Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
240 PM EST Thu Jan 10 2019

Valid 12Z Mon Jan 14 2019 - 12Z Fri Jan 18 2019

Most guidance agrees fairly well that a mean trough aloft over the
Bering Sea into North Pacific as of early day 4 Mon will become
elongated northwest-southeast underneath amplified ridging that
should extend from western Canada through Mainland Alaska and into
the Arctic.  Primary embedded uncertainties relate to individual
systems within/ahead of the overall mean trough and if/when/where
an upper high may close off within the mean ridge.  This pattern
will tend to focus persistent deep-layer flow and moisture into
the Kenai Peninsula/Alaska Peninsula and possibly the extreme
eastern Aleutians.  Resulting precipitation totals will likely be
meaningful though progressive mid-latitude flow across the Pacific
should prevent much of a connection to any low-latitude moisture.

An operational model blend provides a reasonable starting point
for depicting the most likely details early in the period.  There
is still a fair degree of spread for strong low pressure initially
over the Bering Sea.  The 00Z GFS/CMC/UKMET and 12Z/09 ECMWF
cluster the best near the ensemble means over the southern part of
the Bering while the 00Z/10 ECMWF is in the northern part of the
full ensemble spread and the 12Z GFS is in the eastern/northern
part of the envelope.  There is a decent signal for development of
another area of low pressure to the south/southwest of the Alaska
Peninsula by early day 5 Tue but with diverging solutions for
track thereafter.  This part of the forecast seems to have low
predictability as there is likely going to be a fine line between
having the system progress northwestward or remain over the
Pacific.

From this point of divergence, the details remain uncertain from
the Bering Sea through the northeastern Pacific through the end of
the week--favoring a rapid increase of ensemble mean weight in the
forecast.  Latest GEFS/ECMWF means at least provide a coherent
picture with best defined low pressure tending to evolve somewhat
southeastward with time (with a southward trend from yesterday). 
The 00Z CMC mean keeps low pressure farther northward to provide a
little lingering doubt.  Meanwhile within the upper ridge
ECMWF/ECMWF mean solutions have delayed the closing of an upper
high compared to some earlier runs.  The GEFS/ECMWF means are now
fairly similar in showing a closed upper high near the northern
coast of the mainland by day 8 Fri.  Some model solutions still
close off the upper high a little earlier.

A frontal system/possible low pressure will likely come into the
picture over the western part of the forecast domain by Fri as a
broad upper trough with embedded upper low emerges into the
western Pacific.  Given typically low predictability for a day 8
forecast, the ensemble means provide a reasonable starting point
for this system with emphasis on the leading front.  Some
operational models/ensemble members show potential for a better
defined low along the overall frontal zone.

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