Heavy Freezing Spray Warning 185, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
512 PM AKST Wed Nov 24 2021

.ANALYSIS AND UPPER LEVELS...

A large and strengthening upper level low is currently centered
over Bristol Bay and the Lower Kuskokwim Valley this morning.
East of this low, a compact mesolow is evident on satellite south
of Seward. Otherwise, moisture streaming northward ahead of this
feature is running into a very cold air mass. The cold air is
doing its best to squeeze out moisture, but most of the steadiest
snow has been over the eastern Kenai Peninsula, including Seward
and Whittier. A strong shortwave extending south from the low is
over the marine area between the Shumagin and Kodiak. This strong
shortwave rotating northeastward towards Southcentral may
invigorate the snow across the region tonight into Thanksgiving
morning.

An 80 kt jet extends from the southwestern Gulf into the Copper
River Basin. Meanwhile strong northerly flow aloft is coupling
with gusty northerly flow at the surface across the Bering. This
is producing widespread snow shower activity all across the
Aleutians and Alaska Peninsula. Away from the marine areas, the
main story is cold air, with Aniak leading the way in southwest
Alaska dropping well into the 20 below range. Gulkana made it into
the upper teens below zero last night under mainly clear skies.

&&

.MODEL DISCUSSION...

With a very steady-state weather pattern in place across all of
Southern mainland Alaska, the large-scale model agreement is very
good. There is quite a bit of uncertainty on the low placement for
the low approaching the Western Aleutians on Friday, but otherwise
surface high pressure over the eastern half of the Bering by
Friday will keep any effects from that low confined the Western,
and perhaps eventually Central Aleutians by this weekend.

Forecast uncertainty primarily resides with the snowfall forecast
across Southcentral. The mesolow south of Seward is well-agreed
upon to move into Prince William Sound during the day Thursday,
but how far inland its effects extend as regards related snowfall
remain highly uncertain. It is likely to produce scattered snow
shower activity, but timing when or where the individual snow
showers will be is very low confidence.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist for the most
part. However, bands of snow will continue to move from Prince
William Sound over the Kenai and Chugach Mountains into Anchorage.
It's hard to nail down specific times, but there is potential for
brief periods of MVFR ceiling and vis.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Tonight
through Saturday)...

More cold temperatures are on tap for this forecast period. The
forecast high temperature at Ted Stevens Airport for Thanksgiving
Day is 11F, which if observed will make it the coldest high
temperature on Thanksgiving since 2003. There is an upper level
(500 mb) low over Southwest Alaska with numerous embedded
shortwaves. This 500 mb low will move over the Kenai Peninsula
Thursday and north of Anchorage Friday. At the surface there is a
low near Chenega and on the Middleton Island radar there is a
swath of moisture impacting portions of the Gulf Of Alaska and
Prince William Sound. On the Kenai radar there are light returns
over Cook Inlet which is due to cold air being pushed over the
relatively warmer water. This forecast period, the highest
snowfall amounts will be along the coast, especially for Whittier,
where another foot or more could fall this forecast period.
Meanwhile, a trace to a couple of inches of snow are possible from
Anchorage to Homer this evening through Friday. As the upper
level low moves eastward, cold air advection will result in gusty
winds through the gaps and passes of the Alaska Range. The worst
of these winds will impact Kamishak Gap, the Barren Islands and
Shelikof Strait. On Friday as the upper low slowly moves
northeastward, snow showers are likely to linger across Prince
William Sound and portions of Cook Inlet.

&&

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

A longwave trough remains over Southwest Alaska this evening
centered near Sparrevohn. Dense cloud cover noted near the center
is expanding southward with the southern edge approaching Naknek.
This cloud cover and additional clouds streaming southward from
northern portions of the state are expected to mitigate overnight
lows wherever they can persist through tonight. Cold temperatures
over interior Alaska will continue to be one of the main stories
for Southwest over the next couple of days as November proves to
be unseasonably colder. A very cold air mass is in place over the
state with 850 mb temperatures remaining at or below -20 degrees
Celsius for much of Southwest. Interior locations beneath clear
skies should be able to dip into the minus teens with ease while
locations such as Aniak could cool to near -25 degrees. Saturday
will see the longwave trough begin to weaken and lift
northeastward, which should promote quieter conditions over
Southwest. Though calm winds and potentially clear skies will once
again promote favorable conditions for colder overnight lows.

