Winter Weather Advisory 75, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
551 AM AKST Sat Dec 25 2021

.ANALYSIS AND UPPER LEVELS...

A broad high pressure system anchored over the deep North Pacific
is maintaining a ridge that runs northward through the Southwest
Coast. A 135 kt southeasterly jet streak is located over the
eastern Bering while a 105 kt northerly jet streak is positioned
over Southcentral. A strong front is extending north across the
Bering Sea. Radar is already showing returns over the Kuskokwim
Delta and the Bristol Bay area falling as snow (for now) as the
aforementioned front moves across the Bering toward the Southwest.
See the Winter Weather Advisories issued for this area for
additional details. Warm tropical moisture trailing the front,
combined with the broad area of high pressure over the
Aleutians/Bering, is giving way to slightly warmer temperatures
for much of the Aleutian Chain. Over Southcentral, mostly clear to
partly cloudy skies persist except for isolated spots where low
level clouds/stratus linger this morning.

&&

.MODEL DISCUSSION...

Models continue to remain in agreement with synoptic features
through at least Thu morning. Multiple shortwaves associated with
Northwest Pacific lows in the coming week will enhance the
potential for steady rain and stronger winds/gusts for the
Bering/Aleutians and also mostly the Southwest Coast as the front
approaches land. The main forecast challenges precipitation type
and timing. Essentially, multiple advisory/warnings are expected
in the forecast through the Christmas holiday weekend especially
with the potential of freezing rain making its way across the
forecast region this weekend into the new week. Overall, forecast
confidence is good.

&&

.AVIATION...

PANC...VFR conditions will persist through this morning. In the
afternoon through the evening hours, ceilings may drop to MVFR as
lower- level clouds begin to move into the Anchorage Bowl. IFR
ceilings are possible tomorrow as light snow could transition to a
chance of freezing rain.

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

The big story remains the very strong and warm subtropical ridge
sitting over the North Central Pacific. Very warm air is riding up
and over top of the ridge, across the Bering Sea and western
Alaska and toward Southcentral. This will lead to ideal conditions
for freezing rain as we head through the weekend, with the very
warm air riding over top of the cold air in place at the surface.

First off for today, a series of weak upper level short-waves
will continue to transit overhead of Southcentral. To this point,
precipitation has struggled to reach the ground, due to dry air
in the low levels. However, the atmosphere has begun to moisten
up and expect to see areas of light snow today, mainly inland and
along the south to west side of mountain ranges.

A much stronger short-wave will flatten out the ridge and bring
more significant precipitation to the region on Sunday. There is
some uncertainty in the track of this short-wave and southern
extent of the precipitation shield. Compared with the previous
forecast, it appears the short-wave is taking a further north
track. Therefore, have lowered precipitation totals for the Kenai
Peninsula, Anchorage, and southern Mat-Su. The reason this is so
critical is because these southern areas are where the warm air
will intrude and result in freezing rain on Sunday. Despite the
lowering of precipitation amounts, have maintained Winter Storm
Watches in place for portions of the eastern Kenai Peninsula,
Anchorage, and the Matanuska Valley for potential for significant
ice accumulation from freezing rain. Have also added a new Winter
Storm Watch for the Susitna Valley from Talkeetna southward, also
for freezing rain. The biggest uncertainty remains the exact track
of the short-wave and associated freezing rain and ice
accumulation amounts. At this point, the highest ice accumulation
seems most likely across the southern Susitna Valley and much of
the Matanuska Valley. Of course, any ice accumulation at all can
produce difficult travel conditions. Stay tuned as we fine tune
this forecast.

The strong short-wave will exit to the east-southeast Sunday night
through Monday morning, with precipitation largely ending behind
it. There will be some weak cold air advection with passage of the
wave. Light precipitation may linger along inland mountains and
this should be in the form of snow. Despite the cold air advection
aloft, surface temperatures across much of the coastal Southcentral
will remain quite balmy to start the new week.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2: Today
through Monday)...

