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Alaska Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1847Z Oct 22, 2014)
 
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ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
247 PM EDT WED OCT 22 2014

VALID 12Z SUN OCT 26 2014 - 12Z THU OCT 30 2014

THE LOSS OF SOME SATELLITE DATA INGEST INTO NUMERICAL GUIDANCE
APPEARS TO COINCIDE WITH LARGE AND SIGNIFICANT CHANGES IN THE
LARGE-SCALE PATTERN DEPICTED ACROSS THE NORTHERN PACIFIC BY
GUIDANCE WITHIN THE LAST 24 HOURS TO WARRANT A CONSERVATIVE AND
PERHAPS CAUTIOUS APPROACH TO MAKING WHOLESALE CHANGES IN THE
MEDIUM RANGE FORECAST SINCE YESTERDAY.  OTHER FACTORS THAT
CONTRIBUTE TO THE UNCERTAINTY COULD BE THE LINGERING SPREAD WITH
HOW THE LONGWAVE TROUGH ACROSS EAST ASIA PROGRESSES AND/OR
SEPARATES...
WHICH IN TURN INFLUENCES THE TIMING/AMPLITUDE OF DOWNSTREAM
SYSTEMS...AS WELL AS THE INTERACTION OF TS ANA WITH A LARGE-SCALE
SYNOPTIC WAVE TO ITS WEST.  IF THE RESULT IS
SYNERGISTIC...COMPENSATING INFLUENCES COULD OCCUR UPSTREAM FROM
THE ASSOCIATED TROUGH OVER THE GULF OF ALASKA BY THIS
WEEKEND...WHICH WOULD MEAN A LARGER AMPLITUDE RIDGE ACROSS PARTS
OF ALASKA WHICH SOME GUIDANCE SHOWED YESTERDAY.

THESE CONSIDERATIONS ASIDE...THE CURRENT ENSEMBLE MEANS ARE
FASTER/FLATTER SINCE YESTERDAY AND AT TIMES BECOME NEARLY OUT OF
PHASE ACROSS THE BERING SEA AND WESTERN MAINLAND DAYS 5/6 BEFORE
IRONING THEMSELVES OUT AND NEARLY MERGING BY DAY 8.  GIVEN SUCH A
HUGE CHANGE IN THE GUIDANCE AND OTHER CONCERNS MENTIONED
ABOVE...HAVE USED 50 PERCENT OF THE LATEST ENSEMBLE MEANS...00Z
ECMWF ENSEMBLE MEAN/00Z NAEFS BCMEAN/06Z GEFS MEAN...TO DRAW UPON
THE MOST DIVERSE COMMUNITY OF GUIDANCE AVAILABLE...AND THEN BLEND
IT WITH 50 PERCENT WPC CONTINUITY FROM YESTERDAY.  THIS APPROACH
REDUCES THE POTENTIAL FOR FLIP-FLOP WHILE HOPEFULLY INCORPORATING
USEFUL INFORMATION FROM THE LATEST GUIDANCE TO NUDGE THE FORECAST
IN THE RIGHT DIRECTION.  IF AND WHEN GUIDANCE SHOWS BETTER
CONTINUITY OR OTHER REASONS FOR INCREASED CONFIDENCE BECOME
AVAILABLE...LARGER CHANGES IN THE FORECAST CAN THEN BE
INCORPORATED.

THE MAIN IMPLICATION OF THE CHANGES INCLUDED IN THE LATEST
FORECAST IS FOR LESS AMPLITUDE AND FASTER TRANSLATION OF THE
ASSOCIATED SHORTWAVE TROUGHS AND RIDGES. 
THUS...TEMPERATURES/POPS/ETC...SHOULD BE SLIGHTLY MORE MODERATED
AND LESS EXTREME...MOSTLY...COMPARED TO YESTERDAY.

JAMES