Small Craft Advisory 1005, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
512 PM AKST Thu Dec 24 2020

.ANALYSIS AND UPPER LEVELS...

Generally unsettled weather continues across much of Alaska. A
broad upper trough extends southeast from Russia, across the
Bering Sea, and over the Northeast Pacific. Numerous shortwaves
embedded in the flow are supporting numerous surface centers and
driving rain and snow shower actively across the northern Gulf and
snow showers inland across Southcentral. A north moving low and
associated front well south of the AK Pen are spreading wind and
rain to Kodiak Island and the Western Gulf, rain/snow to the AK
Pen, and snow into Bristol Bay. A north moving low over the Y-K
Delta brought some snow and blowing snow to the area earlier
today, but snow and blowing snow have since diminished. A well
defined upper shortwave and cold advection are supporting a low
complex over the Central Aleutians/Central Bering Sea. Strong
northwesterly winds with snow showers are evident over the Central
Aleutians and on the backside of the northernmost low over the
Central Bering Sea.

&&

.MODEL DISCUSSION...
Model data ingest problems with the 12Z GFS and NAM continued,
however it appears that the 18z NAM run may actually get fully
in, fingers crossed. The large storm center which moves to the
Aleutians on Saturday seems to be generally well handled by the
models which are in fair synoptic agreement. Model consensus
then deteriorates later Saturday and Sunday. The American models
hold the storm center to the west whereas the ECMWF/CAN NH still
carry the center further east. Current forecast grids have a
more western solution, so we plan on holding with this solution
for this forecast cycle until things become clearer. The main
differences in models across the Gulf and Southcentral is how
intense they have the low moving northwest across the Gulf Friday
night. The ECMWF/CAN show a stronger low center tracking across
the Gulf Friday night with the American models, particularly the
NAM, showing a much weaker low for now. Leaning toward a weaker
NAM solution at this point.

&&

.AVIATION...
PANC...Generally VFR conditions should persist through tonight.
Northerly winds will be increasing some tonight and Friday. Also
expect potential for LLWS near 1500 ft during this evening.

&&

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

A relatively quiet weather pattern is expected for Southcentral
over the next few days. With the current synoptic pattern having a
ridge of high pressure over the eastern Alaskan interior and low
pressure southwest of Kodiak, offshore flow will be the dominant
wind pattern for tonight. Pressure gradients favor enhanced winds
in Thompson Pass, Matanuska Valley and the Copper Delta tonight
into Christmas morning. The next low will move into the northern
Gulf of Alaska Friday night into Saturday morning. Confidence is
higher today compared to 24hrs ago as there is more agreement on
the placement and intensity of this low at this point in time.
The low is expected to bring rain along the coastal locations
along the northern Gulf and Prince William Sound with snow
expected throughout the inland locations. Temperatures support rain
being confined to the immediate coastal locations with snow for
the remainder of areas that see precipitation. Rain and snow will
gradually taper off Saturday night.

&&

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

Cloud cover and chances for snowfall increase through the
overnight hours as a low south of Kodiak Island this afternoon
lifts northward over Bristol Bay on Friday. Snow will move into
interior Bristol Bay tonight, along with gust northeasterly winds,
which will allow for periods of lower visibility in blowing and
drifting snow. The highest snowfall totals up to 5 inches are
expected from Dillingham to Koliganek through Friday afternoon.
Snowfall will also move into the Kuskokwim Delta and Valley from
Bethel to Sleetmute where a few new inches of snow are possible
for Friday.

Snow will begin to diminish late Friday through Saturday before
the next big system approaches early Sunday. A strong front will
move over the Southwest coast Sunday morning, bringing another
round of strong winds to the coastal locations. There is some
uncertainty as to how impactful this system will be in terms of
blowing snow due to warmer temperatures moving into the region as
the front moves inland. For now, models have precipitation
starting as snowfall, but a quick transition to rain is possible
as the front progresses eastward throughout the day on Sunday.

&&

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

A low south of the AKPEN will continue to lift northward overnight,
bringing strong winds and snowfall to the area. A few inches of
new snowfall are possible from Port Heiden to Pilot Point through
Friday afternoon. Over the central Bering, a complex low with a
small swath of storm force winds will work its way eastward
throughout the day on Friday before diminishing near the
Pribilofs by Saturday morning.

The most dominant weather feature will move into over the western
Bering and Aleutians early Saturday morning as a strengthening low
lifts towards Shemya. Models are in good agreement that storm
force winds will accompany this system crossing into the western
and central Bering on Saturday, with the possibility of seeing a
small area of hurricane-force winds with the initial front and on
the south side of the low. This storm will deepen to 935 mb as the
low center tracks into the central Bering by Sunday morning as its
front extends across the eastern Bering. Widespread rain and snow
are expected across the central to eastern Bering and Aleutians
through Sunday. The biggest forecast uncertainty is how far
eastward this low will track and where the strongest core of winds
will be throughout the day on Sunday. The NAM/GFS solutions are
keeping a stronger low center over the western Bering, while the
EC/Canadian models are much farther east.

&&

.MARINE (Days 3 through 5: Sunday through Tuesday)...

Gulf of Alaska: A strong front from the Bering Sea will track
across the western Gulf waters on Sunday. There was uncertainty
in confidence yesterday, but this now looks like solid gale force
winds with this front. Forecast confidence then drops for Monday
and Tuesday as a series of lows rotate up from the Pacific. Will
likely see continued periods of at least gale force winds with
these low centers.

Bering Sea: Starting Sunday, a large well established low will be
anchored over the southern Bering Sea, with widespread gale to
storm force winds across Bering, Aleutians, and Southwest coastal
waters. Expect a series of lows to track around this larger low
center Monday through Tuesday, but forecast confidence in the
track and strength of individual centers is low. Bottom line,
gale to storm force winds will remain a threat through at least
Tuesday.

&&

.LONG TERM FORECAST (Days 3 through 7: Monday through Thursday)...
Forecast confidence is low to medium at this time with models
rather all over the place with key features. A complex area of
low pressure over the western Bering on Monday morning looks to
remain nearly stationary through Tuesday before heading east
towards the southwest coast on Wednesday afternoon. This will
bring increased winds and precipitation to the area through
Thursday. A North Pacific low should approach the Alaska
Peninsula late Thursday with a secondary low positioned in the
central Bering. Anticipate active weather associated with all of
these systems. A front in the western Gulf moves towards the
northern Gulf coast by Tuesday, bringing an increase in winds and
precipitation to coastal locations and higher elevations as an
area of low pressure forms along the front. The front should then
move inland late Tuesday into Wednesday. After this, another
front associated with a low heading towards the AKPEN from the
North Pacific will enter the western Gulf late Thursday, bringing
another chance for active weather including higher winds and
precipitation to Kodiak Island and the western Kenai Peninsula.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Heavy Freezing Spray: 180,181
Storm Warning: 173,175,178,411,413
Gale Warning: 119,130-132,136-138,150,155,172,174,176,177,412
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BC
SOUTHCENTRAL ALASKA...ED
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...MK/CK