Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
406 PM EDT Sun Mar 25 2018

Valid 12Z Thu Mar 29 2018 - 12Z Mon Apr 02 2018

Guidance has still not quite pinned down the exact timing or
amplitude of a shortwave expected to be crossing the Gulf on day 4
(Thu). The ECMWF/GFS/CMC have all shown significant run-to-run
variability. Given the relatively amplified flow regime in place,
opted to continue favoring the somewhat more amplified solutions
for this shortwave, which included the ECMWF and the 00Z CMC. As a
result, the WPC forecast continues to indicate a low pressure
system nearing western Canada or possibly Southeast Alaska by
early Fri.

Model spread is quite significant across Alaska by next weekend.
There does seem to be at least broad agreement that the pattern
across Alaska and the north Pacific will become much more blocky
by late in the week. There seems to be a general consensus that a
large ridge will develop by late in the week into the weekend,
although solutions vary widely as to exactly where this will occur
and what the effects on Alaska may be. The GFS remains the most
aggressive with developing the upper ridge initially across the
Bering, but then quickly raises heights eastward across much of
mainland Alaska. Meanwhile, the ECMWF is a bit weaker with the
ridge which allows shortwave energy originating in the Arctic to
dig south across Alaska and amplify, perhaps even eventually
forming a closed upper low. Given the significant model/ensemble
spread, ensemble (ECENS/NAEFS) weighting was increased
significantly in the forecast after day 5 (Fri), with
deterministic guidance excluded entirely by day 8 (Mon).

In terms of sensible weather, rising heights across Alaska will
mean gradually increasing temperatures through the medium range
period. Temperatures will reach several degrees above average by
late in the week. Southeast Alaska will see the greatest potential
for precipitation Thu-Fri under the influence of the Gulf low
pressure system. Additional scattered snow showers will be
possible across west central mainland Alaska as a decaying Bering
low lingers. Meanwhile, ridging across east central/northern
Alaska will result in dry and relatively cloud-free conditions
with some potential for an increase in snow shower activity by
Friday or Saturday if Arctic shortwave energy is able to move
southward into the region.

Ryan