Heavy Freezing Spray Warning 179, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
458 PM AKST Sun Nov 21 2021

.ANALYSIS AND UPPER LEVELS...

A broad upper level low sits across the central part of the
state. Weakening low pressure is located to the southeast of
Yakutat, with an attendant front wrapped around to the north and
across western Prince William Sound. This front had a very defined
snow boundary which stretched from Middleton Island, north across
Hinchinbrook Island to just east of Valdez. This area of snow has
slowly been moving east through the day. Farther west, high
pressure spans much of the Aleutian Chain and Bering Sea.
Subsidence across most of western and south central Alaska are
keeping skies clear with temperatures cold. The Copper River
Valley dipped to the -30s this morning, while areas in the Mat-Su
Valley ranged from -23 at Willow to the negative teens in
Palmer/Wasilla. In Anchorage, the usual cold spot in Campbell
Creek only dipped to -16 while Ft Rich fell to -20.

&&

.MODEL DISCUSSION...

Synoptically speaking, models are in good agreement through the
short term. All guidance is similar with a broad upper level low
slowly working down across the Kenai Peninsula by Tuesday, before
drifting west across the Alaska Range by Thursday. Late tonight
and into tomorrow, the first in a series of shortwaves is going to
round the base of a trough protruding from the aforementioned
low, and lift through the Gulf of Alaska. Differences come into
play with the minute differences in the speed and intensity that
this wave moves to the east. All models develop a secondary low in
the Gulf of Alaska tonight and into tomorrow. All models tell
similar stories in drifting this low west somewhere between
Whittier and Seward. The American Models are about 12 to 18 hours
slower than the international models, but all portray similar
intensity and locations.

&&

.AVIATION...

PANC...VFR conditions and light northerly winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Sunday night through Wednesday)...

The surface low that brought heavy snow to Cordova and blizzard
conditions to Thompson Pass has shifted east. Some light snow will
linger over the Copper River Basin as moisture is wrapped around
the north side of the low, but overnight accumulations will remain
between 1 and 2 inches from McCarthy to Glennallen. Outflow winds
are expected to remain elevated as cold air continues to drain
out of the interior areas to the Gulf. Thompson Pass wind chills
will likely remain between -30 and -40 degrees overnight.

The main weather story for the first half of the work week is
that an upper level low is making its way south from the interior
to the Kenai Peninsula, where it will take up residence through
Tuesday before shifting to the west. Later in the week this wave
is likely to amplify the pattern and return elevated chances of
snow to coastal and potentially inland areas. In the short term, a
lee- side surface low is expected to develop overnight along the
north Gulf coast. Considerable uncertainty remains in the overall
synoptic wind field depending on where the low center develops and
where it moves over the next two days. All guidance indicates
that the upper level and surface features will begin to retrograde
on Tuesday as the overall pattern amplifies, but considerable
differences remain as to when Tuesday the surface low will begin
to move from the north Gulf coast into the Prince William Sound or
southwest near Seward. This forecast package represents a low
tracking into the western Prince William Sound overnight Monday,
which will bring snow to coastal areas Tuesday morning.

Due to the considerable amount of cold and dry air from Kenai
City north to the Mat-Su Valleys, any moisture that makes it over
the mountains Tuesday and Wednesday will likely
evaporate/sublimate before reaching the surface. However, a north
Pacific low is expected to move into the Gulf on Wednesday,
bringing a push of warmer air and moisture to the region. Some
uncertainty remains on the track of the low, but most solutions
bring the low into the vicinity of Prince William Sound Wednesday
afternoon, which will increase chances of snow for the populations
centers on Thursday. Coastal cities and mountain ranges are
likely to see snow Tuesday and Wednesday. The pattern shift should
also help to increase temperatures throughout the region and
bring an end to strong low level outflow winds.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 to 3: Tonight
through Wednesday)...

Arctic air continues to keep a firm grip on all of the Southwest,
particularly over the Middle Kuskokwim Valley where temperatures
again plummeted to as low as -40F at Stoney River early this
morning. One more night of the more extreme cold is on schedule
for places including Aniak, Kalskag and other valley locations
across portions of the Kuskokwim Valley/Delta, with lows tonight
between -15 and -25F generally expected once again away from the
coast. Meanwhile, gap winds gusting occasionally over 50 mph are
expected tonight into Monday for the typical locations along the
northern Alaska Peninsula as brisk northwest flow and cold
advection continue.

A deep polar low will swing south towards the northern Gulf Coast
from tonight into Monday, with an embedded east-west trough axis
also shifting south across much of the Southwest by tomorrow
afternoon. This will displace yet another frigid, Arctic air mass
southwest from the Alaska interior, with 850mb temperatures
falling to as low as -30C on Monday over the Middle Kuskokwim
Valley as extremely cold air wraps around the north and west side
of the upper low center. Despite all this, there may be recovery
for temperatures at some spots that have observed the coldest
temperatures over the past few days, namely, for morning lows
early on Tuesday for the Middle Kuskokwim Valley area. This will
be due to the hampered radiational cooling from increasing cloud
cover near the upper low beginning to shift west into the region
by late Monday night. As the aforementioned upper low moves almost
directly overhead, colder than average temperatures (mostly below
0F) and generally dry conditions will continue for much of the
Southwest for the remainder of the period from Tuesday to
Wednesday. The one partial exception will be along southern parts
of the Alaska Peninsula, where a favorable setup for 'ocean'
effect snow could materialize as the coldest Arctic air streaming
south picks up warmth and moisture from ice free water around
Bristol Bay. Intermittent periods of snow on Wednesday may
eventually add up to a few inches of snow accumulation for areas
including Cold Bay and Nelson Lagoon. Additionally, stiff
northerly winds around 30 mph will easily transport the light/dry
snow and lead to areas of blowing snow and lowered visibilities at
times.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 to 3: Tonight
through Tuesday)...

