Small Craft Advisory 75, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
433 PM AKST Tue Jan 25 2022

.ANALYSIS AND UPPER LEVELS...

A shortwave trough and associated weak surface low continue to
push east across Prince William Sound and the Copper River Basin
this afternoon. As the upper-level wave departs, light snow is
coming to an end across the Anchorage Bowl and Mat-Su Valleys.
The snow this morning brought a general 2 to 4 inches to the
Anchorage area with a few localized higher amounts across the
Upper Hillside. Elsewhere, snow continues for Valdez with rain
for Cordova, where a persistent southerly flow is keeping a deep
enough column of warm air near the surface to allow for liquid
precipitation. Temperatures across Southcentral are generally in
the lower 30s. However, temperatures across the northern Susitna
Valley and Copper River Basin are beginning to fall into the mid
20s, a sign that colder air is advecting south in the wake of the
aforementioned trough.

Farther west, a weak upper-level wave and surface trough are
producing a few snow showers across the Kuskokwim Delta while a
ridge builds across the eastern Bering. With a cold dome of high
pressure in place over the eastern Bering, temperatures across the
Aleutian Chain have been quite chilly, with locations from Cold
Bay to Adak and Saint Paul remaining in the teens this afternoon.
For the western Bering, temperatures are climbing into the mid
30s, as southeasterly winds increase ahead of a front associated
with a potent surface low south of Attu.

&&

.MODEL DISCUSSION...

Models are coming into better agreement with what looks to be the
development of a highly amplified blocking pattern beginning
midweek. Even with guidance all showing the establishment of an
omega block from eastern Russia to the Gulf of Alaska, there is
still a fair bit of uncertainty as to how various upper-level
waves on the downstream side of the block interact. The biggest
challenge is how a developing upper-level polar low over interior
Southwest Alaska interacts with a second low lifting north into
the Gulf as the former drops south over the AKPen. Guidance is
keying in on an inverted surface trough over the northern Gulf
with plenty of mid-level moisture streaming into the Southcentral
coast as colder air advects over the region from interior Alaska.
Thus, guidance is in good agreement that there will be widespread
precipitation, mainly in the form of snow, across coastal
Southcentral and over Cook Inlet. The placement and track of
these features, however, will impact where the bands of heaviest
precipitation set up. Yesterday, the GFS was the farthest west
solution, placing the best dynamics over Cook Inlet, while other
guidance was farther east. Today, the 12z runs have trended more
to the west. Thus, the NAM/GEM/EC have increased snow potential
across Cook Inlet while the GFS now has the best dynamics for
heavier snow amounts west of the population centers. While
precipitation looks likely along the coast, any shift in the
placement of the upper-level feature will have a significant
impact on how much snow falls from Homer to Talkeetna Thursday
into Friday.

&&

.AVIATION...
PANC...Largely expect VFR conditions to persist. However, MVFR
ceilings could linger through this evening, with low clouds still
in the vicinity. As is often the case after a precipitation event
and with partial clearing, low clouds and fog could form in
northern Cook Inlet tonight as the atmosphere stabilizes. However,
ridging aloft looks fairly weak, so if low clouds/vis do develop
expect it will be localized. Still, will have to keep a watch out
for the potential for IFR/LIFR conditions in vicinity of the
terminal overnight into Wednesday morning. Northerly flow will
then increase during the day, so anything that forms would
quickly be eroded.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Tonight through Friday)...

Skies are clearing from west to east this afternoon as a more
offshore synoptic flow pattern develops. Temperatures across
Southcentral continue to gradually decrease as cold upper level
air moves in from the west and southwest and mixes down to the
lower levels. A broad upper level ridge over the Gulf is located
in tandem with lingering low level moisture from the previous
system. Thus, areas of patchy fog are possible early Wednesday
morning for Anchorage and Cook Inlet stretching northeast through
Palmer, as well as the Copper River Basin. The calm before the
next storm won't last very long, because come Wednesday and
Wednesday night, a North Pacific surface low is going to lift
northward into the Gulf, spreading precipitation to Kodiak Island
and Southcentral. Low pressure across the Gulf and higher pressure
inland will set up a tighter pressure gradient and induce northerly
surface flow along the coast. At the same time, southerly upper
level flow over the Gulf will work against (and possibly overtake)
the northerly surface flow. If this happens, then inland
Southcentral could see greater snow accumulation beginning
Wednesday night or Thursday. The exact location, timing and amount
of snow will depend on how the system continues to progress.
Blowing snow is possible for Thompson Pass from Wednesday night
through Friday night with this system.

