Heavy Freezing Spray Warning 239, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
453 PM AKST Thu Dec 30 2021

.ANALYSIS AND UPPER LEVELS...

The leading edge of the next storm system to impact mainland
Alaska is in the form of a warm front that is encroaching well
inland into Southwest Alaska, and also across Kodiak Island this
afternoon. This warm front is spreading rain from King Salmon
southward along the Aleutians, Alaska Peninsula and Kodiak Island.
That warm air is being occluded out to the east. In other words,
that warm air is not expected to encroach any further north into
mainland Alaska. Thus, unlike with many recent events, there is
high confidence that all snow will fall with this storm as it
moves northeast further into southern mainland Alaska.

Temperatures are in the 20s across the Kuskokwim Delta and
interior Bristol Bay, above freezing from Egegik south and Kodiak,
and 40s across the Aleutians and AKPEN. Temperatures are into the
20s for Homer and around 20 in Anchorage, teens across the
Matanuska Valley, and below zero for portions of the Susitna
Valley through to the Copper River Basin.

In the upper levels, a large upper level ridge is following along
the Gulf coast from the Alaska Peninsula through to the Panhandle.
A large upper level low is centered west of the Pribilofs. Along
the jets stream, a southwesterly 160 kt jet streak is aligned from
the North Pacific through the Eastern Aleutians and up to the
AKPEN. Meanwhile a northwesterly 150 kt jet is aligned along the
Gulf coast from Cordova to the southern Panhandle.

&&

.MODEL DISCUSSION...

The models are in decent agreement through Saturday afternoon.
While there are some differences on where the center of the Gulf
low is by Saturday afternoon, the areas of impactful weather by
then are in great agreement. There is also great agreement on the
timing of tonight's through tomorrow's snow. A bit of uncertainty
still remains on exact snowfall amounts, primarily on if a
deformation band can add another inch or two around the Cook
Inlet/Anchorage region. That will be determined by where exactly
that relatively small feature sets up. Overall forecast confidence
is high.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist into this
evening. A warm front moving northeastward out of the Gulf will
set off a long duration light snow event starting tonight as the
front stalls over the area. 2 to 4 inches of accumulation are
expected. For most of the event, conditions should stay MVFR,
though certainly any heavier embedded snow showers will briefly
lower conditions to IFR. The snow will persist for much of the
day Friday before gradually diminishing Friday night and evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
This afternoon through Sunday)...

A warm front moving through Kodiak Island tonight will bring rainfall
there tonight. A triple point low has formed just south of the
Kenai Peninsula this will keep cold air across most of
Southcentral with the warmer air staying just offshore of the
coast. This low will spread snow across the Kenai Peninsula and
towards Prince William Sound tonight. An upper level shortwave
will swing towards Southcentral tonight with just enough energy to
produce some light snow late this evening through tonight. Expect
1 to 2 inches with this first wave of snow through tomorrow
morning for most populated areas. On Friday the surface low will
move northeast towards Middleton Island this will continue to
produce snow from Seward, Whittier and Cordova with 1 to 3 inches
in most areas and up to 6 inches in Cordova. An upper level low
will swing out of Southwest Alaska on Friday bringing another
round of snow for the Kenai Peninsula, Anchorage and the MATSU
Valleys with 2 to 4 inches expected through Friday evening.
Behind this low the main upper level trough will swing down from
the interior Friday night and early Saturday morning and may bring
an additional inch or two of snow across the MAT valley and
Anchorage area, with the hillsides possibly getting another 2 to
4 inches through Saturday morning. This system will also bring 2
to 5 inches across the Copper River Basin through Saturday. As the
upper low over the Gulf swings east on Saturday the Northwest gap
winds will strengthen as cold air spills down from the Arctic
with a period of Hurricane force winds near the Kamishak Gap and
Barren Islands on Saturday. Otherwise, strong gap winds will
continue through Sunday from Shelikof Strait and east towards the
Copper River Delta. Wind chill advisories are possible for a few
locations by Saturday and Sunday especially for Thompson Pass and
Broad Pass.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Today
through Sunday)...

A strong front pushing southwest to northeast through Southwest
Alaska this evening is set to gradually weaken through tomorrow.
Across the Kuskokwim Delta, easterly winds gusting up to 45 mph
continue to produce areas of blowing snow with significantly
reduced visibility, especially along the immediate coastline. A
Winter Weather Advisory remains in effect for the Kuskokwim Delta
through 6 am Friday morning. Snowfall totals have increased
slightly, with an additional 4 to 6 inches expected in the
Kuskokwim Delta, with 2 to 4 inches yet to fall across northern
portions of Bristol Bay.

Further south, along the Alaska Peninsula, a more wintry mixed
precipitation event is ongoing. A weak nose of warm air aloft has
moved in just ahead of the front above surface temperatures
stubbornly hovering just below freezing in King Salmon, resulting
in a thin band of freezing rain. Minimal ice/sleet accumulations
are expected due to the short duration of the event for the areas
impacted.

