Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
642 PM EST Thu Dec 09 2021
Valid 12Z Mon Dec 13 2021 - 12Z Fri Dec 17 2021
...Overview...
Guidance generally shows a progressive North Pacific-Bering Sea
stream carrying along multiple lows and frontal systems while more
blocky flow exists in the Arctic. As might be expected, recent
model runs have been waffling on some details within the
progressive flow and near the interface between the two regimes,
but for the moment there is decent consensus over the mainland.
Early-mid week upper ridging over and then north of Siberia should
support an Arctic trough/possible upper low that digs into and
across the northeastern half of the mainland. This feature should
depart as a Bering Sea upper ridge crosses the mainland mid-late
week. The pattern evolution will likely promote some moderation
in temperatures over the state by the latter half of the week
after much below normal readings prevail early in the forecast
period.
...Guidance Evaluation and Preferences...
A composite of 12Z operational models represented consensus well
for significant features during the first half of the period. For
the northern/northeastern mainland upper trough this blend tilted
away from the GFS that was a little weaker than other models, and
the new 18Z run has adjusted slightly toward the preferred
solution. Recent runs have offered some significant differences
for what ultimately happens with this energy, so further change is
certainly possible--but it is good to see the improved clustering
for now. Meanwhile there is good agreement for two strong western
Bering Sea lows that will anchor windy frontal systems (especially
with the first one) that will cross the Bering Sea/Aleutians/North
Pacific. Consensus has trended farther south from yesterday for a
wave that breaks off from the leading front by late Monday/early
Tuesday, with a track now a ways south of the Alaska
Peninsula/Kodiak Island by early Tuesday and continuing
southeastward thereafter.
For the latter half of the period, fairly rapid divergence of wave
details from the western Pacific into the Bering Sea led to a
transition of the initial model blend to a model/ensemble mean
mix. This included splitting ECMWF input between the past two
runs (and keeping the 12Z GFS/CMC) while using the 12Z GEFS/00Z
ECens for the ensemble component. The resulting forecast
downplayed a possible compact wave that could track into the
Bering Sea underneath (and/or interacting with) the main low over
the Bering Sea Wednesday-Thursday, while awaiting better model
agreement and signal in the means. There is a little more
clustering for another system that may reach the western Bering
Sea late in the week. Farther east there is reasonable agreement
in the general pattern with surface pressures over the mainland
trending lower as Bering Sea low pressure approaches, while a
triple point wave ends up near the southern coast of the mainland
early Friday. By late in the week the model/mean blend helped to
balance competing factors, the models possibly leading the means
in being stronger with Bering Sea low pressure versus the higher
surface pressures over the mainland in the means helping to
account for occasional model biases to erode low level cold air
too quickly.
...Weather/Hazard Highlights...
An initial western Bering Sea storm and leading front will produce
strong winds over the north-central and eastern Bering Sea on
Monday while bringing organized precipitation through the
Aleutians and Alaska Peninsula/southwest corner of the mainland.
However those will be the only effects as a frontal wave tracks
southeastward from early Tuesday onward. Lighter precipitation
and somewhat weaker winds should accompany another strong western
Bering Sea storm and leading front extending through the Aleutians
Tuesday-Wednesday. Moisture with this latter system will likely
reach into western and southern parts of the mainland during
mid-late week. A wave expected to reach near the southern coast
by early Friday may help to focus precipitation along the
southeastern coast and Panhandle around that time. Additional
waves may affect the Aleutians late in the week but with low
confidence on details at this time.
Expect below normal temperatures over much of the state early in
the week with best potential for much below normal readings over
the southern half of the mainland. The forecast pattern evolution
should promote 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.
Rausch
Hazards:
- High winds across portions of the Alaska Peninsula and Kodiak
Island, Sun, Dec 12.
- Much below normal temperatures for the southern half of Alaska
including the Alaska Peninsula, Sun-Mon, Dec 12-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