An area of low pressure lifting across the Kamchatka Peninsula is
pushing a warm front into Coastal Southwest this morning. Coupled
with a deep moisture fetch and a shortwave moving downstream of a
ridge over the eastern Bering Sea, this front is bringing a
wintry mix precipitation event across the coastal areas and
interior valleys. For the Y-K Delta, southerly winds is drawing
warmer air along the immediate coast with Toksook Bay, Chevak,
and Kipnuk reporting light steady rain. Further inland, Bethel is
reporting light snow with temperatures climbing above freezing
that will help transition to rain today. Current forecast package
remains on track in the northern areas of Bristol Bay and the
Lower Kuskokwim Valley. Snow is changing to mixing with or to
freezing rain as the atmospheric column of air continues to warm
above the surface. Dillingham, King Salmon, and Iliamna have been
reporting freezing rain for the past few hours. This will still
have the potential for any ice accumulation up to two tenths of
inch today. Please see the Winter Weather Advisories for the
wintry mix of precipitation on our webpage for more details.

By this afternoon, any freezing rain and snow will finally change
over to all rain as southerly winds continue to draw warmer air
down to the surface. As the front exits out of the area, lingering
moisture will keep continued chances of rain for tonight. By
early Sunday morning, another wave riding over the ridge will
bring another round of rain and increasing southerly winds over
the northern Kuskokwim Delta and Lower Kuskokwim Valley. The
aforementioned ridge axis will slide east into the Alaska
mainland, allowing rain to spread south into Bristol Bay from mid-
morning Sunday to Monday. Daytime temperatures will range anywhere
in the low to the mid 40s with overnight lows in the low to mid
30s.

&&

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

An upper level wave will track through the western Bering today
with rain and a swath gale force winds extending across Aleutians
and extending northward into the Pribilofs. Rain is expected to
taper off briefly as the system tracks near the Bering Strait with
winds decreasing to small crafts by Sunday afternoon. Meanwhile,
another wave and well developed surface low (approximately 965 mb)
will track from the North Pacific and into the western Bering
beginning late Sunday. This system will be stronger than the
current one, bringing a second round of rain, widespread gales,
and elevated seas across the the forecast area through Monday.
Additionally, in the western Aleutians near Adak to Attu,
localized storm force winds with hurricane force gusts are also
increasingly likely from late Sunday night through Monday morning
as colder air works in along the backside of the low. A warm
airmass associated with these systems will help maintain
temperatures well above freezing through early next week.

&&

.MARINE (Days 3 through 5 - Monday through Wednesday)...

Bering...Multiple Northwest Pacific lows will bring gale force
winds and large seas between 15 to 22 ft across much of the
Bering/Aleutians through Wed night. A developing low passing near
the eastern Bering could potentially reinforce the wind gradient
over the south of Alaska Peninsula and eastern Aleutians between
Tue night and Wed morning, causing winds to elevate to near
gales Wed night. Very strong gap winds could be felt along the
shores and bays of eastern Aleutians and parts of the Alaska
Peninsula from the latter system. Meanwhile, another deep North
Pacific low will steer gales and elevates seas across the
central/western Aleutians between Wed and Wed night.

Gulf of Alaska...A developing low from the eastern Bering could
potentially reinforce the wind gradient over the Gulf where an
existing low remain parked there for a day or two. Winds will
increase to gales and potentially storm-force with seas up to 20
ft or more. It's still too early to speculate winds higher than
gales for the Gulf next Wed, so will highlight gales for Wed
forecast and monitor this situation closely for future updates.

&&

.LONG TERM FORECAST (Days 4 through 8/Tuesday through
Friday)...

A ridge of high pressure from the North Pacific will continue to
secure its post across the forecast area through the long term,
leading upper lows and frontal systems either tracking north of
the State or forcing their path through the Gulf into the middle
of next week. With the onset of the lows combined with multiple
shortwaves trailing behind the upper troughs, increased snow and
rain/snow mixed are expected for the region. Although the
precipitation-type including the arrival time of the fronts could
be challenging, one thing for sure is that warm tropical moisture
combined with warm southwesterlies behind the fronts will at least
encourage light rain to pass through much of the Aleutians and the
Bering, including parts of the Alaska Peninsula.

Southcentral can expect a more variable weather pattern with cold
conditions at times with brief warm-ups along with occasional
snow with any system that passes through. At first glance, it
appears the weather pattern we have been in with freezing rain for
the Kenai Peninsula and snow for Anchorage on north and east
looks to potentially continue through this timeframe. However,
the exact details will be fine tuned over time and with
consecutive model runs. There is the potential for freezing rain
in Anchorage and the Matanuska Valley in the earlier day (or days)
of the long term.

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Watch: 101, 111, 125, 145.
		 Winter Weather Advisory: 152, 161.
MARINE...Gale Warning: 138, 165, 170, 172-177, 179-181, 185, 411-414.
FIRE WEATHER...NONE.

&&

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