Small Craft Advisory 53, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
545 AM AKST Wed Jan 19 2022

.ANALYSIS AND UPPER LEVELS...

A complex upper-level pattern dominates the region as a broad
longwave trough extends southward from northern Alaska across the
Bering and Aleutians into the North Pacific. Downstream, an
upper-level ridge stretches up from the Northeast Pacific across
Southeast Alaska and into Yukon Territory. There are multiple
shortwaves embedded within the longwave trough. Two of these
shortwaves are situated over the southern mainland, with the
first strung out from south of Kodiak Island into the Interior.
Though waning in influence, this disturbance helped initiate
widespread virga and scattered light snow across the Cook Inlet
region, as well as a band of heavier snow along the western slopes
of the Susitna Valley. The second, more potent shortwave sits
over Southwest Alaska producing a widespread region of light snow
which is pushing east into Southcentral. Heavier precipitation,
primarily snow, is present along the Gulf coast as a 985 mb North
Pacific low and its accompanying front push northward.

A wide expanse of cloud cover is keeping temperatures across
Southern Alaska relatively stable, ranging from the single digits
in the Copper River Basin and the Kuskokwim Delta, to the 10s and
20s elsewhere, with a few coastal locations reaching into the
30s.

Farther west, a vertically stacked low south of Shemya and a
front stretched out across the western and central Aleutians
continues to bring mixed precipitation along with easterly gales
to the region. Northeasterly gales also persist over the northern
Bering with heavy freezing spray along the ice edge around Saint
Matthew Island.

&&

.MODEL DISCUSSION...

Model guidance remains in good synoptic agreement through the
short term. Models have come into increasing alignment with
respect to the shortwave tracking eastward across Southcentral
this morning through tonight, leading to increased confidence in
associated snowfall totals in the Cook Inlet region and the Copper
River Basin. Some uncertainty remains in the lower levels with
regards to the track of the low into the northern Gulf from the
North Pacific through this evening, with recent model guidance
shifting westward. Areas of focus most reliant on the track of the
low include precipitation type (rain or snow) for areas along the
coast east of the low center, as well as the strength of winds
through gaps on the west side of the low.

Beyond Thursday morning, model agreement breaks down amongst
smaller scale features. There is little confidence with the timing
or strength of any specific shortwave or surface low tracking up
from the North Pacific into the southern mainland. Despite this,
agreement on the bigger picture lends confidence to the idea that
these features will generally be weakening as they track north on
strong southerly flow, bringing wetter and warmer weather to much
of Southern Alaska beginning Friday.

&&

.AVIATION...

PANC...VFR conditions with ceilings below 5000' deteriorate to
MVFR ceilings and visibility through the morning hours as light
snow develops over the terminal. The snow is expected to
intensify by mid-morning with IFR ceilings and visibility likely
at times. The potential for IFR conditions persists through the
afternoon until snow begins to taper off, and conditions steadily
improve to VFR Wednesday evening. With the initial cloud deck
clearing out, low stratus or fog will have the potential to
develop over Cook Inlet and affect the terminal Wednesday night
into Thursday morning.

&&

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

There have been some shifts in the track and strength of short-waves
crossing the region today, which will lead to more widespread snow
across Southcentral today. One short-wave in Bristol Bay overnight
will rotate northeastward and head up Cook Inlet this morning,
then take a more easterly track across the remainder of Southcentral
this afternoon through tonight. Meanwhile, a separate short-wave
will lift into the central Gulf today, with moisture streaming
northward to Southcentral (out ahead of the trough moving in from
the west). A corresponding moderate strength surface low will lift
northward across the central Gulf up to the northern Gulf this
afternoon, with a leading occluded front pushing northward toward
the Gulf coast. This will lead to a barrier jet along the north
Gulf coast and heavier precipitation across the Prince William
Sound region. Snowfall accumulation has been increased most places
with this morning's forecast package. However, it still looks like
very light accumulation from the Kenai Peninsula to Anchorage and
the Mat-Su. The heaviest snow will be in Valdez and across the
Copper River Basin. With windy conditions ahead of the front
pushing into the Cordova area, precipitation will likely mix
with or change to rain at times. A Winter Weather Advisory has
been issued for low visibilities in blowing snow through Thompson
Pass, as winds strengthen ahead of the surface low in the Gulf.
All of these features will exit eastward late tonight into early
Thursday, with snow lingering longest from eastern Prince William
Sound to the eastern Copper River Basin.

As advertised, a major pattern change is then in store as we head
through the remainder of the week and into the weekend. An
amplifying trough over the North Pacific will lead to a building
ridge downstream over the East Pacific and extending northward
to Southeast Alaska. A southerly jet stream will set up on
Thursday and remain pointed at southern Alaska for several days.
This will transport numerous short-waves and surface lows
northward from the Pacific along with warm and moist air. This
pattern is favorable for chinook winds, mixed precipitation types,
and ultimately above freezing air temperatures across much of
Southcentral. It creates a lot of messy weather.

So, as far as the details go, a leading warm front ahead of a
surface low tracking toward the Alaska Peninsula on Thursday will
lift northward across the Gulf, then weaken as it reaches the
north Gulf coast. Expect snow as precipitation first reaches the
Gulf coast Thursday afternoon and evening, but with a transition
to rain as warmer air arrives Thursday night into Friday.
Precipitation along the coast will be briefly heavy. Inland areas
may see a quick shot of snow with this, but nothing too
significant.

