Small Craft Advisory 974, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
510 AM AKST Wed Dec 16 2020

.ANALYSIS AND UPPER LEVELS...

The upper level trough from the last couple days is still making
its way northeastward across the Kenai Peninsula and the Alaska
Range, while the upper level ridge remains northeast of Mat-Su
Valley this morning. Cold air advection west of the trough fills
much of Southwest Alaska, with much colder air aloft upstream and
over Southcentral. Steady light snow persists over much of
Southcentral, but conditions should be improving as the trough
continues to lift farther inland today. Over at Southwest Alaska,
both freezing fog and steady light snow continues mostly along
the coastal area. High-level clouds and increased moisture from a
developing upper level system south of the Aleutians will continue
to pass through the eastern Aleutians and AKPEN today.

&&

.MODEL DISCUSSION...

GFS, NAM and ECM are pretty consistent tracking synoptic features
for the short-term. GEM/Reg begins to steer off-track a bit from
the rest of the solutions by Thursday morning. By Friday afternoon,
the other three models start to differ with the placement of
synoptic large scale features approaching both the Gulf and AKPEN
through Friday night. Overall, GFS maintains to be a better solution
for this forecast package.

&&

.AVIATION...

PANC...The heaviest portion of snow is currently making its way
over the terminal. There is a clear back edge moving up Cook
Inlet. As this moves through over the next few hours, expect
fairly rapid improvement. There is an outside chance for some low
stratus and/or fog to form behind it. Cigs are expected to remain
MVFR through the day with light flow allowing moisture to linger.
As flow turns to come out of the north later tonight, this should
bring drying and allow conditions to return to VFR.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through
3/Wednesday through Friday)...

Snow is continuing this morning in much of the Mat-Su Valleys
along and ahead of the axis of a potent, negatively tilted
shortwave as it works its way eastward. The passage of the wave
will mark an abrupt end to the snow from southwest to northeast
later this morning across the western Chugach slopes and the
Anchorage Bowl. The trough axis will also end ongoing snow over
the Copper River Basin slightly later as it sweeps past during
this afternoon and evening. A few areas of lingering low stratus
or patchy fog may linger or form in/around the northern Cook Inlet
and Knik/Turnagain Arms after the snow ends. This would mainly
result from residual moisture left behind the departing system in
the valleys before colder and drier northerly flow increases at
the low levels and likely helps to clear things out by late
tonight.

A second system will enter the picture from Wednesday night to
early Friday as a deep surface low tracks to the south of Kodiak
and then across the central Gulf. The models continue to struggle
with the exact track and strength of this system. Some of the
details of the forecast, especially for Kodiak and Thompson Pass,
will hinge on precisely where the storm ends up. Winds will
increase around the low as it tracks into offshore waters with at
least small craft to low end gales likely across much of the Gulf
as the low tracks eastward on Thursday into early Friday. With
colder air building in over the Copper River Basin by this time,
and with the low aiding the pressure gradient near the Gulf coast,
outflow winds should pick up substantially on Thursday and
Friday. The strongest winds will be mainly through Thompson Pass,
the Valdez Arm and Copper River Delta. Periods of blowing snow are
quite possible in these areas by Thursday evening into early
Friday, but remain highly dependent on the exact track of the
surface low. Additionally, Kodiak City may experience a mix of
rain and snow as the low passes just to the south. A transition
from mainly rain to snow may occur on Thursday afternoon, perhaps
with a quick inch or so of accumulation in town before precip ends
on Thursday evening.

&&

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

An upper level ridge combined with weak surface winds are leading
to patchy freezing fog across portions of Bristol Bay and the
Kuskokwim Delta. There remains uncertainty on the track of a low
moving near Kodiak Island late tonight into early Thursday. In any
case, light snow is expected to overspread the Bristol Bay
region, possibly leading to a few inches of accumulation in the
Dillingham area. Precipitation may shift slightly northward over
the Kuskokwim Delta by Thursday evening, though any accumulations
there would be minimal. Periods of reduced visibility due to
blowing snow are also possible as gusty northeasterly winds will
persist through the day. Scattered snow showers will remain
possible through Friday along the Kuskokwim Delta coast ahead of a
low centered near the Pribilofs. Areas inland and to the south
look to remain dry.

&&

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

A strong, compact low will move across the western Aleutians
today bringing gusty winds and moderate precipitation. Farther
east, a weak upper ridge will lead to benign conditions today
before the aforementioned low's front will quickly track west to
east across the Aleutians on Thursday. While uncertainty exists
into the end of the week, scattered rain/snow showers look to
continue with steadier precipitation near the 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 119 130-131 173 175-177 413-414.
         Heavy Freezing Spray Warning 180-181 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CMB
SOUTHCENTRAL ALASKA...AS/MO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP
MARINE/LONG TERM...MV