Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
338 PM EDT Tue Aug 28 2018

Valid 12Z Sat Sep 01 2018 - 12Z Wed Sep 05 2018

An expansive upper-level ridge which has been persistent across
the north central Pacific for some time is expected to be in the
process of breaking down by this weekend, which will allow a
somewhat more progressive flow regime to take hold across Alaska.
A couple significant shortwaves will affect Alaska during the
period, the first crossing the Gulf Sat-Sun, and the next crossing
the Bering Sea and approaching Alaska during the early to middle
portion of next week. Farther north, flow around the southern
periphery of a persistent Arctic upper low will result in the
potential for relatively small scale (low predictability)
shortwave energy from the Arctic to affect the North Slope and
perhaps portions of the Interior through much of the period. In
the day 4-5 time period (Sat-Sun), model consensus was
sufficiently high to justify use of a majority deterministic
multi-model blend (including the GFS/ECMWF/CMC). This solution
brings a relatively weak low pressure system across the Gulf of
Alaska Sat-Sun. From day 6 onward, confidence in the forecast
gradually decreases. Model spread begins to increase surrounding
the next shortwave/surface low pressure system crossing the Bering
Sea. The 00Z CMC became too fast with this system relative to the
model/ensemble consensus, and its solutions was deemphasized after
day 5. Weighting of ensemble means (ECENS/GEFS) was boosted to
comprise a majority of the forecast during the day 6-8 (Mon-Wed)
time period.

The system crossing the Gulf Sat-Sun will bring the likelihood of
showers to Southeast Alaska. While much of mainland Alaska will be
dry this weekend, the North Slope/Brooks Range and northern
portions of the Interior will see the potential for rain/snow
showers in association with potential Arctic shortwave energy. By
early next week, rain chances will quickly increase across the
Aleutians as the next system crosses the Bering, with
precipitation chances increasing for mainland Alaska by Tue-Wed.
Temperatures are expected to be near or slightly below
climatological norms throughout the period.

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