Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
241 PM EST Sat Dec 08 2018

Valid 12Z Wed Dec 12 2018 - 12Z Sun Dec 16 2018

With the passage of another day the general pattern evolution
still looks similar, as is the forecast approach to depicting
significant features.  Very strong/energetic Pacific flow will be
carrying vigorous systems across the North Pacific into the Gulf
of Alaska region.  These systems will support periods of strong
winds/high waves and most likely lead to highest 5-day
precipitation totals extending from near the Kenai Peninsula into
the Alaska Panhandle.  Meanwhile expect a mean low aloft to
meander over/near the northern Bering Sea for most of the period,
supporting a surface trough and at times (mid-late week in
particular) embedded low pressure along the western coast of the
mainland.  Over recent days the ensemble means have generally
provided the most stable solution for the track and timing of
individual systems.  Thus prefer to maintain a meaningful ensemble
mean influence throughout the period by way of the 06Z GEFS/00Z
ECMWF means, while incorporating as much operational guidance as
possible (today the 00Z GFS/ECMWF/CMC) to enhance system strength
before subsequent manual adjustment.


At the start of the period 12Z Wed there should be a weak compact
low near Prince William Sound, the last vestige of a leading
system.  Models/ensemble members diverge fairly quickly for the
next system in the series--south of the Alaska Peninsula as of
early Wed.  Over the Wed-Thu period recent ECMWF runs have
gradually been trending toward a more consolidated/northward
system which the ensemble means have been showing.  On the other
hand latest GFS runs have become somewhat more split with the
system's energy while other models range within the full ensemble
spread.  By day 5 Thu the preferred model/mean blend yields a
track in the northern half of the envelope.

The next system, likely the strongest, should track south of the
Aleutians around Thu-Fri.  The 00Z cycle of the ECMWF mean in
particular has adjusted deeper into the 950's mb Fri-Sat, adding
confidence to recent operational runs that have reached the 950's
mb as well.  There is still a small minority of operational runs
hinting at a pressure slightly below 950 mb at its deepest point. 
Through late week the operational runs have become better
clustered for timing/detail versus 24 hours ago while ensemble
member clustering has improved modestly.  By next weekend the
storm track will depend upon possible interaction with upstream
Aleutians/Bering Sea flow.  The current most likely track has the
low lifting northward into day 8 Sun and reaching close to Kodiak
Island and the Kenai Peninsula.  This general idea seems
reasonable given the building of a strong upper ridge to the east.

GFS/ECMWF runs and most of their ensembles suggest the next system
to emerge from the western Pacific late in the period will take a
farther south track.  At this time the 00Z CMC and associated
ensembles are the only solutions far enough north to have a
meaningful influence on the Aleutians.

Across the Bering Sea/Mainland/Arctic, confidence is low regarding
a possible compact low that may be over the central Bering Sea as
of early Wed.  GFS runs are somewhat on their own with their more
eastward track.  Signals in guidance through the 06Z cycle were
unclear so the manual forecast kept this wave on the weak side but
the new 12Z ECMWF/UKMET increase potential for a defined wave but
again with a track south of the GFS.  There continues to be decent
consensus on surface low pressure near the western coast of the
mainland in association with the northern Bering upper low with
typical detail/position differences.  Predictability remains low
for shortwave specifics on the eastern side of the upper low and
whether/how an individual shortwave may pull a surface low
northwestward from the North Slope region.  Thus prefer a blended
approach to downplay any individual wave for the time being.

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