Small Craft Advisory 50, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
544 AM AKST Tue Jan 18 2022

.ANALYSIS AND UPPER LEVELS...

A complicated upper level pattern exists across much of Alaska
this morning. A strong Arctic upper level low is present across
the North Slope of Alaska. Over southern Alaska, very weak upper
level ridging is working in tandem with the upper low on the North
Slope creating for mostly southerly to southwesterly flow for the
southern Mainland this morning. As a result, mostly just mid to
high level clouds were streaming across Southcentral with isolated
areas of low stratus or patchy fog this morning. Temperatures
across much of Southcentral are colder tonight than as of recent
with mostly single numbers and teens area-wide. There were also
some pockets of below zero temperatures namely Willow at -1 degree
earlier in the night but they have since warmed up to around 5
degrees.

Southwest Alaska also remains mostly quiet for the time
being due to mostly being caught in between the two upper level
systems lending to weak synoptic flow aloft. Therefore, just some
mid to high level clouds were present this morning, though, there
was some low stratus around as well namely over Sleetmute as obs
there were showing an overcast deck for a couple of hours earlier
in the night. Temperatures were mostly single numbers and teens.
An upper level shortwave moving northward along the Alaska
Peninsula is causing lift out ahead of it with moisture streaming
northward lending to some light snow/flurries around the Bristol
Bay coastline this morning including Dillingham.

Two upper level lows were present spinning near each other south
of the Aleutian Chain this morning sending pulses of energy
northward across the Bering. One of these moved through Saint Paul
Island earlier in the night with 1/2 mi visibility, moderate snow,
and breezy conditions as it passed through. This same wave was
bringing a mix of rain and snow along the Eastern Aleutian
Islands. Heavy freezing spray continues west of Saint Matthew
Island off the ice edge due to northeasterly flow. Lastly, ASCAT
Scatterometer data passed over the low south of Amchitka showing a
tightly wound up low with gale force winds around it especially
on its east side. ASCAT data also confirmed high end gales across
the northern Bering off the ice edge.

&&

.MODEL DISCUSSION...

Model guidance overall remains in good synoptic agreement through
Day 2. However, there continues to be differences especially in
regards to the timing and orientation of various different upper
level shortwaves moving along the Aleutian Chain and up the
Alaska Peninsula and into Southcentral. Therefore, this will have
an impact, while generally small, on the sensible weather details
going forward. Where there is better agreement is on the upper
level features just south of the Aleutian Chain, though, even here
things become fuzzy beyond Day 2 as when looking at the surface
fields, model guidance is all over the place on the next low
moving up from the North Pacific.

&&

.AVIATION...

PANC...VFR conditions are expected over the terminal through this
evening before ceilings begin to lower to around 5000'. Ceilings
are expected to continue lowering during the overnight hours to
MVFR with potentially light snow starting to overspread the area
at this time. Light snow is expected to intensify sometime early
Wednesday morning bringing visibility to MVFR as well. However,
this is low to moderate confidence for light snow. Light northerly
winds are expected for the TAF package.

&&

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

An upper level ridge sitting over the Gulf of Alaska and
Southcentral will shift eastward today, opening the flood gates
to a host of short-wave troughs moving up from the Pacific. As a
result, we'll be moving into a much more active weather pattern
as we head through the week and into the weekend. First off, a
short-wave trough along the Alaska Peninsula will approach
Southcentral today and combine with weak overrunning to produce
areas of light snow. Snow will begin on Kodiak Island this morning
and spread to Southcentral this afternoon through tonight. The
short-wave will weaken as it reaches Southcentral, so snowfall
intensity and resultant accumulation will be quite light. Low
level southeast flow will develop across the Kenai and Chugach
Mountains, but it will be relatively weak, so don't expect much
downslope flow. Still, it may be enough to limit snowfall from the
western Kenai to Anchorage and the Matanuska Valley. Most likely
there will be no more than a dusting for these areas.

Meanwhile, a larger more significant short-wave trough over the
North Pacific will cause a weak surface low to deepen and make
its way northward into the central Gulf of Alaska tonight. Model
guidance has been struggling with the track of this low because of
the plethora of upper level short-waves over the Pacific and
overall complexity of the pattern. There is decent agreement on
the track now, which takes it northward across the central Gulf
Wednesday and then toward Yakutat and the northeast Gulf Wednesday
night as additional short-waves moving in from the Bering kick
everything eastward. Such a low track means the bulk of impacts
from strong winds and heavy precipitation will remain over the
Gulf. Some component of low level offshore flow out ahead of the
low tends to keep precipitation light along the Gulf coast and
Prince William Sound. There will be some heavier upslope flow
(east winds) across the mountains of western Prince William Sound
well out ahead of the low tonight into Wednesday. Precipitation
will then quickly end from west to east as the low exits eastward
Wednesday afternoon through Wednesday night.

