Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
306 PM EDT Sat Aug 25 2018

Valid 12Z Wed Aug 29 2018 - 12Z Sun Sep 02 2018

The first portion of the forecast starts with a compromise among
the 00Z ECMWF/CMC/UKMET and 12Z GFS/06Z parallel (FV3) GFS. 
Leading up to the start of the extended forecast (12Z Wed) there
are lingering question marks over the combined extratropical
evolutions of Cimaron and farther west Soulik--yielding deep low
pressure over the Bering Sea around midweek.  Operational 00Z/06Z
GFS runs are on their own in emphasizing development of the
western short-range low versus the leading low in other solutions.
 The 12Z GFS has trended toward the faster cluster and the 06Z
parallel (FV3) GFS is also with the majority.  Then as the low
approaches western Mainland Alaska by day 5 Thu the guidance is
more mixed due to differences in phasing with Arctic troughing. 
The 00Z ECMWF and its mean begin to lean toward the faster side of
the spread, though the 12Z GFS has also adjusted a little faster
than the guidance median.  The operational model compromise
provides a good intermediate timing that is only slightly faster
than continuity.  This system will generate a period of strong
winds and focused precipitation.

After passage of this system the pattern should become
increasingly dominated by progressive flow around an Arctic upper
low.  Individual solutions and consecutive runs vary significantly
in specifics of upper low position and orientation of overall
troughing.  The 00Z/06Z GFS runs are generally west of consensus
for the trough and/or upper low at least for a period of time,
while the 12Z GFS and 06Z FV3 GFS compare better with the means
into Sat.  ECMWF runs offer other detail possibilities that are
within the realm of the mean pattern.  Given the spread among the
model solutions by the latter half of the forecast period, the
agreeable ensemble means provide an increasingly desirable option
for depicting the most likely aspects of the large scale pattern. 
Thus by days 7-8 Sat-Sun the forecast consists of half 00Z ECMWF
mean and the remainder 06Z GEFS/00Z NAEFS means.

Across the southern Bering/Aleutians/North Pacific, residual
trough energy behind the initial Bering Sea storm may promote some
waviness near or south of the eastern Aleutians by late week. 
This area of waviness may extend into the Gulf of Alaska
thereafter.  Whether from this feature or via separate evolution,
there is decent ensemble support for some degree of surface low
pressure lingering near the southeastern coast during the latter
half of the period.  Upstream the past 1-2 days of GEFS runs have
displayed a pronounced delay in bringing any western-central
Pacific wave toward the Aleutians late in the period.  This is a
nod toward the higher surface pressures of recent ECMWF mean runs.
 Confidence is not very high over this area by next weekend. 
Teleconnections relative to the Arctic upper low show just broadly
cyclonic flow over much of the Pacific and Bering Sea--likely
keeping the pattern suppressed as per latest ensemble mean blend. 
However if the mid-latitude Pacific ridge aloft ends up being a
little more persistent as in some GFS/GEFS/CMC mean runs, upper
troughing to the west could be sharper and bring surface low
pressure toward the Aleutians.

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