A second low and front will then follow closely behind Thursday
night. A series of strong upper level short-waves will drive the
second low and front further northward across southern Alaska on
Friday. This will be the more impactful system, with widespread
precipitation, heavy at times, along the coast and up Cook Inlet
to the Susitna Valley. Depending on the track and strength of the
surface low, which will be headed northward across Southwest
Alaska, gusty winds could move inland across Southcentral. Those
areas most likely to see winds include Anchorage and the Matanuska
Valley, where temperatures could shoot up into the 40s. For areas
which don't see the winds, subfreezing surface temperatures could
hold on longer, meaning there is potential for freezing rain as
the airmass aloft warms during the day Friday. Expect we'll end up
with a mix of precipitation types along with widely varying
temperatures across the region. The greatest threat for heavy snow
will be in the mountains of Prince William Sound Thursday night
through Friday, where multiple feet of snow are possible. Valdez,
where colder air tends to hold in longer, could also see heavy
snow before warming up sometime Friday afternoon. Stay tuned to
the latest forecasts and warnings as we continue to hone in on the
biggest threats from this impending pattern change.

&&

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

An upper level trough tracks south towards the Kuskokwim Delta
through Wednesday night. Shortwaves moving around the trough and
through Southwest Alaska will keep light snow in the Lower
Kuskokwim and interior Bristol Bay valleys through the morning.
Temperatures will drop through the day as cold and dry Arctic air
takes hold. Tonight's lows will range from negatives in the
Kuskokwim Delta to low teens in Bristol Bay.

Thursday marks a transition back towards warmer conditions as a
North Pacific low pushes its front into the Southwest. In Bristol
Bay, light snow is expected ahead of the front over the interior
valleys and upslope regions of the Kuskokwim Mountains. Thursday
afternoon, warm air advection will begin transitioning
precipitation towards a wintry mix, then rain, through Friday. Overall,
much uncertainty remains with how far inland the front will
track, when precipitation type transitions will occur, and where
the heaviest precipitation is expected. Stay tuned as we continue
to refine the forecast.

&&

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

Rain, with some areas of wintry mix, will persist in the Southern
Bering and Western Aleutians through today as a front stalls over
the region. Further north, an upper level trough is bringing
Arctic air southward over the Bering. Where this cold air meets
the warmer ocean waters, unstable conditions will contribute to
snow showers. This may bring periods of heavy snow showers,
blowing snow, and reduced visibilities to the Pribilof Islands
through the day. As the cold air continues southwards, most of the
Bering Sea and Aleutian Chain will see a transition towards a snow
shower regime. This will persist through Friday for all but the
Eastern Aleutians. As a North Pacific low approaches the AK
Peninsula, its front will bring a surge of warm, moist air over
the Eastern Aleutians and Bering. Light snow on Thursday morning
will transition into rain as the front tracks northward through
Friday.

&&

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

Gulf of Alaska...A front extends across the northern Gulf through
Sunday. Forecast confidence starts high but decreases with the
second front's approach. An easterly high end gale force barrier
jet develops along the north Gulf coast, Cook Inlet and Shelikof
Strait diminishes Saturday. Widespread southerly gales spread
over the Gulf behind the front, diminishing Saturday. Seas build
to 20 feet for Saturday, and subside Sunday. A new front enters
the Gulf midday Saturday with gusty winds increasing to southerly
high-end gales over the western Gulf Sunday. Seas to 17 feet
Sunday.

Aleutians / Bering...Arctic high pressure yields to a moderately
strong multi-centered low over the eastern Bering for Sunday.
Forecast confidence starts high, but decreases with the lows'
development. Widespread gales over the central and eastern Bering
diminishes over the Aleutians and Alaska Peninsula late Saturday,
but continue over the central and eastern Bering through Sunday.
Seas to 20 feet subsiding. Gusty winds continue around the lows
over the Eastern Aleutians and Alaska Peninsula. Seas to 15 feet.

&&

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

While models continue to show discrepancies with small-scale
features in the short-term, it appears that guidance is gradually
converging on a large scale pattern change by the end of the
week. The takeaway for now is that much of the forecast area can
expect to see a warming trend over the weekend. A longwave trough
will be making its way eastward into the Eastern Aleutians/Alaska
Peninsula on Friday and eventually into the eastern Bering by
Saturday. Strong southerly flow ahead of the trough will bring an
influx of moisture and warmer temperatures into much of Southwest
and all of Southcentral through early Monday as multiple
shortwaves lift around the trough into Southcentral. Any initial
snow along the coast should transition to all rain Friday into
Saturday with interior Southcentral more likely to experience a
mixed precipitation type. Precipitation will be ongoing at the
start of the period late Friday/early Saturday as a front moves
into the Alaska Peninsula and Southcentral coast. This may
persist along the coastal areas into Monday from the Kenai
Peninsula through Prince William Sound into the Copper River
Delta. For the rest of Southcentral, much of this precipitation
will become filtered out by the mountains, though expect showery
conditions as some moisture makes its way through and further
north. The exception to the above normal temperatures will exist
behind the longwave trough as seasonably colder air filters
southward out of the northern Bering into the Western and Central
Aleutians.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 131.
MARINE...Gale Warning 119 120 130 132 138 155 165 170
                      172-176 178 185 351 352 411-414
         Heavy Freezing Spray Warning 179 185 412 414
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CQ
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KC
MARINE/LONG TERM...KZ/BL

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
319 PM AKST WED JAN 19 2022

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0305 PM     SNOW             12 SSE ANCHORAGE        61.06N 149.76W
01/19/2022  M3.6 INCH        ANCHORAGE          AK   TRAINED SPOTTER

            MIKE FLEMING REPORTED A TOTAL SNOWFALL
            ACCUMULATION FOR THE UPPER POTTER VALLEY AS
            OF 3:00 PM OF 3.6 INCHES.

&&