING. THE ADVISORY ESTIMATE IS INCREASED TO 105
KT. THE OUTFLOW PATTERN REMAINS VERY STRONG...ALTHOUGH THERE HAS
BEEN SOME FLATTENING ON THE WESTERN SIDE LIKELY ASSOCIATED WITH THE
SMALL UPPER LOW. THIS LOW COMPLICATES THE SHORT-TERM INTENSITY
FORECAST.  ALTHOUGH AT THE MOMENT IT MAY BE IMPEDING THE OUTFLOW...
WATER VAPOR IMAGERY SUGGESTS THAT VORTICITY FROM THIS LOW IS BEING
ENTRAINED INTO HELENE...AND THIS KIND OF TROUGH INTERACTION CAN
SOMETIMES PROVIDE A KICK START FOR ADDITIONAL INTENSIFICATION.  THE
OFFICIAL FORECAST IS SLIGHTLY ABOVE THE SHIPS AND GFDL GUIDANCE. 
ALL INDICATIONS ARE THAT HELENE WILL BE AT OR NEAR MAJOR HURRICANE
STATUS THROUGH THE FORECAST PERIOD.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      18/0900Z 22.3N  49.6W   105 KT
 12HR VT     18/1800Z 23.1N  50.4W   110 KT
 24HR VT     19/0600Z 23.7N  51.7W   115 KT
 36HR VT     19/1800Z 24.0N  53.3W   115 KT
 48HR VT     20/0600Z 24.3N  55.0W   110 KT
 72HR VT     21/0600Z 26.0N  57.5W   105 KT
 96HR VT     22/0600Z 28.2N  58.9W   100 KT
120HR VT     23/0600Z 30.0N  60.0W    95 KT
 
$$
FORECASTER FRANKLIN
 
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