Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
332 PM EDT Tue Aug 14 2018

Valid 12Z Sat Aug 18 2018 - 12Z Wed Aug 22 2018

The Alaska medium range period (Saturday - Wednesday) features a
low pressure system approaching the eastern Aleutians and Alaska
Peninsula from the south days 4-6. Consensus shows this low
eventually weakening in response to a second amplifying
trough/upper low entering the Bering Sea by day 6 which moves
relatively slowly across the Bering ending up in a position just
to the north of the western/central Aleutians by day 8. For days
4-5, model/ensemble consensus was sufficient to warrant a majority
deterministic multi-model/ensemble blend of the 12z GFS/00z
ECMWF/00z ECENS/06z GEFS. By day 6 and beyond, differences begin
to arise particularly with the evolution of the Gulf of Alaska
low. The deterministic GFS along with both the ECENS/GEFS means
would support the system quickly weakening and being absorbed into
the broad troughing associated with the more western low entering
the Bering Sea. The latest run of the ECMWF continues to keep some
semblance of a closed low through even day 7 or 8, which also
translates to a noticeable surface low somewhere along or just
south of the southern Alaska coastline. Run-to-run continuity with
the evolution of this system remain poor and confidence on how
this will evolve and any specific sensible weather impacts is low.
For these reasons, an increasing weighting/majority of ensemble
mean guidance was used later in the period. This blend results in
a surface/500 mb pattern similar to previous shift continuity.

Sensible weather wise, precipitation looks to be fairly widespread
across the North Slope and Brooks Range early in the weekend as
Arctic shortwave energy and a frontal boundary cross the region.
To the south, the low pressure system entering the Gulf of Alaska
will likely support gusty winds and widespread/locally heavy rains
for much of southern Alaska and the eastern Aleutians this
weekend, lingering into next week as the low weakens but broad
troughing remains. Meanwhile, the low pressure system in the
Bering Sea by this weekend could result in potential gusty winds
and scattered rain showers for portions of the Aleutians and maybe
even the western mainland.

Santorelli


WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can be found at:

http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html