The eastern coast of Kodiak Island will start to see precipitation
early Wednesday morning. Precipitation is expected to start off
as snow, but a transition to rain or rain/snow may occur sometime
late Wednesday. The exact timing of this transition is still
uncertain. Regardless, accumulating snow is expected in Kodiak
City.

&&

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

As high pressure begins to take hold over the area, clear skies
and colder conditions will occur. Offshore winds will increase on
Thursday due to the pressure gradient tightening between a North
Pacific low approaching the southern Gulf and an Arctic high
pressure centered near the Bering strait. These winds, combined
with cold temperatures, could coincide to produce bitterly cold
wind chills in the -20F to -30F range, especially across the
Kuskokwim Delta on Thursday morning. As cold air advection takes
place and thermal gradients tighten, gap winds near the Alaska
Peninsula and Bristol Bay will increase Friday evening.

&&

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

A broad upper-level trough is draped over the western Bering. A
strong North Pacific low continues to deepen as it stays on track
toward the western Aleutians. It's frontal axis will drive winds
to storm force into areas west of Adak, with gusts near Hurricane
force possible Wednesday morning. A band of rain and snow will
move north with the front, with temperatures warming above freezing.
The parent low pressure will slowly weaken as it moves into the
western Bering on Thursday. However, gusty gale to storm force
winds and periods of rain will continue over much of the western
Bering/Aleutians near the low through Thursday afternoon. A polar
low dropping south over Southwest Alaska, will bring stronger
winds into the eastern Bering with heavy freezing spray returning
near the ice edge and strong gap winds through the Alaska Peninsula
Friday.

&&

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

Gulf of Alaska:

A North Pacific low lifting northward in the Gulf will bring
northwesterly gale force winds from Cook Inlet through Shelikof
Strait from Friday to Saturday afternoon. As the low weakens and
exits out of the Gulf Saturday evening, expect winds less than
35 kts and seas less than 20 ft. Forecast confidence is moderate
to high.

Bering Sea/Aleutians:

A low near the Kamchatka Peninsula will bring localized gale
force winds and elevated seas above 20 ft in the northwestern
Bering Sea on Friday morning.

A strong North Pacific low with an associated front will approach
the southern Bering Sea early Friday afternoon. Widespread gales
with a potential of storm force gusts and elevated seas above 20
ft will extend from the Western Aleutians to the northern Bering
Sea through Saturday. Forecast confidence is moderate to high with
most of the active weather staying west of the Pribilof Islands
and Central Aleutians.

Late Saturday night into Sunday, another North Pacific low will
approach the southern Bering Sea. There is a potential for
another round of gales and elevated seas extending across the same
regions and possibly in the Central Aleutians for the rest of day
Sunday. However, there is model uncertainty with the overall track,
position, and strength associated with this system. Therefore,
forecast confidence is low.

&&

.LONG TERM FORECAST (Days 4 through 7: Saturday through Tuesday)...

Looking ahead in the extended forecast, the latest operational
model solutions seem decently clustered with the synoptic scale
features in the short term, but begin to diverge toward the end of
the period. Synoptically, an omega blocking pattern on Saturday
will be positioned over the western Alaska region with the
amplified ridge axis over the eastern Bering Sea. Downstream of
the ridge, a surface low interacting with a cold polar trough in
the Gulf will keep unsettled conditions for Southcentral with
moderate to heavy precipitation.

As the low exits out of the area, precipitation will taper off
from north to south by late Saturday. Benign conditions with
temperatures near to or below normal will prevail for the southern
mainland as the aforementioned ridge shifts over the state. From
Sunday onward, model spread has increased significantly with the
breakdown of the omega ridge as well as struggling with small to
mid scale embedded systems and interactions. Depending on
consistency of future models runs, unsettled weather may return
for the entire southern state as early as Monday as a storm
system with a good moisture fetch lifts northward into the Gulf.

Out west, a series of deep Pacific lows will move north across
the Western Aleutians, bringing periods of gusty winds, rain/snow
mix, and elevated seas at times into the northern Bering Sea.
Although there is uncertainty in placement, strength, and timing
with these systems, a wet pattern will be the main theme as we
head into the weekend. Some of the precipitation and wind may
extend across the Pribilof Islands, the Central/Eastern Aleutians,
and AKPEN for early next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 177 178.
         Gale Warning 130 132 138 150 155 175 176 185.
         Heavy Freezing Spray Warning 160 165 179 185.
FIRE WEATHER...NONE.

&&

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