Hurricane HELENE

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HURRICANE HELENE DISCUSSION NUMBER  33
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082006
1100 AM EDT WED SEP 20 2006
 
THE SATELLITE PRESENTATION ASSOCIATED WITH HELENE HAS DEGRADED THIS
MORNING AS THE EYE IS NO LONGER EVIDENT AND THE CENTRAL CORE
CONVECTION HAS BECOME LESS SYMMETRIC.  WHILE SATELLITE INTENSITY
ESTIMATES ARE UNCHANGED AT 102 KT FROM BOTH TAFB AND SAB...THE
ACTUAL DATA T VALUES ARE MUCH LOWER AT 77 KT.  THE INITIAL
INTENSITY IS LOWERED TO 90 KT GIVEN THESE DATA.  THE REASONS FOR
THE CURRENT WEAKENING ARE NOT READILY APPARENT. MICROWAVE DATA FROM
YESTERDAY SUGGESTED THAT HELENE WAS ENTERING AN EYEWALL REPLACEMENT
CYCLE WHICH MAY HAVE NOT YET COMPLETED.  ADDITIONALLY...WATER VAPOR
IMAGERY SHOWS THAT UPPER LEVEL OUTFLOW IS NOW RESTRICTED TO THE
SOUTHWEST INDICATING THAT VERTICAL SHEAR AND MIDDLE TO UPPER LEVEL
DRY AIR IS IMPINGING UPON THE CYCLONE.  GIVEN THE AFOREMENTIONED
UNCERTAINTIES...THE INTENSITY FORECAST IS TRICKY.  IF THE EYEWALL
REPLACEMENT CYCLE DOES NOT COMPLETE OR THE SHEAR DOES NOT RELAX...
ADDITIONAL WEAKENING IS POSSIBLE.  FURTHER COMPLICATING MATTERS IS
A LARGE MIDDLE TO UPPER TROPOSPHERIC TROUGH CURRENTLY MOVING OFF
THE EAST COAST OF THE UNITED STATES WHICH MAY PROVIDE A TEMPORARY
SHOT OF DIFFLUENT FLOW ALOFT TOMORROW FOLLOWED BY INCREASING SHEAR.
THEREAFTER...THE CYCLONE IS EXPECTED TO START INTERACTING WITH THE
ATTENDANT BAROCLINIC ZONE ULTIMATELY RESULTING IN THE SYSTEM
BEGINNING EXTRATROPICAL TRANSITION IN 72 HOURS. DURING THE
EXTRATROPICAL TRANSITION...HELENE WILL BE SITUATED NEAR THE RIGHT
ENTRANCE REGION OF A STRONG UPPER-LEVEL JET WHICH MAY PROVIDE
ENOUGH ENERGY TO PREVENT WEAKENING. 
 
HELENE IS MOVING A LITTLE LEFT OF THE PREVIOUS ADVISORY WITH AN
INITIAL MOTION ESTIMATE OF 315/10. THE LONG ANTICIPATED
TURN TOWARD THE NORTHWEST APPEARS TO BE UNDERWAY. GLOBAL MODELS ARE
TIGHTLY CLUSTERED ABOUT A GRADUAL NORTHWARD TURN DURING THE NEXT 36
HOURS FOLLOWED BY AN ACCELERATION NORTHEASTWARD AHEAD OF A LARGE
TROUGH MOVING INTO THE WESTERN ATLANTIC.  THE OFFICIAL FORECAST IS
NUDGED A LITTLE TO THE LEFT AND IS SLIGHTLY SLOWER THAN THE
PREVIOUS ADVISORY THROUGH 72 HOURS AND IS LITTLE CHANGED
THEREAFTER. 
 
EVEN THOUGH HELENE IS FORECAST TO RECURVE OVER THE OPEN WATERS OF
THE CENTRAL ATLANTIC...LONG PERIOD SWELLS ARE ALREADY REGISTERING
AT SEVERAL EAST COAST BUOYS. 

FORECAST POSITIONS AND MAX WINDS
 
INITIAL      20/1500Z 26.0N  56.3W    90 KT
 12HR VT     21/0000Z 27.3N  57.0W    90 KT
 24HR VT     21/1200Z 29.4N  57.3W    95 KT
 36HR VT     22/0000Z 32.3N  56.4W    90 KT
 48HR VT     22/1200Z 35.0N  54.5W    85 KT
 72HR VT     23/1200Z 40.5N  47.0W    85 KT...BECOMING EXTRATROPICAL
 96HR VT     24/1200Z 46.0N  36.5W    65 KT...EXTRATROPICAL
120HR VT     25/1200Z 50.0N  28.0W    55 KT...EXTRATROPICAL
 
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FORECASTER RHOME/AVILA
 
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