Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
210 PM EST Tue Dec 25 2018

Valid 12Z Sat Dec 29 2018 - 12Z Wed Jan 02 2019

The ensemble means agree quite well for the large scale evolution
during days 4-8 (Sat-Wed), with a general upper low migrating from
the northwestern Pacific/western Aleutians into the Bering Sea
while strong ridging builds over Mainland Alaska and western
Canada.  This ridge will rapidly displace a Mainland/Gulf of
Alaska trough on Sat.  The mean pattern should promote a
significant flow of moisture into the northeastern Pacific/Gulf of
Alaska region and heavy precipitation potential from the Alaska
Peninsula to the Panhandle.  Individual storm systems will likely
produce episodes of strong winds and high waves as well.

As for the details, the first priority in the forecast was to lean
away from operational 00/06/12Z GFS runs due to various poor
comparisons to other model/ensemble solutions.  These include
being slower than consensus with the initial Mainland/Gulf trough,
likely being too aggressive in pushing upstream upper
dynamics/surface low pressure eastward into the strong ridge
building over northwestern North America, and straying more
westward then southward with deep low pressure initially near the
western Aleutians.  The 12Z GFS made some adjustments away from
the 00Z/06Z runs but was still not close enough to consensus to
use in the forecast.

Among remaining guidance there is a clear signal that impulses
within fast flow around the southern/eastern side of the mean
upper low will produce multiple energetic systems tracking from
the North Pacific into Bering Sea.  As the overall pattern drifts
eastward the storm track may eventually reach into western
Mainland Alaska--but again less likely into the Gulf as depicted
in GFS runs.  From Sat into Mon the 00Z ECMWF/CMC and 06Z FV3 GFS
provided decent agreement for specifics of the deep system
initially near the western Aleutians while indicating somewhat
looser clustering for North Pacific into Bering low pressure. 
Confidence in any solution is still moderate at best, favoring
some 06Z GEFS/00Z ECMWF mean input (40 percent total) even in the
first half of the forecast period.

By days 7-8 Tue-Wed the forecast incorporated increasing ensemble
weight, 70 to 100 percent respectively, to represent the agreeable
large scale pattern while downplaying increasingly uncertain
detail differences.  The 06Z GEFS mean offered favorably deeper
low pressure but a farther south/southeast track than the 00Z
ECMWF mean or compromise 00Z CMC mean.  For next Wed starting with
a 60/40 blend of the ECMWF mean vs GEFS mean yielded the desired
intermediate position with some manual editing to provide a little
better detail.

Rausch


WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can be found at:

http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html