Blizzard in Alaska, December 2 to 3, 2021

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
432 PM AKST Wed Dec 1 2021

.ANALYSIS AND UPPER LEVELS...

The pattern remains amplified across the North Pacific with a deep
upper level trough over the Alaskan mainland. An upper level low
inside of the aforementioned upper trough is currently over the
northern Gulf and is remaining nearly stationary. An upper ridge
currently extends across the western Bering/Aleutians. A frontal
system is currently moving eastward across the Eastern Aleutians.

Closer to the surface, radar imagery shows convective snow showers
moving northwestward from the northern Gulf into Prince William
Sound. Light snow is currently moving southwestward across Cook
Inlet. Radar also shows light snow showers moving southeastward
across the Kuskokwim Delta.

&&

.MODEL DISCUSSION...

Model solutions are in good synoptic agreement through the short
term and are all on track with the main Gulf low departing to the
south by late Thursday. There is also good agreement that a cold
Arctic airmass moves southward over Southcentral for the end of
the week. Farther west, models are consistent with a front
arriving over the western Aleutians early Thursday and tracking
eastward, ultimately reaching Southwest Alaska by Friday. One
notable change is the strength of the front by the time it reaches
the southwest coast. Model solutions have trended slightly
stronger with this as it lifts inland over Southwest, so will be
keeping an eye on future runs for comparison. In the meantime,
the biggest forecast challenge continues to be the weaker upper
level waves circulating around the main Gulf low that have
continued to produce snow showers across Southcentral.

&&

.AVIATION...PANC...

Expect mostly MVFR with conditions dropping to IFR intermittently.
Snow showers are expected to continue through the afternoon
before diminishing this evening. Winds will shift to northerly by
midnight and are expected to remain out of the north through
Thursday.

&&.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Friday night)...

Current radar indicates patches of snow showers continue to spin
over the northern Gulf and Prince William Sound with additional
areas of light snow falling over the Kenai Peninsula and portions
of the Anchorage Bowl. A lingering trough over the Gulf has
ushered a disturbance or two into Southcentral over the past 24
hours allowing for several inches of accumulating snow. On
satellite a low tracking northward along the Panhandle is taking
advantage of some upper level support downstream of the Gulf
trough and a thick cloud layer is quickly expanding westward and
over the Copper River Basin.

While light snow may linger into the evening hours for various
locations, the bigger story into Thursday and through the end of
the week will continue to be the very cold temperatures filtering
into Southcentral from the northwest. Overnight lows across the
Anchorage Bowl will dip back into the teens tonight, the single
digits on Thursday night, and below zero by the weekend.
Meanwhile, overnight lows for coastal regions will drop into the
teens tomorrow night. The dense cold air mass will also promote
elevated outflow winds and heavy freezing spray through the bays
and passes of Shelikof Strait and Kamishak Bay tonight with
potential gusts to 60 knots west and southwest of the Barren
Islands and to the southwest and northwest of Shuyak Island.
Elevated northwest winds will persist through Thursday afternoon
and gradually diminish through the early morning hours on Friday.

Areas further north will also feel the effects with Resurrection
Bay, Passage Canal and Thompson Pass likely to experience gales
into Thursday night. The upper level trough over the Gulf will
begin to exit the region to the southeast by Friday, allowing cold
air to spread further south. This will see northerly outflow
winds begin to impact the bays and passes along the northern Gulf
from Prince William Sound to the Copper River Delta. Peak
sustained winds by midday Friday are expected to range from 40 to
50 knots out of Valdez Narrows, Valdez Arm and the Copper River
Delta, with potential 30 to 40 knot winds impacting the town of
Valdez Friday into Saturday. Heavy freezing spray will be a
concern into the weekend.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: tonight
through Saturday night)...

