Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
746 PM EDT Sat Sep 18 2021

Valid 12Z Wed Sep 22 2021 - 12Z Sun Sep 26 2021

...Stormy Pattern from the North Pacific into the Gulf of Alaska...

...Guidance and Predictability Assessment...

The model and ensemble consensus shows a mean trough aloft
extending from Mainland Alaska southward/southwestward while an
Arctic upper low likely drifts north and then west.  The overall
pattern should support a storm track across the North Pacific into
the Gulf of Alaska.  The most confident systems will be a
short-range storm in the process of weakening along the
southeastern coast as of the start of the period early Wednesday
and a trailing North Pacific storm whose triple point development
likely tracks into the Gulf by Thursday-Friday.  Then latest
guidance is developing a more pronounced signal for another storm
that could track south of the Aleutians and Alaska Peninsula late
week into the weekend.

Early in the period a blend of 12Z operational models represented
the most common elements of guidance.  Clustering was good for the
weakening low initially along the southeastern coast while some
detail differences remained for the trailing strong North Pacific
storm.  For locations along the southern coast/Panhandle, the main
consideration was that the 12Z CMC tracked the triple point low
(evident by late Wednesday-early Thursday) farther south than
other guidance.  Thus the blend under-weighted the CMC.  The 12Z
GFS was also a bit more consolidated than other solutions with a
track closer to Kodiak Island, and the blend toned down this
aspect as well.

By late week through the weekend, recent model runs have been very
inconsistent and ensemble means very ill-defined for a trailing
North Pacific system.  However latest 00Z-12Z model runs have been
strengthening their signal for such a storm, supported by some
combination of separate pieces of energy emerging from Kamchatka
and dropping south/southeast from Siberia.  Today's forecast
transitioned the aforementioned early model blend toward about
half models/half means for days 7-8 Saturday-Sunday.  This
provided an intermediate starting point between the deep depiction
in the 12Z models and ill-defined pattern in the means.  Among the
means available at the time of forecast preparation, only the CMC
mean offered some hint of a wave.  The new 12Z ECMWF mean now has
a weak wave while the 18Z GFS still has a strong storm.

...Sensible Weather...

The southeastern coast/Panhandle will see only a very brief lull
in significant precipitation on Wednesday, between a weakening low
reaching the southeastern coast by the start of Wednesday and a
trailing storm initially south of the Aleutians.  This latter
storm will generate a broad area of brisk to strong winds from the
southern Bering Sea/Aleutians/North Pacific into the Gulf of
Alaska, with organized precipitation to the north of the low track
midweek and potentially moderate to heavy activity directed into
the southeastern coast/Panhandle Thursday-Friday.  Most guidance
suggests the greatest precipitable water anomalies should be
confined to the southern Panhandle.  The remnants of West Pacific
Tropical Cyclone Chanthu may provide some input for this system. 
The next storm in the series may bring another episode of strong
winds and organized precipitation to the Aleutians and vicinity
late week into the weekend.  Effects on the Aleutians/Alaska
Peninsula will be very sensitive to storm evolution/track which
has moderate confidence at best given model inconsistency before
clustering improved somewhat today.  Away from far southern areas,
most locations should see mostly light/scattered precipitation or
dry conditions.  Best potential for some periods of light activity
would be over the North Slope.  The mean trough aloft will keep
temperatures below normal over much of the state.  Coldest
anomalies should be over Southcentral and near the Brooks Range.

Rausch


Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

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

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