As we continue to Thursday, the long-wave trough will amplify over
the North Pacific, leading to development of a corresponding high
amplitude ridge downstream over the East Pacific and Southeast
Alaska. This will lead to a strengthening southerly jet and
transport of warm and moist air from the Pacific toward southern
Alaska. This pattern tends to lead to chinook winds, warm
temperatures, and mixed precipitation types. While there is
currently uncertainty in the details, it looks favorable for a
warm front to make its way northward to Kodiak Island and the
Gulf Thursday and then continue to Southcentral Thursday night.
Precipitation will start as snow, but would expect a fairly quick
change to rain along the Gulf coast, with heavy snow likely in the
mountains. Moderate to heavy snow will also be possible over the
Susitna Valley thanks to deep southerly flow, while chinook winds
crossing the Chugach and Kenai Mountains should keep precipitation
lighter. Stay tuned, as this will likely be the first of multiple
impactful storms moving up from the Pacific.

&&

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

A weakening front lingers across the Bering through Tuesday. Light
snow will continue along the Kuskokwim Delta and Bristol Bay
coasts through Tuesday afternoon. Further inland, an upper level
trough moving south across the state will bring light snow to the Lower
Kuskokwim and interior Bristol Bay valleys through Wednesday.
Snow accumulations will remain minor, with about 1 to 4 inches
expected. Temperatures drop Wednesday as the upper level trough
continues to descend across the state. However, temperatures look
to increase again on Thursday as a North Pacific low approaches
the Southwest.

&&

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

A compact low will continue to move towards the Western
Aleutians. Behind the low, a warm front will track across the
Aleutians on Tuesday, bringing a quick shot of light to moderate
rain. The front stalls just north of the Chain.

By early Wednesday morning, cold air surging southward across the
Bering will produce snow showers as it meets the warmer waters of
the Bering Sea. Periods of heavy snow, combined with robust
northeasterly winds, are expected to reduce visibilities at times
over the Pribilofs. Most of the Bering and Aleutians transition to
a showery regime by Wednesday night as the cold air continues
tracking southward. However, this will be short-lived over the
Eastern Aleutians as a North Pacific low approaches Thursday.

&&

.MARINE (Days 3 through 5 - Thursday through Saturday)...

Gulf of Alaska: A front will advance northeast across the Gulf on
Thursday, and then approach the North Gulf Coast by Thursday
evening. Expect a narrow zone of easterly gale force winds to
develop just offshore with a barrier jet, or between Prince
William Sound and Middleton Island and south of the Kenai
Peninsula from late Thursday into Friday morning. Northeast winds
to gales will also be possible near the southern Cook Inlet and
near the Barren Islands during the same timeframe. The winds will
diminish on Friday as the front falls apart, with weaker southerly
winds and seas of 15 to 20 ft spreading over the rest of the
Gulf. Another weak front or low will move into the western Gulf on
Saturday, though currently expect winds with this next system to
be under gale force with seas less than 20 ft across the entire
Gulf Saturday. However, forecast confidence is notably lower for
this second system at this time.

Bering Sea: Northeasterly winds will drop below gale force early
in the day on Thursday, with seas less than 20 ft and winds less
than 35 kt overall expected to prevail through Friday. A North
Pacific low will approach the central Aleutians from the south on
Saturday, potentially sending easterly gale force winds into much
of the southern Bering. Confidence through Friday is high, but
becomes much lower for the North Pacific low and any winds to gale
force on Saturday.

&&

.LONG TERM FORECAST (Days 4 through 7: Friday through Monday)...

Struggles with more near-term pattern complexity are keeping much of
the long range out of focus for the moment, although the largest
and most important synoptic driver still looks on track to keep a
warm/wet theme in play for much of the southern mainland. Models
continue to shift wildly around for what will be an ongoing
interaction between two systems at the start of the period on
Friday over the Bering. A polar low drifting out into the northern
Bering from the Seward Peninsula will likely become entangled
with another low pivoting north past the eastern Aleutians and
southern Alaska Peninsula, with the two centers eventually at
least partially merging and moving north and one system towards
the Bering Strait through Saturday. Consistency across models and
between runs for this evolution both remain poor, resulting in
diverging solutions for much of the smaller scale details for
nearly the entire outlook area. One feature that remains better-
resolved is the strong longwave ridge that will develop earlier in
the week over the eastern Gulf and much of the eastern mainland,
with the consensus being that this ridge will remain in place into
early next week. A series of shortwaves will attempt to work
around this ridge from the North Pacific through Monday as they
traverse deep, warm southerly flow, with each system helping to
draw enough warm air north to change snow over to mainly rain
along the immediate Gulf coast. Rounds of mixed precipitation are
also likely for the eastern Aleutians, Alaska Peninsula, and
Southwest as each system moves north around the ridge. However,
precipitation will be more light and showery over parts of
interior Southcentral due to the southerly cross-barrier flow and
downslope drying. Above normal temperatures over Southwest and
Southcentral continue to be anticipated throughout the entire
extended period.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Special Weather Statement 195.
MARINE...Gale Warning 411 412 413 119 177 178 185 Heavy Freezing
Spray Warning 412 414 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MV
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KC
MARINE/LONG TERM...AS