Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
414 PM EDT Thu Mar 22 2018

Valid 12Z Mon Mar 26 2018 - 12Z Fri Mar 30 2018

The large scale pattern from the north Pacific and Alaska into
western North America is expected to shift toward a blockier setup
during the medium range, although the specifics remain somewhat
unclear and variable from model to model. Models/ensembles show
good agreement that a relatively deep (~980 hPa) low pressure
system will be located across the eastern Bering Sea on day 4
(Mon), with an occluded front weakening as it moves into southern
mainland Alaska, and warm/cold fronts crossing the Gulf. This
system is expected to weaken relatively quickly, leaving a
lingering barotropic low near the western Alaska coast through the
middle of next week.

Farther east, models show a bit more disagreement even from the
start of the forecast period. The origin of the dissensus revolves
around the strength of an upper ridge axis extending from the
Canadian west coast north to the Arctic Ocean. The ECMWF along
with the CMC continued to favor a stronger ridge, with the ECMWF
the strongest showing a cutoff upper high over the Arctic. The
GFS, on the other hand, has been persistent in showing a weaker
ridge axis which allows for the westerly flow to be much more
progressive and less amplified. The ECMWF/CMC have relatively
strong support from a large number of ECENS and CMCE/NAEFS
members, while the GEFS resembles the GFS. Based on the ensemble
consensus, opted to lean away from the GFS and base the forecast
on a blend of the 00Z ECMWF/CMC/ECENS/NAEFS. Confidence was
sufficient to use a majority of deterministic guidance in the
blend on days 4-5. This results in a slower and somewhat stronger
shortwave (relative to the GFS) crossing the Gulf Tue night/Wed.
Later in the forecast period, additional spread arises, although
broken down along the same lines. The GFS is very aggressive with
building a ridge across the Bering Sea by mid to late next week,
with a deep cutoff low over the North Pacific, with perhaps a Rex
Block formation. The ECMWF/CMC are substantially weaker with the
ridge, although they still show some degree of ridging. Ensemble
spread becomes larger by this time, and thus weighting of ensemble
means were boosted to a majority of the forecast beyond day 5.

In terms of sensible weather, rising heights across Alaska will
mean gradually increasing temperatures through the medium range
period. Initially near average temperatures early next week will
reach several degrees above average by mid-week. Precipitation
should be fairly widespread across southern/southeastern Alaska
Mon-Wed under the influence of the deep Bering low and weakening
front system, and the next low pressure system entering the Gulf
by Wed. Meanwhile, ridging across east central/northern Alaska
will result in dry and relatively cloud-free conditions at least
through mid-week.

Ryan