Small Craft Advisory 972, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
429 PM AKST Tue Dec 15 2020

.ANALYSIS AND UPPER LEVELS...

An upper level trough is in place along the western Alaska coast
while an upper level ridge extends across the eastern part of the
state along the Alcan border. Moisture associated with a low in
the southeastern Gulf continues to be advected inland over
Southcentral. Light snow showers are falling across much of the
area from the Kenai Peninsula north to the Mat-Su Valleys. Light
snow also extends eastward over the Copper River Basin. Meanwhile
farther west, a low centered over Adak is slowly tracking
eastward, spreading rain and cloud cover across the Aleutian
Chain. As the trough centered over the western half of the state
slowly lifts eastward, snow showers will continue over the
Kuskowim Delta and Bristol Bay region this afternoon.

&&

.MODEL DISCUSSION...

Model solutions are in good agreement through the short term. By
Wednesday afternoon, there is disagreement regarding the position
of a low tracking across the Alaska Peninsula and over Kodiak
Island into the Gulf. Farther west, models continue to struggle to
resolve a complex low over the Bering through the end of the week.

&&

.AVIATION...

PANC...Light snow will continue overnight. A period of heavier
snow is expected around 9-12z which should cause the worst
conditions within the TAF window.

&&

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

Moisture on the backside of a low in the eastern Gulf, continues
to propagate across Southcentral this afternoon. This will
allow for a continuation of the snowfall we've seen today through
the overnight hours and into tomorrow morning/early afternoon.

Tomorrow morning into the early afternoon, a strong, upper level
shortwave looks to provide more lift. This, combined with upslope
flow, will provide for an increase in snowfall intensity for the
western Chugach early tomorrow morning for a 3 to 6 hour period.
Snowfall intensity, due to the shortwave, will also increase along
the coastal areas of Prince William Sound, as it moves from west
to east through the day on Wednesday.

Another system is on track to approach the southwestern Gulf
Wednesday night. As the low moves towards Kodiak Island, a wintry
mix of precipitation is anticipated. Additionally, gale force
northeasterly winds are expected down Cook Inlet and into Shelikof
Strait. Offshore flow will continue to strengthen with the
increasing pressure gradient as this system approaches the
northern Gulf Coast. Expect gusty winds through the Matanuska
Valley, Copper River Delta, Thompson Pass and Valdez waters.

Temperatures will remain on a downward trend through the forecast
period for the Copper River Basin as cold air aloft settles into
the Basin.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Tonight
through Friday)...
Areas of very light snow will persist tonight, primarily over
interior Southwest Alaska, as an upper trough swings through.
A weaker trough will follow behind on Wednesday, maintaining low
clouds, patchy fog, and a few flurries across the region. There
has been a change in the forecast for a storm tracking south of
the Alaska Wednesday and into the Gulf of Alaska Thursday. This
storm will take a further north track, with support from a
negatively tilted trough aloft. As a result, expect snow to
overspread much of Bristol Bay Thursday afternoon through
evening. Could also see snow make it up to the Kuskokwim delta,
though confidence in that is lower. Nonetheless, accumulation
will be light. Some locally gusty northerly winds from roughly
Dillingham westward will also produce some blowing snow, which
will reduce visibilities at times. This will all quickly wind
down Thursday night into friday as both the low and upper trough
exit. Friday then looks quiet as a ridge briefly moves overhead.
Never really see any significant low level winds or temperature
advection, so low clouds and patchy fog will likely remain in
place.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3/Tonight
through Friday)...
Progressive cyclonic flow aloft, with a plethora of embedded
short-waves will lead to development of multiple compact lows over
the next few days. Forecast confidence in the track of most of
these storms is quite low. Fortunately, they will generally move
through any one area pretty fast and the stronger winds will be
confined to a small area near the low center. At the moment, it
appears the strongest of these lows will be one that tracks from
the North Pacific to near Adak Wednesday night, then eastward to
south of the Pribilof Islands Thursday night. Expect high end
gales on the south side of this low. The overall pattern will
produce rain and snow showers, with steadier precipitation near
low centers.

&&

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

Gulf of Alaska: There is a very good agreement that a gale force
low will impact the Gulf on Friday. Most of the gale force wind
activity is expected to be on the southern side of the low center.
Model guidance spreads out after this with the possibility for
relatively benign conditions across the Gulf whereas the other
models are showing at least small craft westerlies across the Gulf
Saturday through Sunday which is currently what we are leaning
towards. There is potential for these to be gale force as well
but there is still uncertainty for this.

Bering Sea: For Friday and Saturday, generally benign conditions
are expected across the Bering with winds below 30 kts and seas
less than 15 feet with some rather small compact lows. The big
weather maker is a rather large gale force storm system that is
expected to impact the Bering Saturday evening through the day
Sunday which all model guidance agrees on. As a result of the
winds, waves (greater than 15 feet) will be on the increase as
well.

&&

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

The weather pattern during the first half of the forecast time
period continues to show strong Arctic high pressure locked in
over the Interior of Alaska east to the Yukon Territory. The
subtropical jet continues to remain fairly active during this time
frame as well with multiple storms trying to move northeastward
and into the cold air that is locked in across the interior areas
of Alaska and the Yukon. Latest data suggests that coastal areas
of the eastern Kenai Peninsula, Kodiak Island, and Prince William
Sound area have the best chance to see any meaningful
precipitation during this timeframe. Some of the energy from these
systems may occasionally send some light snow into the Anchorage
area and across Southwest but this time frame looks to remain on
the colder side with temperatures averaging below normal. The
exact track and intensity of any of these systems in this time
frame continues to remain highly uncertain and will become more
clearer over the coming days.

Farther west, model guidance has since come into much better
agreement across the Bering and for Southwest Alaska for a rather
large low to impact the area. The details of this system remain
uncertain as this time but the threat is there for it bring a
decent amount of warmer air northward towards Southwest Alaska
Monday and Tuesday potentially creating precipitation type issues
for this time period. Stay tuned as we continue to hone in on the
details of this system in the coming days.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gales 130 138 172 175-177.
FIRE WEATHER...NONE.

&&

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