Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
630 PM EST Sat Dec 12 2020
Valid 12Z Wed Dec 16 2020 - 12Z Sun Dec 20 2020
...Guidance and Uncertainty Assessment...
Today's guidance is reasonably consistent with the forecast of the
large scale pattern while also maintaining the idea that embedded
features will be more difficult to resolve several days ahead of
time. Continue to expect a broad weakness aloft across the Bering
Sea and Mainland Alaska into northern Canada, south of a strong
ridge over the Arctic. The best consensus still suggests that
multiple pieces of energy from various directions (especially
drifting eastward into the Bering and retrograding across the
mainland) may converge into an upper low over the northern Bering
by late week/weekend. Meanwhile shortwaves embedded within
progressive North Pacific flow will provide some complexity, in
light of typically low predictability of details in fast flow and
uncertain interaction with the broad weakness immediately to the
north.
From Wed into early Fri, recent model runs have started to show a
signal for what could be fairly vigorous surface development
reaching within an area encompassing the eastern Aleutians/Alaska
Peninsula, southeastern Bering Sea, and Gulf of Alaska. Overall a
blend of the 12Z/00Z ECMWF and 12Z/06Z GFS runs provided the best
intermediate solution for this system while balancing the desire
for good detail versus accounting for continued uncertainty. By
early Thu the UKMET was a bit faster and the CMC slower. After
early Fri confidence in specifics further decreases. The best
model/ensemble clustering shows separate surface lows persisting
through the weekend over the western coast of the mainland/eastern
Bering and over the Gulf of Alaska. Trending the aforementioned
operational model blend to a model-mean blend (12Z GEFS/00Z ECens
for the means) best represented the most common themes of guidance
at the surface and aloft for next weekend.
...Weather/Hazards Highlights...
During Wed-Fri a potential storm system may bring an episode of
enhanced precipitation and brisk/strong winds to some areas from
the eastern Aleutians into the Alaska Peninsula/southwest mainland
and continuing along the southern coast into the Panhandle.
Uncertainty over the storm's evolution/track keeps confidence in
the lower half of the spectrum for specifics of coverage and
intensity of precipitation. Low pressure may linger over the Gulf
of Alaska through the weekend but moisture in its vicinity will
likely decrease. Expect dry conditions or light/scattered
precipitation over most other parts of the mainland while the
Aleutians/Peninsula likely see periods of mostly light activity
behind the aforementioned system. Winds over the mainland should
be strongest mid-late week, corresponding to the tightest gradient
between high pressure over the Arctic and low pressure reaching
into the southeastern Bering/Gulf. Below normal temperatures
should prevail over a majority of the mainland, especially for
highs. Parts of the south may be modestly above normal and other
areas may see pockets of above normal readings for lows. Most of
the Panhandle should see below normal temperatures for highs but
again with a more mixed picture for low temperature anomalies.
Rausch
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