Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
635 PM EST Fri Dec 11 2020
Valid 12Z Tue Dec 15 2020 - 12Z Sat Dec 19 2020
...Guidance and Uncertainty Assessment...
Models and ensembles are similar for the broad pattern evolution
but suggest low confidence/predictability for a lot of the details
that may be important over some areas. Consensus shows a strong
upper ridge/high over the Arctic while progressive flow prevails
over the Pacific. Between the ridge and Pacific flow there will
be a broad weakness aloft likely to extend from Kamchatka through
the Bering Sea, Mainland Alaska, and into northern Canada. One of
the more agreeable upper lows within this weakness seems most
likely to drift near Kamchatka into the western Bering while
another meanders over northern Canada. Other upper lows may form
as well, such as over or near the northern/eastern Bering Sea as
some guidance has been suggesting toward late next week.
Low-predictability specifics of individual Pacific shortwaves and
possible interaction with features embedded within the broad
east-west weakness aloft (which have their own uncertainties) lead
to below average confidence in resolving surface low/front details
for most of the period.
Early in the period there is some degree of clustering on a
concentrated surface low that may track near or just north of
Haida Gwaii (12Z GFS a bit on the strong/northeast side of the
spread) while a wave initially over the Aleutians lifts into the
Bering Sea. A modest shortwave continuing eastward from
over/south of the southwestern mainland/Alaska Peninsula may
produce a short-lived system over the Gulf of Alaska by early Wed.
Due to a delay in model data, the 12Z ECMWF was not available in
time for preparation of today's 500mb and surface forecast. A
blend among the 12Z GFS-CMC-UKMET/00Z ECMWF reflected the most
common ideas of guidance and offered some improved detail over the
means. After Wed individual models/consecutive runs essentially
devolve into a random assortment of various surface lows over the
North Pacific/Gulf/Bering. There is a general signal that low
pressure may try to consolidate over or near the Gulf toward next
Fri or Sat. Lack of model agreement for specifics favored a
transition toward greater 12Z GEFS/00Z ECMWF mean input late in
the period.
...Weather/Hazards Highlights...
Expect windy conditions over portions of the Interior next week,
with the mainland in the gradient between strong Arctic high
pressure and various low centers over the Bering Sea and Gulf of
Alaska. Most precipitation during the period should be confined
to locations from the Aleutians through the far southern mainland
and Panhandle. Intensity/timing/coverage details are difficult to
resolve at this time, though guidance is generally signaling some
increase in precipitation over the southern coast and Panhandle by
late next week/early weekend as low pressure consolidates over the
Gulf. Other systems/fronts may provide one or more episodes of
focused activity over the Aleutians/Peninsula. Below to well
below normal temperatures will become more widespread with time
over a majority of the state as heights within the east-west
weakness aloft trend lower. Offshore surface low will likely keep
temperatures below normal over southern areas and the
Panhandle/eastern Interior as well.
Rausch
Hazards:
- High winds across portions of mainland Alaska, Mon-Tue, Dec
14-Dec 15.
- Much below normal temperatures across portions of mainland
Alaska, Wed-Fri, Dec 16-Dec 18.
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards 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