Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
437 PM EDT Mon Aug 27 2018

Valid 12Z Fri Aug 31 2018 - 12Z Tue Sep 04 2018

A persistent Arctic low with low amplitude mean flow will be
present across much of northern mainland Alaska through the
extended periods- although ensemble means have a fairly decent
handle, there continues to be enough differences with the
shortwave details to have a moderately low confidence forecast. To
the south, low pressure will be present over the Bering Sea with a
ridge extending into the interior from the North Pacific. Model
solutions suggest this feature will evolve into an open trough,
possibly with a closed low, that will flatten the ridge by the end
of the extended periods. However, a lot of noise arises especially
beyond day 4 resulting in too many differences on specifics for
areas downstream of the Aleutians. Low confidence on the exact
timing of fronts beyond day 5. 

Based on best early-period consensus the forecast starts with an
operational model blend (12Z GFS and 00Z ECMWF) along with equal
weighting of the ECWMF and GEFS ensemble means- with each day
decreasing the weighting of the deterministic ECWMF/GFS and
increasing that of the ensemble means. The ensemble mean blend for
the large scale flow at higher latitudes provided a conservative
starting point in light of conflicting guidance farther south.

Campbell


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