Prognostic Meteorological Discussion
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FXAK02 KWNH 171859
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ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
159 PM EST SUN DEC 17 2017

VALID 12Z THU DEC 21 2017 - 12Z MON DEC 25 2017

DETERMINISTIC AND ENSEMBLE GUIDANCE CONTINUE TO SHOW A STRONG
SIGNAL THAT UPPER RIDGING BUILDING IN THE EAST/NORTHEAST PACIFIC
WILL EXTEND NORTHWARD INTO THE ALASKA REGION, DOMINATING THE
WEATHER PATTERN LATE THIS WEEK AND THROUGH NEXT WEEKEND.  THERE
REMAINS SOME UNCERTAINTY IN THE EXACT AXIS AND AMPLITUDE OF THE
BUILDING RIDGE, WITH THE LATEST 00Z ECMWF/ECMWF ENSEMBLE MEAN
ADVERTISING A MORE AMPLIFIED AND FARTHER WEST RIDGE THAN THE 00Z
GFS/GEFS MEAN BY NEXT WEEKEND.  ANOTHER AREA OF FORECAST
UNCERTAINTY IS WITHIN THE WESTERN PERIPHERY OF THE BUILDING RIDGE,
WHERE DETERMINISTIC GUIDANCE CONTINUES TO SHOW RUN-TO-RUN
INCONSISTENCIES WITH THE EVOLUTION OF APPROACHING
SHORTWAVES/SURFACE LOWS (BOTH WITH FEATURES LIFTING TOWARDS THE
EASTERN/CENTRAL ALEUTIANS AND WITH ENERGY FROM A PERSISTENT UPPER
LOW IN FAR EASTERN RUSSIA CROSSING INTO THE BERING SEA).

THE WPC FORECAST STARTED WITH A BLEND HEAVILY WEIGHTED TOWARDS
DETERMINISTIC GUIDANCE, BUT THEN MORE OF THE MEANS WERE
INCORPORATED BY DAY 6(SATURDAY) IN ORDER TO ACCOUNT FOR THE
UNCERTAINTY WITH FEATURES APPROACHING THE WESTERN EDGE OF THE
BUILDING RIDGE.  ALSO, THE TREND IN ENSEMBLE MEMBERS OVER THE PAST
FEW MODEL CYCLES SUPPORTS THE ECMWF/ECMWF ENSEMBLE MEAN`S IDEA OF
A MORE AMPLIFIED/FARTHER WEST RIDGE THAN WHAT IS CURRENTLY SHOWN
IN THE 00Z GFS/GEFS MEAN, SO MORE WEIGHT WAS GIVEN TO THE
ECMWF/ECMWF ENSEMBLE MEAN THAN THE GFS/GEFS MEAN.


GERHARDT


$$





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