Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
223 PM EST Sat Feb 17 2018

Valid 12Z Wed Feb 21 2018 - 12Z Sun Feb 25 2018

Models and ensembles show general agreement on a gradual
weakening/flattening of the North Pacific upper ridge by the
middle/end of next week, with the pattern across Alaska becoming
much progressive. A persistent upper low over eastern Asia will
continue to eject numerous shortwaves east toward Alaska. The
small scale of many of these waves along with complex interaction
occurring at times with energy of Arctic origin leads to quickly
increasing uncertainty by late in the extended period.

A number of deterministic solutions, including the 00Z ECMWF/CMC
and 12Z GFS showed sufficient agreement early in the period (Wed)
to justify use of a majority deterministic multi-model blend on
Day 4. These solutions show a shortwave and associated surface low
crossing the Bering Sea Wed before moving into mainland Alaska Wed
night/Thu and interacting with Arctic upper-level energy across
eastern Alaska Thu night. Models show another shortwave crossing
the Bering Sea Fri/Fri night, reaching the Gulf on Sat, as well as
additional areas of Arctic energy spreading southward into Alaska
as the upper ridge weakens. Spread becomes quite large by that
time however, and models (the GFS in particular) show relatively
poor run-to-run continuity. Given the gradually increasing
uncertainty, a gradual trend toward heavier ECENS/NAEFS weighting
was shown through the forecast period, with majority ensemble mean
weighting from day 6 (Thu) onward.

In terms of sensible weather, this will be a fairly snowy pattern
for much of Alaska, with periods of relatively high pops even
across interior areas of central/northern Alaska. The Aleutians
along with areas along the Gulf of Alaska coast in
southern/southeastern Alaska will see periodic rain/snow and gusty
winds as the series of systems traverse the island chain.
Temperatures, while initially above average (especially across
central/northern Alaska), will gradually decrease through the
extended period as the North Pacific ridge flattens and heights
across Alaska fall, allowing colder, Arctic air to take hold.

Ryan