Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
225 PM EST Fri Dec 07 2018

Valid 12Z Tue Dec 11 2018 - 12Z Sat Dec 15 2018

At least in the large scale there is still good
agreement/continuity during the forecast period.  Consensus
continues to show very strong Pacific flow carrying along a series
of progressive but strong Aleutians/North Pacific systems that
should gravitate toward the Gulf of Alaska.  At the same time a
mean low aloft should persist over the vicinity of the extreme
northern Bering Sea.  This mean low will likely support some
degree of surface reflection near the western coast of the
mainland with best potential for a defined low near Norton Sound
mid-late week.  The best signal for heaviest 5-day total
precipitation still extends from the eastern Kenai Peninsula
through the Alaska Panhandle.  Some enhanced activity will be
possible over the Aleutians depending on exact track of individual
systems.  These systems will likely produce periods of strong
winds as well.

For today's forecast of individual northern Pacific systems,
operational model runs appear to show a little more scatter than
yesterday.  However ensemble members display a similar degree of
spread and clustering versus 24 hours ago and their means are
comparable in principle at most valid times.  Thus preference is
to rely on the 06Z GEFS/00Z ECMWF means as a base to provide
stability while incorporating the most similar model runs
depending on system and time frame.  The 00Z ECMWF run does not
compare well to the means for most aspects of the forecast and
thus could not realistically be included in the blend.  It strays
somewhat eastward with the upper low north of 60N latitude.  In
addition it is more split with energy/surface development for the
leading two North Pacific/Gulf systems (days 4-6 Tue into Thu) and
for a time slower than consensus for the deep system tracking near
the Aleutians around Thu-Fri.  In contrast, it remains to be seen
if the leading two systems are as deep as depicted in the 12Z GFS
but that run's general evolution and track align better with the
ensemble means so that run provides the operational input for the
Tue-Wed period.  Looking at new 12Z guidance, the ECMWF has at
least trended closer for the leading Gulf system on Tue but the
UKMET/CMC do allow for a farther southeast track than the 12Z GFS
and perhaps even the means.  The next system has adjusted a
fraction toward preference in the ECMWF while the UKMET/CMC a
little closer to the means. 

From about day 6 Thu onward the 00Z GFS/CMC are closest to the
ensemble means for the very deep system tracking along/south of
the Aleutians as well as the trailing day 8 Sat system, so the
blend uses some 00Z GFS/CMC input while toning down the 12Z GFS
influence later in the forecast.  A further note on the late week
Aleutians system, ensemble means have held fairly steady in the
960's mb over the past day--but in response to a somewhat weaker
trend for upper dynamics the latest operational runs only reach as
low as the 950's mb versus 940's mb in some earlier runs.  As a
result a manually adjustment model/mean blend yields a depth that
is slightly weaker than in yesterday's forecast.

Elsewhere, there is fairly good agreement for the general surface
troughing near the western coast of the mainland but with some
spread for embedded low pressure that is most likely to exist
mid-late week.  The ensemble means provide a reasonable
intermediate starting point for this aspect of the forecast. 
There is little support for the 00Z CMC's idea of low pressure
lifting into the Arctic toward the end of the period so its weight
in the blend due to preferences in the Pacific stream had to be
kept low enough not to have an adverse impact on the forecast over
northern latitudes.

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