Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
319 PM EDT Sat May 12 2018

Valid 12Z Wed May 16 2018 - 12Z Sun May 20 2018

The ensemble means continue to provide the most consistent
depiction of the overall pattern evolution through the period
(Wed-Sun).  They show varying combination of energy over eastern
Bering/Aleutians, plus separate Pacific systems, ultimately
leading to a slow moving mean low aloft positioned near the
eastern Aleutians/Alaska Peninsula by next weekend.  As this
interaction occurs expect a downstream ridge aloft to build over
western Canada/Mainland Alaska mid-late week and persist into the
weekend.  For the first couple days operational guidance provides
some refinement of embedded details compared to the means, but
then solutions diverge enough to lower confidence in specifics.

GFS trends over the past day have led to somewhat more agreement
on one or more well defined Bering Sea-Aleutians
shortwaves/compact upper lows that may interact with/influence a
leading Pacific system as well as another system tracking out of
the western Pacific--even if perhaps not to the point of
generating a separate Bering surface system as far westward as
seen in the 00z ECMWF.  There is also some degree of clustering
for a small upper low tracking over the western/northern mainland.
 This feature will be deflected around the upper ridge building
over the mainland and western Canada.  The 00z CMC may be too far
southeast with the track of the mainland upper low.

After early Fri recent GFS runs have been more aggressive than
other solutions in bringing the core of upper dynamics from a
leading Pacific system across the Alaska Panhandle.  The 00z
ECMWF/CMC suggest more shortwave energy could pass into/through
the ridge aloft than in the ensemble means but historical biases
at least favor holding onto more ridging than depicted in the GFS.
 Ensemble members also argue against the GFS surface system that
tracks into the Gulf of Alaska around Fri-Sat.

Aside from a faster trend in consensus, guidance has had decent
continuity in principle for the second Pacific system whose
evolution may be affected by lingering energy near the Aleutians. 
Models/means get there in different ways (including a
low-confidence third Pacific wave in the 00z ECMWF) but there is
better than average agreement with the general idea of low
pressure tracking south of the Alaska Peninsula by day 8 Sun. 
Among the ensemble means the ECMWF mean has been trending steadily
deeper with the second system--in the direction of the operational
guidance--while the GEFS/NAEFS means are still rather weak. 
Therefore by mid-late period prefer to give considerably more
weight to the ECMWF mean for the ensemble component of the
forecast.

Supported by flow around the above Pacific systems, the best
model/ensemble consensus over the past day has been fairly stable
in signaling enhanced moisture flow into areas from the Alaska
Peninsula to the Kenai Peninsula during at least portions of the
period.  At the same time this majority suggests less moisture
reaching the Panhandle than forecast by a number of GFS runs.

Over the past couple days GFS runs have tended to be deepest with
any possible closed upper low over the Arctic, leading to weaker
surface high pressure north of Mainland Alaska.  GEFS means agree
with other operational models and means that Arctic high pressure
should be stronger than seen in GFS runs.

For days 4-5 Wed-Thu an operational model blend of 40 percent 00z
ECMWF and the rest spread evenly among the 00z CMC/00z UKMET/12z
GFS provided added detail within the overall pattern recommended
by the ensemble means, while downplaying less confident aspects of
any run over a particular area.  From Fri onward, combining
preferences for mid-late period features led to starting with a
blend of the ECMWF mean/ECMWF/CMC/06z GEFS mean in order from most
to least weight.


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

Rausch