Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
329 PM EDT Mon Jun 18 2018

Valid 12Z Fri Jun 22 2018 - 12Z Tue Jun 26 2018

Expect some persistence of mean troughing over the extreme western
mainland/eastern Bering Sea, with continued uncertainty over the
position/depth of an embedded upper low on a day to day basis. 
There is somewhat better agreement for a system tracking across
the northeastern Pacific, with arrival of upper support helping to
erode the downstream ridge initially extending from western Canada
into parts of Mainland Alaska.  At the same time anticipate mean
troughing over the Arctic with an average of most similar guidance
tending to place the northwestern mainland near the southeast
periphery of the mean trough.  Over recent days guidance has been
having considerable difficulty in resolving forecast details
across the western-central Pacific/Aleutians, as upper low energy
over Asia in the short range emerges over the northwestern Pacific
and leading flow evolves toward a progressive/low amplitude regime.


Over the past day guidance has displayed some degree of
convergence for the northeastern Pacific system.  Recent trends
toward greater emphasis on the trailing of two shortwave features
aloft has trimmed away the northern side of the prior guidance
spread for the surface low (represented by yesterday's GFS and
GEFS/ECMWF ensemble means).  The southern side of the spread is
little changed but now dominated more by the CMC and some of its
ensemble members.  As of the time of 12Z GFS arrival the 06Z-12Z
GFS/00Z ECMWF now represent the best consensus for surface low
track, between the southern Alaska Panhandle and Haida Gwaii. 
Timing differences still exist though.

A model/ensemble mean blend looks best for the upper low expected
to be over or near the eastern Bering Sea for at least the first
half of the period.  Solutions range from ECMWF runs that stray
southward of most other guidance and into the Gulf of Alaska, to
the GFS which keeps the feature nearly stationary and the 00Z CMC
that eventually brings the main core of energy into the mainland. 
The ECMWF mean hints at the more progressive operational run but
still holds onto some elements of the other ensemble means that
keep the overall upper trough axis just off the western coast of
the mainland.  Such an axis has better continuity though position
of the embedded upper low has varied from day to day.

Recent GFS/ECMWF runs and their means provide fairly similar ideas
for upper troughing expected to exist over the Arctic, albeit with
some spread for exact position of the trough.  ECMWF-based
guidance is generally a little eastward of the GFS/GEFS. 
Differences are well within typical guidance error, favoring a
model/ensemble compromise that yields a forecast similar to
yesterday.

Guidance variability and spread keep confidence low with respect
to the forecast over the western-central Pacific and Aleutians. 
There is some loose agreement on one area of low pressure
approaching the western Aleutians late in the period corresponding
to the main core of upper support departing from eastern Asia late
in the short range period.  However individual models/ensemble
members are very diverse for the pattern farther east.  Over the
past 24 hours the GFS/GEFS have changed significantly from a
suppressed pattern (still favored by the ECMWF/ECMWF mean) to
having lower pressures cross the Aleutians (which CMC/CMC ensemble
runs have tended to lean toward).  The evolution of North Pacific
flow aloft toward a progressive/low amplitude regime may allow for
a faster moving weak feature to stream ahead of the main low but
perhaps not to the fast/northern extent of the GFS/GEFS.  Starting
with a blend of GFS/GEFS and ECMWF/ECMWF ensemble solutions would
provide some degree of continuity with a moderately suppressed
leading pattern but noticeably lower surface pressures across the
Aleutians than forecast by the ECMWF-based guidance.

Combining considerations for each area of interest, the forecast
started mostly with varying weights of the 12Z GFS/00Z ECMWF and
00Z NAEFS/ECMWF means.  The GFS component was split among 12Z/06Z
runs late.  The blend gave operational runs 80 percent weight on
day 4 Fri followed by a gradual increase of ensemble mean input
with time.

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