&&

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

Continued light snow showers are forecast for the Pribilof
Islands, the Alaska Peninsula, and the Eastern Aleutians. As cold
surface temperatures are drawn southward and over the warmer
Bering, ocean effect snow will continue to develop and impact the
various regions through Thursday. Elevated gusts will promote
visibility less than a mile at times within snow showers. Wind
chills for the Aleutians, Pribilof Islands and the Alaska
Peninsula will remain in the teens and potentially dip into the
single digits tonight. Wind and snow will gradually taper off
throughout the day on Friday as a trough over Southwest begins to
weaken and move out of the region. The next low pressure system
and front is forecast to move into the Western Aleutians by
Saturday, bringing a potential rain/snow mix as well a renewed
threat for small craft and gale winds. Models have not been in
particularly good agreement with the placement of the low nor its
overall development. A ridge of high pressure will be the main
complicating factor helping to impede the low's progress. As a
result, impacts at the start of the weekend should be limited to
the Western and possibly the Central Aleutians only.

&&

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

Gulf of Alaska...Cold Arctic air through gaps along the Gulf
coast may produce high end small craft to gale force winds
through Monday. The Gulf will likely be impacted by a deep low
moving out of the northeast Pacific on Monday. While the track
is uncertain, Gales are likely across much of the Gulf waters
with the possibility of Storm force winds along the leading
front.

Aleutians/Bering Sea...High pressure ridge exists over the
eastern Bering Saturday through Sunday morning. A front off a
North Pacific low enters the western Bering on Saturday and
pushes eastward, encompassing a majority of the Bering by Sunday
afternoon. Seas 15-20 ft are likely along the Central/Western
Aleutian coastal waters Saturday and Sunday, with winds near
Gale force (Central Aleutians) and at or above Gale force
(Western Aleutians). Winds and waves begin to diminish by
Monday as the low moves toward the Gulf of Alaska.

&&

.Long Term Discussion (Days 4 through 7: Saturday through
Wednesday)...

The cold pattern continues as the Arctic airmass holds strong.
Expect temperatures to remain frigid, reaching into the -20’s
for inland areas across Southern Alaska. A digging trough will
help force these winds through gaps in the Western Alaska Range
and Kamishak Bay, producing increased wind and the potential for
heavy freezing spray. High amplitude ridging persists over the
eastern Bering on Sunday, until the front off an incoming North
Pacific low forces it onshore over Southwest Alaska. The low
tracks south of the Aleutian Islands in its journey towards the
Gulf of Alaska. The front moves along the Chain, producing gusty
winds and falling precipitation to the area Sunday and Monday.
With another surge of Arctic air on the back side of this the
potential for snow and blowing snow is high.

The front enters the Gulf by Monday, bringing rain and snow to
the Gulf and coastal regions. The parent system looks to develop
a secondary system in the base of its trough late Monday
night/early Tuesday morning, and merge. The new system moves along
the Southeast Coast through Tuesday before reaching the North
Gulf Coast Tuesday night. There is uncertainty in the exact tracks
of these lows across the Gulf. However, the pattern looks
favorable for periods of heavy snow along the coastal mountains
beginning Monday and continuing through at least mid week. There
might be enough warm air at sea level to cause snow to mix with or
change to rain.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 130.
         Gale Warning 120 131 132 138 139 150 155 165.
         Heavy Freezing Spray 121 129 130 137 138 139 141 150
                              160 165 180 181.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JW
SOUTHCENTRAL ALASKA...PS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BL
MARINE/LONG TERM...KM

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
345 AM AKST Thu Nov 25 2021

AKZ181-185-195-260600-
Alaska Peninsula-Eastern Aleutians-Pribilof Islands-
Including the cities of Cold Bay, Sand Point, Unalaska, Nikolski,
and Saint Paul
345 AM AKST Thu Nov 25 2021

...OCCASIONALLY HEAVY SNOW SHOWERS AND STRONG GUSTY WINDS EXPECTED
TO CONTINUE THROUGH THIS EVENING ACROSS THE EASTERN BERING...

Snow showers and gusty northwesterly winds are expected to
continue across the Alaska Peninsula, Pribilofs, and Eastern
Aleutians through this evening. Arctic air continues to move off
the Alaska mainland and over the relatively warm water of the
Bering. Thus, the atmosphere will continue to be moistened and
contribute to snow shower development through this evening as the
pattern remains stagnant.

Wind gusts ranging from 30 to 40 mph this morning will slowly
subside to 20 to 30 mph by this evening. As a result, any falling
snow combined with the gusty winds could reduce visibility to as
low as one half mile at times. Locations that continue to be
impacted by these adverse conditions include False Pass, Port
Heiden, Cold Bay, Dutch Harbor, and Saint Paul.

Widespread snow shower activity throughout the morning and into
the afternoon will become more isolated in nature during the
evening.