Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
658 PM EST Tue Nov 23 2021
Valid 12Z Sat Nov 27 2021 - 12Z Wed Dec 01 2021
...Arctic cold over much of the Interior through the weekend
followed by moderation over the east...
...Overview and Guidance Evaluation/Preferences...
The models and ensemble means generally agree on the large scale
evolution, with flow around an initial mainland upper low curling
around to the west and then south over or near the Bering Sea,
leading to some interaction with dynamics/low pressure tracking
across the North Pacific. This would ultimately draw what could
be a fairly strong system toward the Alaska Peninsula or Gulf of
Alaska by the early to middle part of next week with a
corresponding increase of precipitation over the Panhandle and
possibly parts of the southern coast depending on the low track.
A downstream ridge aloft should extend from western Canada into
the eastern mainland early-mid week as well. The forecast pattern
will initially favor well below normal temperatures over many
areas, followed by some moderation over eastern areas while cold
anomalies persist over the west.
Although guidance is similar for the broad pattern, there are
still various embedded detail uncertainties that significantly
lower confidence in important specifics. These include issues
with North Pacific/Aleutians upper dynamics and associated low
pressure, possible development farther east, precise evolution of
Bering Sea-mainland upper low/troughing, and the interaction of
the two primary streams. Guidance comparisons through the 12Z
cycle recommended starting with a blend of the 00Z/12Z ECMWF runs
and the 12Z GFS/CMC from day 4 Saturday through early day 6
Monday, followed by reducing GFS input and incorporating some 12Z
GEFS/00Z ECMWF means (up to 40 percent total) while maintaining
some weight of the CMC and ECMWF runs. Main considerations were
to lean away from the 12Z UKMET through its run (farther north
North Pacific/Aleutians evolution during the weekend; but the 18Z
GFS trended north as well) and to minimize GFS influence after
early Monday when 06Z/12Z runs brought low pressure into Bristol
Bay--farther northwest than most other guidance. This northwest
scenario is in the minority among ensemble members but within the
spread. On a percentage basis more CMC members offered that
possibility relative to the GEFS/ECMWF members. The 18Z GFS
trended more suppressed for a while before eventually looping its
surface low into Bristol Bay from the east. The 12Z ECMWF mean
provided reasonable continuity.
Farther north, consensus shows upper ridging to the north of
eastern Siberia while an upper low/trough prevail to the northeast
of the mainland. Guidance is undecided over how much flow around
the western side of this trough could reach farther southwestward.
For most of the period the mainland should be in the surface
gradient between high pressure over the Arctic/northern coast of
the mainland and the combination of an initial Gulf system and
then stronger low pressure approaching from the North Pacific.
...Weather/Hazard Highlights...
Through the weekend expect many areas to see very cold
temperatures, especially over the south/southwest where readings
may be 20F or more below normal. The Panhandle into far southeast
corner of the mainland as well as parts of the north could see
slightly above normal lows though. Transition of the upper
pattern toward a trough near the west coast and ridge extending
from Canada into the eastern mainland should promote a moderating
trend over eastern areas while the west remains well below normal.
Expect light to locally moderate precipitation to persist along
the Panhandle and southern coast during the weekend with low
pressure initially over the Gulf of Alaska. From the weekend into
the start of next week North Pacific low pressure should spread an
area of precipitation as well as the potential for strong easterly
winds across the Aleutians. Winds will be sensitive to exact low
track, with a farther north Aleutians track (and lighter winds
over the islands) lower in probability but still possible. Either
way, winds over the Aleutians should strengthen from the north
after the low passes by. As this low or separate leading
development extend into the Northeast Pacific/Gulf of Alaska next
week, rain/snow should trend heavier at least over the Panhandle.
Uncertainty over specifics of flow aloft and track of low pressure
leads to lower confidence in the westward extent and intensity of
precipitation (ranging between heavy and fairly light) along the
southern coast/Peninsula. The consensus mean pattern should favor
at least some moisture reaching the southern coast.
Rausch
Hazards:
- Heavy precipitation across portions of mainland Alaska, Mon, Nov
29.
- Heavy precipitation across portions of the Alaska Panhandle,
Tue, Nov 30.
- Much below normal temperatures across portions of mainland
Alaska, Fri-Mon, Nov 26-Nov 29.
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