Heavy Freezing Spray Warning 199, NWS Anchorage, AK

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

.ANALYSIS AND UPPER LEVELS...

The main synoptic feature is a negatively tilted shortwave trough
with a strong surface low lifting north of the Western Aleutians.
Based on the CIMSS MIMIC imagery, a deep fetch of southerly
moisture of 0.5-1.00 inch of precipitable water is extending
northward to Nunivak Island. A warm front extending east from this
low is accessing this moisture source, providing precipitation
across the Aleutians, Pribilofs, Alaska Peninsula, and southern
portions of Bristol Bay. Northwesterly gale force winds can be
seen on the ASCAT passes in response to a thermal gradient as
polar air wraps around the low. Meanwhile, a ridge of high
pressure extends from eastern Siberia southeastward along the
Southwest AK coast and into the northwestern Gulf.

Over Southcentral, a weak upper level wave moving downstream of
the ridge is beginning to develop a surface low south of the Kenai
Peninsula, with light precipitation remaining primarily offshore.
This wave will have minimal impact and expected to exit into the
southern Gulf later today. Otherwise, mid to high level clouds are
streaming ahead of the frontal system for areas of south of
Anchorage. Latest observations are reporting benign conditions
with overnight temperatures ranging anywhere between the teens to
the low 30s. Lastly, a trough with an embedded polar low centered
over the Beaufort Sea extends across the North Slope.

&&

.MODEL DISCUSSION...

The latest operational model runs are in good agreement with the
placement and intensity of synoptic scale features at both the
surface and in the upper levels. As the warm front advances east
today, some uncertainty remains for a wintry mix of precipitation
for parts of the northern Alaska Peninsula and Bristol Bay. This
includes the potential for a brief period of freezing rain, sleet,
and snow between Pilot Point and King Salmon. For Southcentral,
model solutions are in a consensus with a triple point low forming
along the frontal boundary near the southern coast of the Kenai
Peninsula on Friday morning. The differences, however, is the
exact precipitation timing and storm total amounts, but confidence
is high that this will be a snow event. Models are also keying on
an Arctic outbreak of cold air across the Alaska mainland,
leading to gusty winds for the bays and passes, heavy freezing
spray, as well as cold wind chills as we head into the weekend.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist into this
evening. Snow will move into the area tonight, bringing MVFR to
IFR reductions to ceiling and visibility at the terminal.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Thursday through Saturday)...

A warm front moving into the Kodiak area today will bring some
rainfall there by this afternoon. This front will develop a
triple point low south of the Kenai Peninsula this afternoon
which will keep the bulk of the warmer air from moving inland or
even to the north Gulf coast. With this low moving near Middleton
Island overnight, it will allow colder air to move into
Southcentral as a cold upper level trough moves into the area from
the west. The front aloft will override the colder surface air
over Southcentral tonight bringing in some snowfall. This snowfall
is expected to be generally light.

The pattern get a little more interesting Friday as the main
upper level low digs into Southcentral bringing much colder air to
the region. This will keep the snowfall going for most of the day
Friday. Most of this snowfall will also be light and general
amounts across Southcentral should be 2 to 4 inches for the
entire event. There is some disagreement on the timing of the
upper level low moving through. That matters since the snowfall
will generally end for an areas after the low moves past.

For Saturday, the pattern changes to a cold and windy one. Expect
skies to clear out, but gusty northerly winds will develop in the
normal places as the 850 mb temperatures plunge 15 to 20 degrees
from Friday to Saturday. This will also cause large areas of Heavy
freezing spray from Cook Inlet southward through Shelikof Strait
and the Alaska Peninsula.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 2: Today
through Friday night)...

A strong low moving across the Bering will send a front onshore
with light to moderate snowfall moving inland from southwest to
northeast through this evening. Across the Y-K Delta, easterly
winds will gust up to 40 mph will combine with falling snow and
produce significant areas of blowing snow and cause significantly
reduced visibility, especially along the immediate coastline. A
Winter Weather Advisory remains in effect for the Y-K Delta beginning
by noon today for the expected snow and blowing snow with the
front. Snowfall totals with the main round of snow continue to be
anticipated in the 4 to 6 inch range for the Y-K Delta, with lower
amounts of 2 to 4 inches expected across northern portions of
Bristol Bay.

