Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
657 PM EST Mon Dec 13 2021
Valid 12Z Fri Dec 17 2021 - 12Z Tue Dec 21 2021
...Likely precipitation with significant amounts and strong
southwesterly winds over the western mainland Saturday night
through Monday...
...Overview...
Guidance continues to show a significant pattern change, from a
west-to-east succession of shortwaves and associated surface
systems to a strong Pacific upper ridge whose northern periphery
will progress from the Bering Sea into Mainland Alaska and
Northeast Pacific during Saturday-Monday. The gradient between
the ridge and dynamics associated with a system tracking east of
Kamchatka through eastern Siberia and into the Arctic should
ultimately lead to strong westerly flow aloft over much of the
state by next Tuesday. Flow ahead of the surface system will
likely bring widespread precipitation and strong southwesterly low
level winds into the western mainland during the latter half of
the weekend into early next week. The pattern change will also
promote a significant warmer trend for temperatures.
...Guidance Evaluation and Preferences...
With today's UKMET trending deeper to recent runs of other models,
clustering has further improved for the early Friday Bering Sea
storm expected to weaken as it tracks into the far western
mainland by early Saturday. The 12Z model consensus serving as
today's forecast blend for at least the first half of the period
continued the trend of recent days in being a little
deeper--consistent with steadily improving track and timing
clustering--though the models show signs of starting to stabilize
for strength. This system should lead to a weak Gulf wave that
tracks southeastward offshore the Panhandle during the weekend,
after a leading weak wave that takes a similar track Friday into
early Saturday.
Individual model runs have been varying with details of the storm
forecast to track from the western Pacific to east of Kamchatka
and eventually into the Arctic. The guidance average has adjusted
somewhat eastward versus yesterday when the system passes near
Kamchatka, while some models now suggest there could be two lows
that eventually consolidate, and there have been timing issues as
well. For the latter the 00Z ECMWF appeared notably fast. Into
day 7 Monday a primary emphasis on the 12Z operational models in
the forecast blend had decent agreement from the means, and in
spite of the low's track adjustment east of Kamchatka it still
ended up near continuity northwest of the mainland by early
Monday. The new 12Z ECMWF mean was close to this scenario as well.
After early Monday the models have been showing increasing spread
and run-to-run variability for the shape of the northern periphery
of the strong Pacific upper ridge and details of shortwave energy.
These differences aloft corresponded to variation in where a
surface front sets up between the mainland and Aleutians/North
Pacific. Decreasing confidence in specifics led to increasing
total ensemble weight to 40 percent by day 8 Tuesday, with
somewhat more 00Z ECMWF mean versus the 12Z GEFS mean as the
former continues to be somewhat stronger and closer to the models
for upper ridge strength.
...Weather/Hazard Highlights...
Light to moderate precipitation and brisk to strong winds should
accompany the system tracking from the central Bering Sea into the
western mainland Friday-Saturday. Brisk winds could reach the
southwestern coast of the mainland depending on exact low track.
From late this week into the weekend a couple weak waves tracking
southeastward from the Gulf of Alaska may produce light to
moderate precipitation though consensus is trending noticeably
lighter over the Panhandle.
The flow ahead of a strong storm forecast to track from east of
Kamchatka through eastern Siberia and into the Arctic should
spread a broad area of moisture and strong winds across portions
of the Aleutians, Bering Sea, and then the western mainland during
the weekend into early next week. Over the western mainland from
the Brooks Range southward, during Saturday night through Monday
precipitation (of varying types) may be heavy over terrain favored
by strong southwesterly winds. Areas with a combination of snow
and strong winds could see blizzard conditions. Precipitation and
winds should trend lighter on Tuesday.
Aside from some below normal temperatures over the Panhandle,
eastern mainland, and southern coast, the majority of the mainland
should see temperatures trend to above normal levels with the
highest anomalies likely to be over western areas around
Sunday-Monday.
Rausch
Hazards:
- Heavy precipitation across portions of mainland Alaska, Sat-Sun,
Dec 18-Dec 19.
- Heavy snow across portions of mainland Alaska, Sun, Dec 19.
- High winds across portions of mainland Alaska and the Aleutians,
Sat-Sun, Dec 18-Dec 19.
- Much below normal temperatures across portions of mainland
Alaska, Thu, Dec 16.
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