Tropical Storm HELENE

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TROPICAL STORM HELENE DISCUSSION NUMBER  43
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082006
1100 PM EDT FRI SEP 22 2006
 
HELENE IS LOOKING SOMEWHAT LESS TROPICAL IN SATELLITE IMAGERY THIS
EVENING.  ONLY LIMITED DEEP CONVECTION REMAINS NEAR THE CENTER...
WITH THE BULK OF THE COLDER TOPS NOW WELL TO THE NORTH AND
NORTHWEST OF THE CENTER.  SATELLITE INTENSITY ESTIMATES ARE 65 KT
FROM TAFB...55 KT FROM AFWA...AND EXTRATROPICAL FROM SAB.  QUIKSCAT
DATA FROM 22Z SHOWED 50-60 KT WINDS SOUTHEAST OF THE CENTER...WITH
ONE QUESTIONABLE-LOOKING 65 KT VECTOR IN THE SOUTHWEST QUADRANT. 
BASED ON THE AVAILABLE DATA...HELENE IS DOWNGRADED TO A 60-KT
TROPICAL STORM.  THE INITIAL AND FORECAST WIND RADII HAVE BEEN
EXPANDED CONSIDERABLY BASED ON THE QUIKSCAT DATA.

HELENE TURNED TO THE RIGHT BETWEEN 18Z AND 00Z...BUT SINCE THEN
LOOKS LIKE IT MAY BE TURNING BACK TO THE LEFT.  THE INITIAL MOTION
IS A SOMEWHAT UNCERTAIN 070/20.  HELENE SHOULD CONTINUE GENERALLY
EAST-NORTHEASTWARD TO NORTHEASTWARD FOR 72 HR IN DEEP SOUTHWESTERLY
FLOW.  AFTER THAT TIME...THERE IS SERIOUS DIVERGENCE IN THE MODEL
FORECASTS.  THE 12Z ECMWF...THE 18Z GFS...AND THE 18Z NOGAPS ALL
TURN HELENE SOUTHWARD AND PASS NEAR OR OVER THE AZORES.  ON THE
OTHER HAND...THE 12Z UKMET...THE 18Z GFDL...AND THE 18Z GFS
ENSEMBLE-MEAN ALL FORECAST HELENE TO CONTINUE NORTHEASTWARD.  SINCE
THE 18Z GFS AND NOGAPS RUNS SHOW MAJOR CHANGES FROM THEIR PREVIOUS
RUNS THAT TOOK HELENE NORTHEASTWARD...IT IS TOO SOON TO MAKE MAJOR
CHANGES TO THE FORECAST TRACK.  THUS...THE NEW TRACK CALLS FOR A
CONTINUED NORTHEASTWARD MOTION AFTER 72 HR.  THE NEW TRACK IS SOUTH
OF THE PREVIOUS TRACK AND IN BEST AGREEMENT WITH THE UKMET.

SATELLITE IMAGERY SHOWS COLD AIR STRATOCUMULUS CLOUDS WEST OF THE
CENTER...SUGGESTING THE THE COLD FRONT THAT THE LARGE-SCALE MODELS
HAVE BEEN FORECASTING TO INTERACT WITH THE STORM MAY BE A LITTLE
STRONGER THAN PREVIOUSLY EXPECTED.  THIS HAS LED TO AN EARLIER
EXTRATROPICAL TRANSITION THAN FORECAST 24-48 HR AGO.  ALTHOUGH THE
LARGE-SCALE MODEL GUIDANCE SUGGESTS THAT HELENE MAY KEEP A WARM
CORE OF SOME TYPE FOR THE NEXT 3-4 DAYS...IT IS EXPECTED THAT
ENOUGH COLD AIR WILL ENTRAIN INTO THE STORM TO COMPLETE
EXTRATROPICAL TRANSITION IN 12-24 HR.  HELENE WILL LIKELY
STRENGTHEN SOME AS AN EXTRATROPICAL LOW THROUGH 36 HR...THEN
GRADUALLY WEAKEN THEREAFTER.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      23/0300Z 36.0N  50.6W    60 KT
 12HR VT     23/1200Z 37.0N  47.1W    60 KT...BECOMING EXTRATROPICAL
 24HR VT     24/0000Z 38.5N  42.6W    65 KT...EXTRATROPICAL
 36HR VT     24/1200Z 40.0N  38.0W    65 KT...EXTRATROPICAL
 48HR VT     25/0000Z 42.1N  33.7W    60 KT...EXTRATROPICAL
 72HR VT     26/0000Z 45.5N  26.0W    55 KT...EXTRATROPICAL
 96HR VT     27/0000Z 49.0N  18.0W    45 KT...EXTRATROPICAL
120HR VT     28/0000Z 53.0N  12.0W    40 KT...EXTRATROPICAL
 
$$
FORECASTER BEVEN
 
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