Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
250 PM EDT Sat Aug 11 2018

Valid 12Z Wed Aug 15 2018 - 12Z Sun Aug 19 2018

Model solutions continue to present a complex and difficult
forecast for Alaska during the extended range. At least early in
the forecast period, model consensus was sufficient on days 4-5
(Wed-Thu) to justify use of a slight majority of deterministic
guidance (ECMWF/GFS/CMC) as a basis for the forecast. At the large
scale, models/ensembles continue to show some support for breaking
down the persistent upper ridging across eastern Asia, but this
looks to be a rather slow process, with a large degree of chaos
introduced into the flow during the process. Additional
complexities are added by the potential arrival/interaction of
Arctic shortwave energy with additional energy (at least partially
of tropical origin) crossing the North Pacific. As a result of
these factors, forecast confidence rapidly decreases after day 5.
Ensemble means (ECENS/NAEFS) were used as a majority of the
forecast starting point starting on day 6, with deterministic
guidance eliminated entirely by the end of the forecast period.

Precipitation appears fairly common across much of Alaska through
the extended period with persistent upper troughing migrating from
the Bering Sea toward the Gulf of Alaska, and relatively small
scale upper-level disturbances traversing the flow. By next
weekend, precipitation may increase further in coverage across
southern mainland and southeastern Alaska, as a potential low
pressure system enters the Gulf (although confidence is extremely
low in this portion of the forecast). Farther north, confidence
has increased a bit with respect to Arctic shortwave energy
penetrating southward across the North Slope, increasing
precipitation chances across that region from Wed onward, with
sufficient cold air in place to support a mix of rain and snow.


Ryan


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