Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
156 PM EST Sun Feb 18 2018

Valid 12Z Thu Feb 22 2018 - 12Z Mon Feb 26 2018

Models and ensemble continue to show a fairly progressive pattern
taking hold across Alaska during the medium range as the North
Pacific upper ridge flattens a bit. This will allow multiple upper
shortwaves originating in an area of negative height anomalies
across northeastern Asia to move east toward Alaska, amplifying as
they cross the Bering Sea, and eventually moving into mainland
Alaska or the Gulf. The relatively small scale of these waves,
along with potential interactions occurring at times with energy
or Arctic origin leads to gradually increasing uncertainty by
mid/late in the medium range forecast period.

A number of deterministic solutions, including the 00Z ECMWF/06Z
GFS showed sufficient agreement early in the period (Thu) to
justify use of a majority deterministic on Day 4. These solutions
show a shortwave and associated surface low reaching the west
coast of mainland Alaska early Thu before some degree (perhaps
less than shown in recent days) of interaction with Arctic
shortwave energy occurs Thu night and the shortwave dives
southeastward toward Southeast Alaska. General model agreement
also exists that another amplifying shortwave will cross the
Bering Sea Thu night/Fri. One change from yesterday is a trend
among most of the deterministic guidance toward a more pronounced
triple point low developing near the Aleutians Thu night/Fri, and
this trend is reflected in the latest forecast. The 00Z ECMWF/06Z
GFS showed general agreement on the timing/track of this surface
low, reaching the west coast of mainland AK by early Sat morning.
The 12Z GFS was much faster than consensus or its previous run
with this system, and thus was not used in the forecast blend. As
the upper wave continues east, another low looks likely to develop
across the Gulf of Alaska on Sat, before rapidly occluding off the
western Canada/Southeast Alaska coast. Finally, yet another upper
shortwave looks to cross the Bering/Aleutians Sat night/Sun. While
deterministic solutions all show general agreement on the
existence of this system, there are variations in terms of timing
and track of the system. Although, this system seems to show a bit
less spread among the models/ensembles than would be expected for
a Day 7-8 time frame.

Given the described considerations, the WPC forecast followed a
similar approach to that of recent days. The forecast was
initially based on a heavily deterministic 00Z ECMWF/06Z GFS blend
on Day 4, with a shift to majority ensemble mean (00Z ECENS/NAEFS)
weighting from Day 6 onward. Confidence toward the latter portion
of the period seems slightly improved compared to yesterday, thus
a 30 percent component of deterministic solutions was carried
through to Day 8.

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 those areas. 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 air to take hold.

Ryan