Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
308 PM EDT Sat Jul 21 2018

Valid 12Z Wed Jul 25 2018 - 12Z Sun Jul 29 2018

Today's forecast represents some degree of similarity to yesterday
but also with some adjustments and/or improved definition.  D+8
multi-day means are remarkably similar with their height anomaly
centers over much of the Northern Hemisphere, which provides some
confidence in the large scale pattern forecast through next
weekend even though some details will likely take time to gain
clarity.

The most consistent/agreeable part of the forecast is with the
deep-layer low just south of the Alaska Peninsula at the start of
the period early Wed.  The upper low should persist into Fri and
the surface reflection at least through Thu.  On the other hand,
upstream there has been considerable uncertainty over the details
of dynamics aloft heading into the Bering Sea/Aleutians in the
late week/weekend time frame.  This energy may interact in some
fashion with the system near the Peninsula, and consists of at
least a couple features so there is potential for significant
adjustments in future cycles.  However latest/recent GFS/ECMWF
runs and their means are starting to suggest that this mid-late
period evolution may lead to a fairly strong system over the
southern or eastern Bering Sea by next weekend.  A compromise
among model/ensemble mean ideas seems reasonable, with the means
trending stronger over the past day (supporting operational model
input) but forecast uncertainty and verification recommending some
inclusion of ensemble guidance.  The leading system will tend to
focus moisture along the southern coast while the trailing one
should support a farther westward moisture focus, from the
Aleutians into the Peninsula.

Farther north/east, guidance has trended much stronger with a
midweek Arctic system whose trailing cold front will settle into
the northern mainland.  An operational model compromise looks
reasonable aside from keeping the 12Z GFS out of the mix as it
strayed weaker/faster than even its 00Z/06Z runs which were
somewhat ahead of the 00Z ECMWF/CMC.  Compared to yesterday,
stronger upper troughing to the south of the Arctic system delays
the progression of high pressure that tracks from the north of the
mainland into northwestern Canada.  Guidance is in the process of
converging for the strength of the trailing upper ridge, and there
is fairly good continuity with respect to this ridge ultimately
connecting with the ridge initially in place from the
Panhandle/western Canada into the eastern mainland--leading to a
fairly strong mean ridge from western Canada through the northern
mainland by next weekend.  As this occurs expect the surface
boundary over northern areas to lift northeastward as a warm
front. 

The preferred forecast blend incorporated consensus ideas of
operational guidance for days 4-5 Wed-Thu to yield better
definition than the means while accounting for lingering
detail/strength uncertainties.  For days 6-7 Fri-Sat the 00Z ECMWF
and recent GFS runs were close enough to the means in principle to
merit keeping their solutions around 70 percent of the blend while
introducing some 06Z GEFS/00Z ECMWF mean for the remainder.  By
this time frame the 12Z GFS could be included as a minority blend
component.  Then for day 8 Sun, moderate increase in spread for
the Bering Sea system and emerging differences between the model
and mean clusters for position of the western North America mean
ridge aloft recommended trending the blend to a 50/50 weight among
models and means.

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