Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
651 PM EST Fri Dec 10 2021
Valid 12Z Tue Dec 14 2021 - 12Z Sat Dec 18 2021
...Overview...
Today's guidance generally maintains the previously advertised
pattern evolution during the period. Initial moderately amplified
and progressive flow across the North Pacific/Bering Sea should
extend into the mainland around midweek or so, with an upper ridge
displacing an Arctic/Mainland Alaska trough in place as of early
Tuesday. Expect a brief flattening of mean flow late in the week
and then renewed amplification as a deep storm near Kamchatka
builds an upper ridge into the Aleutians/Bering Sea by next
Saturday. The ridge should in turn begin to deepen a trough from
the mainland southward. This forecast pattern will carry along a
series of lows/frontal systems that will tend to bring varying
amounts of moisture to the Aleutians, western/southern parts of
the mainland, and the Panhandle. In addition temperatures should
moderate after the well below normal readings forecast early in
the week.
...Guidance Evaluation and Preferences...
A composite of the 12Z operational models provided better than
average continuity from yesterday for significant features between
Tuesday and early Thursday. Meaningful model discrepancies become
evident by early Thursday, with additional divergence for systems
over the Bering Sea and Pacific thereafter in the brief period of
flatter mean flow as well as where amplifying features set up by
Saturday. Thus the early-period model blend transitioned steadily
toward somewhat greater emphasis on the means by day 8 Saturday in
light of very good agreement among the means for large-scale
features.
One area of notable uncertainty involves what may become of a
mid-latitude Pacific wave that could get picked up by the frontal
system extending from the midweek Bering Sea low. The wave would
then contribute to the surface low that consensus has been
indicating over the Gulf of Alaska by early Friday. The majority
clustering over the past couple days has been showing at least the
warm front even if not a very well defined anchoring wave, while
latest GFS runs (which keep the wave suppressed) stray flatter
than most other guidance (including the GEFS mean) for the overall
upper trough. Thus some indication of such a wave seems
reasonable. Interestingly the new 18Z GFS has become even more
pronounced with its flatter/faster flow aloft. On the other hand
the 12Z ECMWF mean maintains a decent hint of the ECMWF scenario.
Meanwhile there is now enough support to include a wave which the
majority brings to the western Aleutians by early day 6 Thursday
and into the Bering Sea thereafter--though still with a lot of
spread for specifics by that point. Again the GFS is on the fast
side of the envelope. Ensemble members are diverse enough for the
means not to have a defined wave but do suggest a front about
where the non-GFS cluster shows it. This feature should quickly
lead to another Gulf wave by early next weekend.
Keeping modest influence of the operational guidance late in the
forecast provides a little enhancement to the upper ridge expected
to build over the Aleutians/Bering Sea by Saturday. Latest GFS
runs have been on the deeper/slower side of the full guidance
spread for the amplifying trough to the east.
...Weather/Hazard Highlights...
Shortwave energy and a weak surface wave may support light to
locally moderate precipitation along the southern coast/Panhandle
on Tuesday. A Bering Sea storm and its leading front will bring
light precipitation across the Aleutians Tuesday-Wednesday and
light-moderate activity into western and southern parts of the
mainland during mid-late week. Still expect a wave to reach the
Gulf of Alaska or southern coast by early Friday and help to focus
precipitation along the southeastern coast and Panhandle around
that time. However precip totals are uncertain due to lack of
confidence in whether this low will reflect just northern stream
dynamics or have a contribution from a mid-latitude Pacific wave
(which would lead to somewhat higher amounts). Another
wave/frontal system may bring light to locally moderate
precipitation to the Aleutians and western/southern mainland as
well as the Panhandle late in the week.
After below normal temperatures cover much of the state on
Tuesday, with some well below normal readings possibly lingering
in the southern half of the mainland, there should be a moderating
trend from southwest to northeast during the latter half of the
week. This could allow for a few pockets of slightly above normal
highs to develop by Friday while coverage of above normal lows
could be greater at that time. A slight cooling trend from the
northwest is possible by next Saturday.
Rausch
Hazards:
- Much below normal temperatures across portions of mainland
Alaska and the Aleutians, Mon, Dec 13.
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