The primary inclement weather-maker for the course of the next
couple days will struggle to make headway against an entrenched
reservoir of modified Arctic air and high pressure holding tight
across most of the Bering. A potent North Pacific low will slow
down as it encounters the ridge to the north tonight, eventually
stalling out in place well south of the Aleutians on Monday.
Nonetheless, the contrasting temperature and pressure between
these two features will still drive fairly strong east to
northeast winds along the Aleutians and far southern Bering, with
winds peaking just above storm force on the Pacific side of the
central Aleutians for Monday afternoon and evening.

To the north, widely scattered snow showers will spread south and
west on Monday and Tuesday as an upper level wave moves south
into the eastern Bering. Meanwhile, winds along the Aleutian Chain
will subside below gale force by midday Tuesday as the
aforementioned North Pacific low begins to fill and shift back
south, away from the forecast area. An even colder air mass will
spill into the Bering from the western mainland from late Monday
well into Wednesday as the ridge in place retrogrades towards the
far western Bering and as a deep upper low takes its place over
the eastern Bering. The advancing frigid air, combined with 25 to
35 kt northeast winds, will facilitate development of heavy
freezing spray that will become more widespread over the next few
days, with the highest threat focused along the sea ice edge near
Southwest Alaska and northwest of St Matthew Island.

&&

.MARINE (Days 3 through 5: Wednesday through Friday)...

Gulf of Alaska: A North Pacific low tracks northward across the
Gulf towards Prince William Sound Tuesday night through Wednesday
mornings. The associated front pushes against the North Gulf Coast
late Tuesday night/early Wednesday morning, producing a round of
gale force winds that will persist through at least early
Wednesday afternoon. The low moves into the Sound and dissipates
by Wednesday night. A second North Pacific low moves into the
Southern Gulf Thursday morning, taking a similar track as its
predecessor. Widespread areas of westerly high end gales follow in
the wake of the system, with increasingly gusty winds through the
passes of the Western Alaska Range; winds out of Kamishak Bay and
through Shelikof Strait will likely see some of the strongest
gusts. Additionally, cold, arctic air pulled through the gaps in
the mountains may filter down into the lower level and could end
up producing areas of freezing spray becoming heavy at times.

Bering Sea: Expect winds less than 35 kts and seas less than 20
ft through early Friday morning. A front enters the western Bering
late Friday morning, carrying increasingly gusty gale force
winds. A low develops at the base of the trough, south of the
Aleutian Chain Friday night and will move towards the coastal
waters through the night. Winds along the northern edge of the low
will likely reach high end gale force, with the GFS hinting at
the possibility of low end storm force winds by early Saturday
morning along the Central Aleutian waters. Confidence for winds
higher than gale force remains low at this time.

&&

.LONG TERM FORECAST (Days 4 through 7: Thursday through Sunday)...

A North Pacific low pushes into the Gulf Thursday morning and
quickly moves towards Prince William Sound. There is some model
discrepancy, mainly between the GFS and Canadian/EC, with the GFS
significantly further south until Friday night. The Canadian/EC
move the low into the Gulf by Thursday night and stall it, while
the GFS moves slower northward and reaches PWS by Friday night.
The low then moves onshore over Eastern Kenai, weakens, and
dissipates through the weekend. Most areas will be cold enough to
see any falling precipitation in the form of snow, however, areas
immediately along the North Gulf coast may be influenced enough by
warm southerly air to see temperatures rise above freezing, and
thus could experience a transition to rain or a rain/snow mix at
times. Additionally, heaviest precipitation will be along the
North Gulf Coast and Prince William Sound regions, with some light
precipitation spilling into the southern Copper Basin as well as
the Anchorage/Mat-Valley, and western Kenai areas. Confidence for
timing of precipitation remains low at this time.

The cold arctic airmass will persists across the State through
the long term. However, the initial round of southerly onshore
flow as the system approaches the coast will help push some of the
cold air back, allowing a slight increase to temperatures. Cold
air will eventually wrap back around the low to, once again,
decrease temperatures across Southcentral.

Southwest will essentially stay in stasis, with the same overall
pattern persisting through the long term. Dry, arctic air
continues to flow out of the Interior, keeping Southwest precip-
free, with frigid, subzero temperatures through the extended
timeframe. A front reaches the western Bering late Friday morning,
producing gusty low end gale fore winds. A secondary low develops
at the base of the trough, south of the Aleutians Friday night.
Areas of high end gales along the northern edge of the system move
across the Central and Western Aleutians through Sunday night.
The GFS is hinting at areas of low end storm force winds on
Saturday as well, however, confidence remains low that far out in
time.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning: 131 (Thompson Pass)
MARINE...Storm Warning: 176
		 Gale Warning: 127 130-132 138 150 173-175 177 178 352
		 Heavy Freezing Spray: 121 127 129 130 138 150 160 180
								181 185 414
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SS
SOUTHCENTRAL ALASKA...CJ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AS
MARINE/LONG TERM...KM