Tonight, the front is expected to become fully occluded as the
parent Bering low approaches from the west. Cooling temperatures
aloft will put an end to any mixed precip, with more showery snow
drifting across the entire region on Friday as the low weakens
and drifts southeast towards the Alaska Peninsula. Still, areas of
freezing drizzle/mist will be possible in some areas with the mid
levels (including the dendritic growth zone, an important layer
for snow formation) drying out, while low levels remain
saturated. Snow and any freezing drizzle will end from north to
south as drier air filters in and as the low crosses into the
Gulf. Expect rapidly falling temperatures as the snow and cloud
cover clear out by Friday evening and strong northerly flow
develops, ushering in a frigid, Arctic air mass.

A strong ridge develops over the Bering as low pressure moves off
into the Gulf of Alaska, driving strong northerly winds through
the Lower Kuskokwim Valley and portions of Bristol Bay east of
Dillingham. Cold wind chills below 40 degrees below zero are
expected to result, beginning Saturday night. These wind chills
linger in the coldest locations through the day, and are expected
to return over a wider area on successive nights into early next
week.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: Today
through Sunday)...

A large low pressure system sits in the central Bering this
evening, with its attendant front spiraling to the east and south
across Southwest Alaska and the Alaska Peninsula. Rain continues across
the central Aleutians tonight, with snow showers further north in
the Pribilofs. A mix of rain and snow will progress across the
western Aleutians on the back side of the low into Friday. Winds
around the low will peak later this evening into tonight, with
a swath of storm force winds moving through the central Aleutians
at the same time. Cold air wrapping into the low will help
destabilize the marine layer on the southwest side of the low,
with the potential for hurricane force gusts to materialize in
Adak and Atka before midnight. The low will weaken to gale force
as it moves into the eastern Bering and dives across the Alaska
Peninsula on Friday, with rain showers becoming snow showers
across the entire Aleutian chain as much colder air filters in.

Widespread gales also develop Friday night over the far
western Aleutians/Bering along the western periphery of a strong
ridge setting up over the central Bering. Strong cold air
advection on the eastern side of the ridge strengthens gap winds
through the eastern Aleutians and Alaska Peninsula early Saturday
morning. Locations south of the Alaska Peninsula from Sitkinak to
Castle Cape are expected to see these winds reach hurricane force
by Saturday night. Heavy freezing spray will also become
increasingly widespread over the northern and eastern Bering by
Friday night, with particularly high ice accretion rates possible
south of the Alaska Peninsula. Winds begin to diminish on Sunday
as the axis of strongest cold advection shifts eastward and
weakens.

&&

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

Bering...Enhanced northerly winds across the Eastern Bering
should be coming to an end on Sunday, and with it the threat for
heavy freezing spray, as a tightened pressure gradient shifts
inland of the coast and into the Gulf. Meanwhile, a highly
amplified ridge will be lifting northward over the Bering along
with strong southerly flow over the western Bering. This should
promote small craft conditions over the eastern Bering while
widespread gales develop over the western Bering Sunday into
Monday, including the Western and Central Aleutians. A very cold
air mass will remain in place over interior Alaska and the
advection of cold air and associated outflow winds is forecast to
persist across the Kuskokwim Delta and Bristol Bay coastal areas.
By Tuesday high pressure over the Bering will lift further north
allowing a trough over interior Alaska to retrograde and amplify
westward into the southern Bering. Guidance suggests a pocket of
intense cold air advection may accompany this trough,
overspreading Bristol Bay and Alaska Peninsula by midday Tuesday.
Intense northeasterly winds will become an increasing concern
along with a returning threat for heavy freezing spray for the
eastern Bering.

Gulf of Alaska...The main features over the Gulf of Alaska during
the 3 to 5 day period will concern a large trough of low pressure
centered over the southern Gulf and an arctic air mass inland of
the Gulf Coast. Weather put into effect by these features will
include snow showers scattered across the Gulf as well as the
continued presence of strong outflow winds spilling into the Gulf.
Gale to storm force northerly winds will likely exist across all
of the gaps, bays and passes on Sunday with a high threat for
freezing spray also existing. Very rough seas to potentially high
seas in some locations of 15 to 25 feet are forecast for much of
the Gulf Sunday into early Monday. Low pressure will begin
retreating southeastward and into the North Pacific by early next
week helping to slow down previously strong cold air advection
into the Gulf. This should allow for winds and seas to finally
begin diminishing Monday into Tuesday.

&&

.LONG TERM FORECAST (Days 4 through 8: Monday through
Thursday)...

A ridge of high pressure is forecast over the eastern Bering early
next week. This ridge will separate northerly winds and a cold
arctic air mass to the east of the ridge from southerly flow over
the Aleutians and strong warm air advection into the northern
Bering to the west of the ridge. Conditions across Southwest and
Southcentral are expected to remain cold and dry with little in
the way of any precipitation. Through the latter half of the week
high pressure over the Bering is forecast to slowly lift further
north, eventually breaking down as a shortwave trough moves from
interior Alaska into the Alaska Peninsula. Cold temperatures
previously kept inland by the ridge are forecast to sweep westward
across the Bering with previously warm temperatures over the
Aleutians to advect northward with the high. As a result, gusty
northeasterly winds are likely to occur with the cold air
advection along with snow showers and a threat of heavy freezing
spray.

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 155.
MARINE...Storm Warning 130 131 138 175-178 185.
		 Gale Warning 119 150 155 160 170-174 179-181.
		 Heavy Freezing Spray Warning 121 129 130 131 136-139 150
									  160 180 181 185.
FIRE WEATHER...NONE.

&&

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