Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
302 PM EDT Sun Aug 26 2018

Valid 12Z Thu Aug 30 2018 - 12Z Mon Sep 03 2018

The beginning of the extended forecast continues to be dominated
by Bering Sea low pressure that has its origins from former
tropical systems Soulik and Cimaron.  This low will bring a period
of fairly strong winds and focused precipitation.  Short range
details have adjusted over the past 24 hours, with consensus now
showing greater focus on the trailing of two waves (ex-Soulik). 
This has resulted in somewhat slower timing than yesterday's
consensus and reflects an adjustment closer to the ideas of some
earlier GFS/GEFS runs.

Across higher latitudes the ensemble means maintain fairly good
agreement and consistency regarding a persistent Arctic upper low
with surrounding low-amplitude mean flow extending across a
majority of Mainland Alaska.  Individual runs of operational
models have been varying with shortwave specifics, so confidence
is not very high for exact frontal position/embedded waves along
the southern fringe of the Arctic stream.  Thus aside from early
in the period a blend of the ensemble means would be the best
starting point for the forecast.

Some aspects of the forecast are more challenging along and south
of 60N latitude.  Already by day 5 Fri significant differences
emerge regarding energy aloft behind the initial Bering Sea storm.
 This energy comes from eastern Siberia/Kamchatka in the short
range time frame, with a general signal that it should evolve into
at least a trough if not an upper low over/near the Bering Sea. 
The 00Z ECMWF has a minority of its ensemble members showing
similar ideas but otherwise most guidance suggests that this
feature will be slower and/or weaker than forecast by the ECMWF. 
ECMWF runs have been very inconsistent with this feature as well. 
As a result preference is to exclude the 00Z ECMWF from the
forecast after Thu.

Besides the uncertain impact this feature has downstream from the
Aleutians/Bering into the Gulf of Alaska, after day 5 Fri guidance
diverges with respect to east-central Pacific ridging aloft which
would affect the amplitude/sharpness of upstream troughing--and
how much surface low pressure may reach into the Aleutians.  Over
the past 2-3 days operational GFS runs have kept the ridge quite
strong, leading to sharper upstream troughing and a strong push of
surface low pressure into the Aleutians/Bering.  On the other hand
the ECMWF-ECMWF mean weaken/retrograde the upper ridge and keep
higher surface pressures over the Aleutians and vicinity.  Overall
thinking is that the most likely scenario will lie between these
two extremes.  Teleconnections relative to the Arctic upper low do
favor eventual flattening of northern Pacific flow but strength of
the upper ridge through late week suggests it could take longer to
weaken than in the ECMWF/ECMWF mean.  A nearly even blend of the
06Z GEFS mean, 00Z NAEFS mean (includes some CMC mean that weakens
the ridge a bit more than the GEFS late), and 00Z ECMWF mean
provides a reasonable intermediate starting point that tilts 2/3
away from the ECMWF scenario but also tempers any GFS Aleutians
low pressure.  Thus far GEFS mean trends have suggested
weaker/delayed low pressure if it were to exist.

Based on best early-period consensus the forecast starts with an
operational model blend (00-06Z GFS and 00Z ECMWF/UKMET/CMC) day 4
Thu and then replaces the ECMWF with its mean for day 5 Fri. 
After that time the forecast transitions toward the aforementioned
ensemble mean blend to reflect the means' agreement/consistency
for large scale flow at higher latitudes and a conservative
starting point in light of conflicting guidance farther south.

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