Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
757 PM EDT Sat Oct 23 2021

Valid 12Z Wed Oct 27 2021 - 12Z Sun Oct 31 2021

...Overview and Guidance/Predictability Assessment...

The medium range forecast over Alaska shows a persistent pattern
with an upper low northwest of the state, with rounds of troughing
tracking around it. One such shortwave trough is forecast to push
across the Alaska Peninsula Wed-Thu, before deeper troughing
establishes itself south of the Aleutians in the northeast Pacific
on Friday and tracks slowly eastward over the weekend. Model
agreement starts to go awry particularly with this second trough
and its associated surface low. The 12Z UKMET quickly becomes a
strong/fast/northern outlier near the Aleutians Thu-Fri, while the
12Z CMC is somewhat slower/south. The 12Z GFS and ECMWF align
fairly well to begin with, but by Friday into the weekend the GFS
shows a northern track of the surface low closer to the Alaskan
coast (and thus copious amounts of precip) compared to the farther
south ECMWF. The ensemble mean solutions have somewhat of a middle
ground placement of the low, so using these seemed most prudent
for now.

Farther upstream, a possibly closed low is forecast to move east
across eastern Russia from midweek onward, while a southern stream
low of possibly tropical origin tracks northeast but well to the
south of the Aleutians. These features may end up combining as
particularly the CMC shows, or stay separate, but confidence is
low with this evolution.

Therefore, WPC's forecast for Alaska began with a deterministic
model blend heavily favoring the GFS and ECMWF early in the
period, but gradually transitioned to using a blend of mainly the
GEFS and EC ensemble mean solutions by Friday into next weekend.

...Weather/Hazard Highlights...

As a relatively weak low pressure system tracks across the Bering
Sea Wed and toward Southcentral Alaska Thu-Fri, precipitation and
possibly gusty winds should increase somewhat from the eastern
Aleutians to Southcentral, while generally rainy conditions
continue for the Panhandle with convergent flow. Then by Friday
into the weekend, the amount of precipitation expected will be
dependent on the surface low track in the Northeast Pacific/Gulf
of Alaska. Heavy amounts are possible if the low takes a northern
track more like the 12Z GFS, but lighter to moderate amounts are
forecast with a southern low track. Meanwhile, generally quiet
conditions are expected for much of mainland Alaska, with
occasional snow showers along the southwestern mainland and the
North Slope and Arctic Coastal Plain.

Expect above normal temperatures to dominate much of interior
Alaska to start the medium range period next Wednesday. A slow
cooling trend is anticipated for the entire area as heights aloft
are forecast to slowly decrease due to the slow approach of the
broad cyclonic circulation over the Gulf. The southwestern
mainland may see temperatures well below average with this
cooldown, with highs 10-20F below normal, and temperatures should
become below normal farther east into the Panhandle as well.

Tate


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