Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
654 PM EST Tue Dec 14 2021
Valid 12Z Sat Dec 18 2021 - 12Z Wed Dec 22 2021
...Likely precipitation with significant amounts and strong
southwesterly winds over the western mainland Saturday night
through Monday...
...Overview...
A significant pattern change is still expected over Alaska early
next week. After a shortwave trough comes across the mainland
Saturday, ridging from the surface to upper levels will set up
across the Pacific, with its northern periphery spreading
northeastward from the Bering Sea into Mainland Alaska and
Northeast Pacific during Saturday-Monday. This pattern will lead
to strong, moist, and unseasonably warm southwesterly flow over
much of the state for the early part of next week, causing the
potential for gusty winds and heavy precipitation along with much
milder temperatures.
...Guidance Evaluation and Preferences...
At the beginning of the period Saturday, there is reasonably good
model agreement for a shortwave trough to track quickly across the
mainland, which should spin up a weak surface low along the Gulf
of Alaska coast tracking eastward, while another surface low
weakens offshore of the southern Panhandle. Then, guidance
continues to have a good signal for the aforementioned Pacific
ridging coming into the state and directing southwesterly flow
initially to the Aleutians/western mainland and spreading
eastward. However, there are some model differences especially
west of the state with the details of low pressure systems. The
12Z ECMWF and UKMET indicate two lows moving through the Bering
Sea and eastern Siberia that eventually consolidate, along with
timing differences. The 12/06Z GFS runs seemed to match well
clustered ensemble means' low position best, so favored those
along with lesser proportions of other deterministic guidance for
the early part of the forecast period throughÂaroundÂMonday.
By Tuesday and Wednesday, an upper low establishing itself to the
north of Alaska may send energy southward and create a shortwave
trough passing through the state cutting through the ridge axis
(with additional ridging behind it moving into western Alaska
early Wednesday). General trends since yesterday include a
stronger shortwave, with the 12Z GFS and ECMWF agreeing reasonably
well on its position, and the cold front moving faster across
Alaska ahead of it. The 12Z CMC did not line up as well with the
shortwave axis, but with the GFS/ECMWF agreement, felt it was best
to trend stronger with the shortwave and a surface low
strengthening offshore of the Alaska Panhandle by early Wednesday.
The WPC model blend included the GEFS and EC ensemble means by day
7 and quickly increased their proportion to 50% ensemble means and
50% deterministic GFS/ECMWF on day 8 amid increasing model
uncertainty.
...Weather/Hazard Highlights...
Precipitation is likely for the Aleutians and the western mainland
beginning Saturday given the increasing deep layer moist
southwesterly flow into the state west of the Pacific ridge, which
should spread into central parts of the mainland by Sunday. Over
the western mainland from the Brooks Range southward, during
Saturday night through Monday precipitation (of varying types, but
mainly snow) may be heavy over terrain favored by strong
southwesterly winds. Brisk low level winds are expected, which
along the coast may even lead to areas of coastal flooding. Areas
with a combination of snow and strong winds could see blizzard
conditions. Precipitation and winds should trend lighter on
Tuesday after the cold front passes.
With the relatively warm inflow for Alaska, western areas of the
mainland will see above normal temperatures on Saturday,
increasing and spreading eastward through early next week.
Temperatures could be 20 to as much as 40 degrees above average,
with some temperatures rising above the freezing mark especially
in the southwest part of the state. The Panhandle will be the main
area with below normal temperatures through the first half of next
week, while parts of the southeastern mainland could be around
normal.
Tate
Hazards:
- Heavy snow across portions of mainland Alaska, Fri-Sat, Dec
17-Dec 18.
- Heavy snow across portions of mainland Alaska, Sun-Mon, Dec
19-Dec 20.
- High winds across portions of mainland Alaska and the Aleutians,
Fri-Sat, Dec 17-Dec 18.
- High winds across portions of mainland Alaska, Fri-Mon, Dec
17-Dec 20.
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