Residual snow showers linger across portions of Southwest Alaska
through the rest of the evening as weak shortwaves pulls moisture
into the region. Cold temperatures remain, however, with increased
cloud cover, especially during the overnight hours, temperatures
have been overall warmer than we have seen recently. A dry
pattern ensues beyond tonight and will last through tomorrow night
when a Kamchatka front approaches the Southwest coast. Snow
pushes into the Kuskokwim Delta and Bristol Bay Coast (including
the AKPen) by Friday afternoon. Light snow showers may spill into
the rest of Bristol Bay and the Kuskokwim Valley early Saturday
morning but will likely be short-lived. As it stands now, there is
the possibility for blowing snow along the immediate coast that
may reduce visibilities some, however, this will depend on how
much warm air makes it to the coast. Should temperatures raise
high enough, the snow will likely be too warm/wet for any
significant blowing snow to occur.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3:
tonight through Saturday night)...

A shift in the position of the Gulf low further west near Kodiak
will keep winds gusty along the southeast Bering as well as
through waters south of the AKPen through at least tomorrow
evening. High end gales are likely south of the AKPen, with a
period of storm force gusts, tonight before beginning to gradually
diminish. A strong front extending off a Kamchatka low pushes into
the western Bering late tonight. A band of storm force winds track
along the front through tomorrow, diminishing to high end gale
just west of the Pribilof Islands. Widespread gale force winds
dominate a large area of the Bering both along and behind the
front as it pushes towards the Southwest coast with small craft
winds elsewhere. The front weakens significantly as it moves into
the eastern Bering, winds diminishing once again to small craft or
lower closer to the Southwest coastal waters. High pressure moves
back in behind the front, pulling warmer air across the
western/central Bering and along the Western/Central Aleutians.
Thus, expect these areas to see periods of rain or a rain/snow
mix.

&&

.MARINE (Days 3 through 5/Saturday through Monday)...

Bering/Aleutians...

Forecast confidence remains low for the 3-5 day period due to
significant differences in model agreement with respect to the
placement and intensity of a low moving across the Bering Sea.
This low is likely to produce widespread gale force winds during
this period.

Gulf of Alaska...

Forecast confidence remains above average for Saturday and Sunday
as winds will remain below gale force with no high seas expected.
Confidence decreases on Monday as a front moves into the Gulf and
there are significant differences on the placement of this front
between guidance.

&&

.LONG TERM FORECAST (Days 4 through 7/Sunday through Wednesday)...

A low moves across the Bering Sea towards Alaska on Sunday. Gale-
force winds are expected around the low and along its front, with
models hinting at some areas of storm-force winds. Its front,
supported by strong mid-level vorticity aloft, produces a broad
swath of mostly snow (with some mix and rain) as it enters
Southwest Alaska on Sunday. Warm air advection with the front
will increase temperatures in the Southwest to near the freezing
point. As the low occludes, a triple point low forms off the AKPEN
early Monday morning. The parent low lingers near the Seward
Peninsula while the triple point low moves north along the Gulf
towards the Prince William Sound by Tuesday morning. There are
some model disagreements with the track and speed of the triple
point low. Regardless, models agree that it will bring increasing
winds across Southcentral Monday night through Tuesday. Warm air
advection associated with this low will also increase temperatures
throughout Southcentral. Precipitation looks to remain mostly
snow over land, with localized areas of heavier snow possible for
Kodiak Island, the Eastern Kenai Peninsula, and Prince William
Sound.

Confidence declines further by Tuesday afternoon. The GFS and
ECMWF support another low developing from a shortwave and tracking
across the western Bering into Alaska. While the tracks differ,
models agree that the low enters western Alaska by Wednesday
morning, with its front bringing snow and rain across the
Southwest. Another strong low approaches the Western Aleutians by
Wednesday afternoon, with models hinting at possible storm-force
winds.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm: 130 131 177 178 411-413.
Gale: 132-138 150 173-176 179 181-352 414.
Heavy Freezing Spray: 121 127 129-150 160 180-185.

&&

SYNOPSIS AND MODEL DISCUSSION...ED/KO
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KM
MARINE/LONG TERM...ED/KC