Tropical Storm HELENE

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TROPICAL STORM HELENE DISCUSSION NUMBER  44
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082006
500 AM EDT SAT SEP 23 2006
 
A SMALL BURST OF CONVECTION WITH CLOUD TOPS TO -60C HAVE PERSISTED
OVER THE LOW-LEVEL CENTER FOR MORE THAN 6 HOURS...INDICATING THAT
HELENE STILL POSSESSES WARM-CORE TROPICAL CHARACTERISTICS. DVORAK
SATELLITE CURRENT INTENSITY ESTIMATES ARE T3.5/55 KT FROM TAFB AND
XT3.5/55 KT FROM AFWA. GIVEN THE FORWARD SPEED IS NOW 22 KT...THE
ADDITIONAL TRANSLATIONAL SPEED EASILY SUPPORTS 60 KT FOR THE
INTENSITY...ALTHOUGH THE INTENSITY COULD BE HIGHER.
 
THE INITIAL MOTION IS 060/22. THERE IS NO SIGNIFICANT CHANGE TO THE
PREVIOUS FORECAST TRACK OR REASONING. AFTER THE SLIGHT EASTWARD JOG
ON THE PREVIOUS ADVISORY...HELENE APPEARS TO HAVE RETURNED TO A
BASE COURSE OF 060 DEGREES...AND THAT GENERAL MOTION IS FORECAST TO
CONTINUE THROUGHOUT THE PERIOD BASED ON THE GFDL...GFDN...NOGAPS...
GFS...AND GFS ENSEMBLE MODELS. HOWEVER...THERE ARE SOME DISPARITIES
BETWEEN THOSE MODELS AND THE UKMET AND ECMWF MODELS. BY 72 HOURS...
THE LATTER TWO MODELS DROP A PIECE OF STRONG SHORTWAVE ENERGY INTO
THE BASE OF THE LARGE-SCALE TROUGH THAT IS CAPTURING HELENE....AND 
DEVELOP A CLOSED LOW THAT DROPS SOUTHWARD OVER THE AZORES. THE
OTHER MODELS DO THE SAME EXCEPT THAT THEY MERELY WEAKEN HELENE AND
KEEP THE CYCLONE MOVING EAST-NORTHEASTWARD TOWARD THE BRITISH ISLES
RATHER THAN RAPIDLY DEEPENING HELENE AND TAKING THE CYCLONE
SOUTHWARD OVER THE AZORES. GIVEN THAT HELENE IS A DISTINCT ENTITY
THAT IS STILL VERTICALLY DEEP...IT SEEMS UNLIKELY THAT THERE WILL
BE ENOUGH TIME FOR A NEW MID-LEVEL LOW THAT IS FORECAST TO DEVELOP
SOUTH OF HELENE TO CAPTURE THE CYCLONE AND FORCE IT SOUTHWARD IN
72-120 HOURS LIKE THE UKMET AND ECMWF MODELS ARE INDICATING.
 
COLD AIR STRATOCUMULUS CLOUDS ARE POISED TO BE DRAWN INTO THE CENTER
OF HELENE...AND EXTRATROPICAL TRANSITION IS POSSIBLE AS EARLY AS
THE 12-H FORECAST PERIOD. BY 24 AND THROUGH 48 HOURS...A SHOT OF
BAROCLINIC ENERGY BY THE AFOREMENTIONED STRONG SHORTWAVE TROUGH
SHOULD HELP TO INTENSIFY HELENE AS INDICATED BY ALL OF THE MODELS.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      23/0900Z 37.2N  48.2W    60 KT
 12HR VT     23/1800Z 38.2N  44.8W    60 KT...BECOMING EXTRATROPICAL
 24HR VT     24/0600Z 39.7N  40.3W    65 KT...EXTRATROPICAL
 36HR VT     24/1800Z 41.5N  35.8W    65 KT...EXTRATROPICAL
 48HR VT     25/0600Z 43.3N  31.7W    60 KT...EXTRATROPICAL
 72HR VT     26/0600Z 46.8N  23.9W    55 KT...EXTRATROPICAL
 96HR VT     27/0600Z 50.4N  16.5W    45 KT...EXTRATROPICAL
120HR VT     28/0600Z 55.0N  10.0W    40 KT...EXTRATROPICAL
 
$$
FORECASTER STEWART
 
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