Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
642 PM EST Sun Dec 12 2021
Valid 12Z Thu Dec 16 2021 - 12Z Mon Dec 20 2021
...Overview...
Models and ensemble means maintain a mean upper low over or near
the Sea of Okhotsk for most of the period while advertising a
pattern change downstream. Initially there will be progressive
flow across the North Pacific and Bering Sea/Alaska with a
succession of shortwave troughs and ridges, with the troughs
supporting Bering Sea systems and waves over the Gulf of Alaska.
Then from the weekend into the start of next week a strong upper
ridge will build over the central and northern/northeastern
Pacific. The northern extension of this ridge will progress from
the Bering Sea through the mainland. Expect a strong jet to set
up between the ridge and upstream low/trough. As this new pattern
takes shape a strong storm should track from near Kamchatka to
northwest of the mainland.
...Guidance Evaluation and Preferences...
Continuity is good in principle with the initial Bering Sea system
and leading front nearing the mainland as of the start of the
period early Thursday as well as the wave that reaches the Gulf of
Alaska by early Friday. Most models have suggested a separate
wave to the south will stay sufficiently far south not to have an
influence on the southern Panhandle, with the UKMET being the
exception. Meanwhile a slower trend in latest GFS runs toward the
prior majority has led to better timing agreement for the system
forecast to track from the western Aleutians through the Bering
Sea and into the mainland Thursday-Saturday. Today's forecast
continues recent deeper trends seen in most models, though the
UKMET was noticeably weaker than the others. Also some latitude
spread arises by early Saturday. Consensus still shows a wave
near the southeastern coast and Panhandle during the weekend.
As the large scale pattern changes during the latter half of the
period, the past couple days of model runs have been consistent in
showing a stronger Bering Sea into mainland upper ridge versus the
ensemble means, with the ECMWF mean tending to be a little
stronger than other means. There have been some differences in
timing of strong low pressure tracking from near Kamchatka to
northwest of the mainland, as well as for how much shortwave
energy could try to dampen the northern side of the ridge.
However the recent model continuity for the upper ridge through at
least Sunday, plus the past couple ECMWF runs and 12Z GFS/CMC
being sufficiently agreeable overall and fitting within the
general ensemble mean template, favored keeping a majority weight
of operational guidance even into day 8 Monday.
The 12Z models, with the least weight on the UKMET, provided the
starting point for the forecast during the first half of the
period. With guidance comparisons offering continued support for
operational runs thereafter, by day 8 Monday the blend maintained
75% operational model weight (12Z GFS/CMC and 12Z/00Z ECMWF) with
the rest provided by the 00Z ECMWF mean given its stronger
solution for portions of the upper ridge.
...Weather/Hazard Highlights...
Each of two Bering Sea systems and subsequent Gulf of Alaska waves
will bring generally light to moderate precipitation to the
Aleutians, western/southern parts of the mainland, and the
Panhandle during the latter half of the week. Briefly heavier
activity may be possible over a few isolated locations along the
southern coast and Panhandle. The system tracking from the
western Aleutians to the mainland Thursday-Saturday may produce an
episode of brisk to strong winds over the Bering Sea, and possibly
into parts of the western mainland depending on the exact
evolution/track of the low. Flow ahead of a storm forecast to
track from near Kamchatka to northwest of the mainland should
spread a broad area of moisture and strengthening winds across
portions of the Aleutians, Bering Sea, and eventually the mainland
during the weekend into the start of next week. Some locations
over favored terrain may see enhanced precipitation in that time
frame.
Below normal temperatures over the eastern part of the state on
Thursday will give way to warmer temperatures moving in from the
southwest, with western areas likely to be above normal by the end
of the week. Anomalies for min temperatures should be greater
relative to those for highs. Departure of Saturday low pressure
may lead to a brief dip in temperatures over the mainland but then
the flow ahead of the storm well to the west should renew the
overall warming trend early next week, with the northwestern half
of the mainland possibly reaching well above normal values. The
Panhandle will tend to remain below normal while a mix of
above/below normal readings prevail in-between.
Rausch
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