Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 PM EDT Thu Jul 05 2018

Valid 12Z Mon Jul 09 2018 - 12Z Fri Jul 13 2018

The ensemble means have been consistent in showing a persistent
mean trough aloft over or just west of western Mainland Alaska. 
This trough will initially consist of a strong eastern Bering Sea
system (extratropical Prapiroon) and short range Arctic energy
dropping into the western side of the overall trough.  After some
significant adjustment/spread in prior days, guidance has
stabilized for the handling of the Bering system.  By the middle
of next week upstream high latitude flow should feed into the mean
trough.  00Z ECMWF/CMC runs (plus the UKMET through its 144hr
duration) compare fairly well to this overall evolution.  On the
other hand 00Z-06Z GFS runs are more amplified and closed with the
upstream flow, ultimately kicking out the initial Bering upper low
energy faster than the majority cluster.  While the 12Z GFS does
not show the closed upstream feature of prior runs, its farther
west trajectory actually causes the overall pattern to deviate
more significantly from consensus.  Thus except for a small 00Z
GFS component early in the forecast, today's blend leans away from
operational GFS data.

Model/ensemble guidance continues to suggest a fairly high degree
of uncertainty in details across the North Pacific and
Aleutians/Bering Sea by the latter half of the period.  The best
defined feature that can be keyed upon is an upper low forecast to
be west/southwest of Kamchatka as of early Mon and eventually
drift into the northwestern Pacific, possibly ejecting to the
northeast (with corresponding surface system) as depicted in the
00Z GFS/ECMWF/CMC.  Expected formation of the upper low to the
south of a closed high over Russia suggests below average
predictability for the ultimate evolution/path/timing of the upper
low.  Not surprisingly the ensemble means are less defined with
the feature due to very wide spread among individual members. 
Whether from this energy or another shortwave, there is room for a
weak surface feature to be embedded within the flow so a
toned-down version of the aforementioned operational cluster seems
reasonable for depicting this potential.

Based on the above considerations the forecast starts with an
operational model blend early in the period to capture details of
the system likely to be over the southeastern Bering Sea as of
early Mon.  Then the blend starts to incorporate increasing weight
of the 06Z GEFS/00Z ECMWF means, by days 7-8 Thu-Fri reaching
60-70 percent total input while holding onto some representation
from the 00Z ECMWF/CMC.  The primary change in continuity is for
better definition of a front reaching the northwestern mainland by
Wed in connection with the upstream energy aloft feeding into the
mean trough position.  The best consensus of guidance continues to
show highest 5-day precipitation totals along the southern coast
and Panhandle regions while western and northern areas also see
meaningful amounts aided by low pressure and a wavy front
respectively.

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