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Tropical Storm HELENE

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TROPICAL STORM HELENE DISCUSSION NUMBER   7
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082006
1100 PM AST WED SEP 13 2006
 
INFRARED SATELLITE IMAGERY THIS EVENING INDICATES CONTINUED GRADUAL
DEVELOPMENT WITH A DISTINCT CURVED BAND ON THE NORTHWESTERN
SEMI-CIRCLE.  BASED UPON UNANIMOUS DVORAK T-NUMBERS OF 2.5 AND
EARLIER AMSU INTENSITY ESTIMATES...THE INTENSITY IS ANALYZED AT 35
KT AND IS GIVEN THE NAME HELENE.
 
THE TROPICAL STORM APPEARS TO HAVE SPED UP WITH AN INITIAL MOTION OF
285/19...THOUGH THIS IS SOMEWHAT UNCERTAIN GIVEN THE SPREAD IN
POSITIONS.  HELENE SHOULD CONTINUE WEST-NORTHWESTWARD FOR THE NEXT
DAY UNDER THE INFLUENCE OF A DEEP-LAYER RIDGE.  BETWEEN 36 AND 72
HOURS...A MID TO UPPER TROUGH INDUCES A WEAKNESS IN THE RIDGE AND
THIS MAY ACT TO STEER THE SYSTEM TOWARD THE NORTHWEST.  AFTER THIS
TROUGH BY-PASSES HELENE...A WEST-NORTHWESTWARD TRACK SHOULD RESUME
AT DAYS 4 AND 5.  ALL RELIABLE GUIDANCE SHOWS THIS STAIR-STEP
SCENARIO...WITH NOGAPS BEING A MODEST OUTLIER TO THE SOUTHWEST OF
THE PACK.  THE FORECAST TRACK IS NEAR THE MODEL CONSENSUS AND IS TO
THE RIGHT OF THE PREVIOUS FORECAST.

WHILE THERE IS A LARGE SAHARAN AIR LAYER TO THE NORTH AND WEST OF
HELENE...TOTAL PRECIPITABLE WATER ANALYSES FROM CIRA SUGGEST THAT
THIS VERY DRY AIR IS NOT REACHING HELENE'S INNER CORE.  THE
SYSTEM'S PROJECTED TRACK SHOULD TAKE IT OVER WARM WATER...THROUGH
ABUNDANT MOISTURE AND INTO LIGHT VERTICAL SHEAR FOR THE NEXT THREE
DAYS.  SHIPS AND THE FSU SUPER-ENSEMBLE THUS BRING HELENE UP TO
ABOUT 100 KT IN THREE DAYS.  THE GFDL...ON THE OTHER HAND...IS
SUBSTANTIALLY WEAKER...LIKELY BECAUSE IT DEPICTS MORE SHEAR BEING
IMPARTED ON HELENE DUE TO THE AFOREMENTIONED MID TO UPPER TROUGH. 
AT DAYS 4 AND 5...ALL GUIDANCE SUGGESTS THE SHEAR WILL PICK UP AND
LIMIT FURTHER INTENSIFICATION.  THE INTENSITY FORECAST IS CLOSE
TO...BUT JUST BELOW...BOTH SHIPS AND THE FSU SUPER-ENSEMBLE.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      14/0300Z 13.3N  32.7W    35 KT
 12HR VT     14/1200Z 13.7N  35.0W    45 KT
 24HR VT     15/0000Z 14.7N  37.4W    55 KT
 36HR VT     15/1200Z 15.9N  39.5W    65 KT
 48HR VT     16/0000Z 17.4N  41.0W    75 KT
 72HR VT     17/0000Z 20.0N  43.5W    90 KT
 96HR VT     18/0000Z 21.5N  45.5W    95 KT
120HR VT     19/0000Z 22.5N  49.0W    85 KT
 
$$
FORECASTER AVILA/LANDSEA
 
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