Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
225 PM EST Fri Feb 16 2018

Valid 12Z Tue Feb 20 2018 - 12Z Sat Feb 24 2018

Models and ensembles show general agreement on a gradual weakening
of the North Pacific upper ridge by the middle of next week, with
the pattern across Alaska becoming gradually more 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.

The ECMWF and GFS showed sufficient agreement early in the period
(Tue) to warrant use of majority deterministic solutions during
that time frame. Both solutions (along with support from the CMC)
showed a shortwave crossing the Bering Straits before quickly
moving southeastward across Alaska as energy spills around the
northern side of the upper ridge as the (initially deep) surface
low weakens and moves north into the Arctic Ocean. In its wake,
another smaller-scale shortwave and associated surface low look to
cross the Bering Sea Tue night/Wed before moving into mainland
Alaska Wed night/Thu and interacting with Arctic upper-level
energy across eastern Alaska Thu night. Models show additional
shortwaves crossing the Bering Sea Fri/Sat, as well as additional
areas of Arctic energy spreading southward into Alaska as the
upper ridge weakens, but spread becomes quite large by that time
and models show relatively poor run-to-run continuity. Given the
gradually increasing uncertainty, a gradual trend toward heavier
ECENS/GEFS 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 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, 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