Hurricane HELENE

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HURRICANE HELENE DISCUSSION NUMBER  19
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082006
1100 PM EDT SAT SEP 16 2006
 
THE INITIAL MOTION ESTIMATE IS 300/09 KT...DESPITE A SHORT TERM
MOTION OF 290/09 BETWEEN 18Z AND 00Z. THE LATTER MOTION IS BELIEVED
TO BE A SHORT TERM WOBBLE AS THE CENTER REORGANIZES WITHIN THE DEEP
CONVECTION. THE MODELS REMAIN IN EXCELLENT AGREEMENT ON HELENE
MOVING WEST-NORTHWEST TO NORTHWESTWARD FOR THE NEXT 36-48 HOURS
TOWARD A WEAKNESS IN THE SUBTROPICAL RIDGE...AND THEN MAKE A SLIGHT
WESTWARD BEND IN 48-72 HOURS. AFTER THAT...THE MODELS DIVERGE QUITE
SIGNIFICANTLY WITH THE GFDL...UKMET...CANADIAN...AND NOGAPS MODEL
TAKING HELENE NORTHWARD...WHEREAS THE GFS...GFS ENSEMBLE...THE
BAMS...AND THE STATISTICAL-DYNAMICAL MODELS KEEP THE HURRICANE ON A
MORE WESTWARD COURSE AS A MID-LEVEL RIDGE BUILDS TO THE NORTH AND
NORTHWEST OF HELENE OFF THE U.S. MID-ATLANTIC COAST BY DAY 5. WATER
VAPOR IMAGERY AND 00Z UPPER-AIR DATA INDICATE THE UPPER-LOW THAT
HAS RECENTLY MOVED OFF THE U.S. MID-ATLANTIC COAST HAS BEEN MOVING
EASTWARD SIMILAR TO THE GFS SOLUTION...RATHER THAN DIGGING
SOUTHEASTWARD TOWARD BERMUDA AS FORECAST BY THE OTHER MODELS. WITH
THE STRONGEST WINDS NOW INDICATED BY WATER VAPOR IMAGERY AND
UPPER-AIR DATA TO BE ON THE SOUTH AND EAST SIDE OF THE UPPER-LOW...
THE 18Z GFS RUN MAY BE INDICATING A SIGNIFICANT CHANGE IN THE MODEL
FORECAST TRACKS COULD BE FORTHCOMING. HOWEVER...UNTIL THE OTHER
MODELS MAKE A CHANGE SIMILAR TO THE GFS...THE OFFICIAL TRACK WAS
ONLY NUDGED SLIGHTLY WESTWARD...BASED MAINLY ON THE MORE SOUTHERLY
INITIAL POSITION...AND IS SIMILAR TO BUT A LITTLE SLOWER THAN THE
NHC MODEL CONSENSUS AT 96 AND 120 HOURS.
 
AN EARLIER WINDSAT MICROWAVE OVERPASS INDICATED HELENE POSSESSED A
WELL DEVELOPED LOW-LEVEL EYE FEATURE. SINCE THEN...THE RAGGED EYE
FEATURE NOTED PREVIOUSLY IN CONVENTIONAL SATELLITE IMAGERY HAS
ERODED...ALTHOUGH THE PAST COUPLE OF IMAGES SUGGEST THAT A BANDING
EYE FEATURE MAY BE FORMING. THE INITIAL INTENSITY WAS INCREASED TO
70 KT BASED ON A BLEND OF SATELLITE ESTIMATES FROM TAFB AND SAB.
ONLY SLOW INTENSIFICATION IS EXPECTED AS HELENE CONTINUES TO BATTLE
PERIODIC INTRUSIONS OF DRY AIR. THE UPPER-LEVEL FLOW PATTERN AND
UPSTREAM SSTS FAVORS MORE SIGNIFICANT INTENSIFICATION...BUT IT IS
TOO DIFFICULT TO PREDICT AT THIS TIME JUST WHEN AND IF HELENE WILL
EVENTUALLY OVERCOME THE EFFECTS OF THE MID-LEVEL DRY AIR.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      17/0300Z 19.5N  47.5W    70 KT
 12HR VT     17/1200Z 20.2N  48.5W    75 KT
 24HR VT     18/0000Z 21.2N  49.7W    80 KT
 36HR VT     18/1200Z 22.1N  50.8W    85 KT
 48HR VT     19/0000Z 22.9N  52.3W    90 KT
 72HR VT     20/0000Z 24.1N  55.0W    90 KT
 96HR VT     21/0000Z 26.0N  57.0W    85 KT
120HR VT     22/0000Z 29.0N  58.5W    80 KT
 
$$
FORECASTER STEWART
 
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