Tropical Storm HELENE

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TROPICAL STORM HELENE DISCUSSION NUMBER  11
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082006
1100 PM EDT THU SEP 14 2006
 
THE INITIAL MOTION REMAINS 285/13...BASED PRIMARILY ON MICROWAVE
SATELLITE FIX POSITIONS. THE 18Z NHC MODEL GUIDANCE HAS LESS SPREAD
NOW THAN PREVIOUS ADVISORIES...WITH THE GFS AND NOGAPS MODELS
HAVING SHIFTED MORE WESTWARD AND CLOSER TO THE UKMET 12Z SOLUTION.
THE NEW 18Z UKMET RUN IS ALSO MUCH SLOWER NOW AND VERY SIMILAR TO
THE 18Z NOGAPS MODEL RUN. THE KEY PLAYER DURING THE FORECAST CYCLE
IS THE MID-LEVEL SHORTWAVE TROUGH CURRENTLY OVER THE NORTHEASTERN
U.S. THAT IS FORECAST BY ALL OF THE MODELS TO DIG SOUTHEASTWARD AND
CUTOFF INTO A LOW PRESSURE SYSTEM. HOWEVER...THE UKMET AND NOGAPS
MODELS ARE NOW WEAKENING THE LOW AND LIFTING IT OUT SOONER THAN THE
GFS...ECMWF... GFDL...AND CANADIAN MODELS. GIVEN THAT THE
MID-LATITUDE FLOW ACROSS THE NORTHERN U.S. AND THE NORTH ATLANTIC
IS FORECAST BY ALL THE MODELS TO BE FAST ZONAL FLOW...LESS
AMPLIFICATION OF THE TROUGH SEEMS MORE REASONABLE...WHICH SHOULD
DECREASE THE EFFECT OF ANY NORTHWARD STEERING FLOW ON THE WAST SIDE
OF THE TROUGH. THE RESULT IS THAT THE STEERING FLOW SHOULD WEAKEN
CONSIDERABLY BY 96 AND 120 HOURS AS MID-LEVEL RIDGING BUILDS SLOWLY
WESTWARD TO THE NORTH OF HELENE. THE OFFICIAL FORECAST TRACK IS A
LITTLE SOUTH OF THE PREVIOUS TRACK AND IS CLOSE TO THE GUNA AND
CONU MODEL CONSENSUS FORECAST TRACKS.

HELENE HAS BEEN HAVING A DIFFICULT TIME MAINTAINING CENTRAL DEEP
CONVECTION FOR MORE THAN A JUST FEW HOURS AT A TIME DUE TO A LARGE
SLUG OF DRY AIR PUNCHING INTO THE SOUTHERN AND EASTERN QUADRANTS OF
THE CYCLONE. SINCE THIS PATTERN SEEMS TO BE UNLIKELY TO CHANGE ANY
TIME SOON...THE INTENSITY FORECAST WAS DECREASED SLIGHTLY DURING
THE FIRST 72 HOURS SIMILAR TO THE SHIPS MODEL. AFTER THAT... A
SLIGHTLY FASTER RATE OF INTENSIFICATION IS FORECAST AS PER THE GFDL
AND FSU MODELS...WHICH SEEMS REASONABLE SINCE HELENE WILL BE MOVING
OVER 28C AND WARMER SSTS AND ALSO BE UNDERNEATH A VERY FAVORABLE
UPPER-LEVEL OUTFLOW REGIME.

FORECAST POSITIONS AND MAX WINDS
 
INITIAL      15/0300Z 14.5N  38.9W    40 KT
 12HR VT     15/1200Z 15.3N  40.8W    45 KT
 24HR VT     16/0000Z 16.4N  43.2W    50 KT
 36HR VT     16/1200Z 17.5N  45.2W    60 KT
 48HR VT     17/0000Z 18.5N  46.7W    70 KT
 72HR VT     18/0000Z 20.3N  49.4W    80 KT
 96HR VT     19/0000Z 22.4N  51.9W    90 KT
120HR VT     20/0000Z 24.5N  54.0W    90 KT
 
$$
FORECASTER STEWART
 
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