Looking to the south closer to the Alaska Peninsula, the details
remain elusive for a more wintry mixed precipitation event that
will unfold in several short stages today. Current thinking is
that the leading edge of frontal precipitation will still start
out as snow over the northernmost parts of the Alaska Peninsula
and southern Bristol Bay later this morning. However, a weak nose
of warm air aloft is expected to move north just ahead of the
front above surface temperatures likely still hovering at or just
below freezing, setting up the potential for a myriad of
precipitation types ranging from snow, sleet, freezing rain and
rain. This wintry mix is most likely to occur with a transition
zone that will work north between near Pilot Point and King Salmon
over the course of the day. Fortunately, the layer of cold air at
the surface will remain shallow, and temperatures should easily
warm above the freezing mark and change the mix over to rain by
late this afternoon. Very little ice/sleet accumulations are
expected due to the short duration of the event for the areas
impacted.

By Friday morning, the front will become fully occluded as the
parent Bering low approaches from the west. Cooling temperatures
aloft will mostly 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 as strong northerly flow
develops and quickly ushers a frigid, Arctic air mass south,
plunging the entire area back into a deep freeze (below 0F) to
kick off the new year.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 2:
Today through Friday night)...

A large low pressure system has made its way from the North
Pacific into the Bering this morning, with its attendant front
spiraling to the east and south across the Pribilofs and out into
the Alaska Peninsula. This will send mainly rain across the
eastern Aleutians and Pribilof Islands through this afternoon,
with a mix of rain and snow progressing across the western
aleutians on the back side of the low. Winds around the low and
it's associated front will peak later this afternoon and evening,
with storm force winds developing over the western Aleutians and
southwest Bering, as well as near St Matthew Island, through late
tonight. Cold air wrapping into the low from the low just above
the surface will help destabilized the marine layer near the
corridor of strongest winds, supporting Hurricane force gusts
roughly between Adak and Atka with gusts to 70 mph over land. 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 with much
colder air filtering in with strong northerly flow developing.
This will set the stage for strengthening gap winds through the
eastern Aleutians and Alaska Peninsula, with gusts increasing to
storm force by late Friday night. At the same time, another front
will bring southerly winds increasing to storm force over the far
western Aleutians/Bering. Heavy freezing spray will also become
increasingly widespread over the northern and eastern Bering
through Friday night.

&&

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

Bering...A low that tracks across the AKPEN Friday will bring
northerly winds across the Central and Eastern Bering Saturday.
Winds will peak at high- end gales on the backside of the low. As
the low exits into the Gulf, storm- force gap winds and heavy
freezing spray are possible south of the AKPEN Saturday through
early Sunday. In the Western Bering, a strong front will bring
southerly gales and areas of storm-force winds Saturday through
Sunday. Seas will be mostly 8-15 ft except 15-25 ft where gales
are expected. A ridge of high pressure will build over the Bering
Sunday night through Monday.

Gulf of Alaska...Gusty outflow winds intensify to gales early
Saturday and persist through Sunday, with storm-force winds likely
through Kamishak Gap. Heavy freezing spray is possible with these
strong and cold outflow winds that will continue through Monday.
Seas will increase to 15-25 ft Saturday though Monday.

&&

.LONG TERM FORECAST (Days 4 through 8: Sunday through Wednesday)...

An upper-level ridge in the Bering moves eastward into Alaska
through Tuesday, transitioning the weather towards cold and mostly
dry conditions. As a surface low, developed by an upper-level
trough located over Alaska, moves out of the Gulf through
Saturday, snow will slowly taper off. Cold, northerly gap winds
out of the AKPEN and northern Gulf Coast will continue through
Monday due to high pressure over the interior and ridging in the
Eastern Bering. Cold air advection and clearer skies at night will
allow temperatures to drop below negative during the weekend.

Behind the upper-level ridge, a strong warm front brings rain and
southerly winds as it tracks across the Aleutians through
Tuesday. Some uncertainty remains with how quickly the front will
move east. However, temperatures warm by Tuesday/Wednesday across
Southwest as the front approaches.

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory: 155.
MARINE...Storm Warning: 155 165 170 172-174 178 185 411 413.
Gale Warning: 175-177 179 180 181 412 414.
Heavy Freezing Spray Warning: 180 181 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MF
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AS
MARINE/LONG TERM...NS