Small Craft Advisory 964, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
404 PM AKST Sun Dec 13 2020

.ANALYSIS AND UPPER LEVELS...

A frontal system with a couple of embedded lows is the primary
weather maker for Southcentral and especially Southwest Alaska
this afternoon. One of the lows is in the southeastern Gulf. The
second, much more impactful low is moving across Kodiak Island.
This is keeping rain and breezy conditions ongoing in Kodiak. As
that moisture moves northwestward along the front, snow is
ongoing across portions of Bristol Bay, mostly west of Igiugig.
Lighter snowfall rates are happening along the Kuskokwim Delta
from the same front. It's a bit warmer from King Salmon south, so
rain is being reported along the Alaska Peninsula.

For Southcentral, most of the precipitation is staying out over
the Gulf, with even most coastal locations not yet reporting
steady precipitation. Temperatures across the area are holding in
the 20s and 30s, with Homer and Seldovia holding on to the 40s.

Away from the frontal system, the Bering is characterized by
widespread shower activity with an expansive cumulus field over
almost all of the Bering. With temperatures near or just above
freezing area-wide, a rain/snow mix is most likely with any
associated precipitation, though any accumulations would be very
short-lived.

&&

.MODEL DISCUSSION...

Model agreement remains good for the short-term portion of the
forecast, lending confidence to timing and intensity of the
precipitation affecting the area over the next couple days. There
have been some notable shifts in the forecast for snow by Tuesday
across Southcentral, with a good bit of disagreement on how much
moisture makes it into Anchorage, the Mat-Su, and Western Kenai
Peninsula. There is enough agreement that chances of snow have
been increased, but even using the most robust solution, should
only amount to an inch or two. Meanwhile, a few other solutions
have just flurries or under an inch, so that particularly remains
a point of low confidence.

&&

.AVIATION...
PANC...VFR conditions and light northerly winds will persist.
Confidence is increasing for light snow Monday night into Tuesday,
but with emphasis that the snow will be light, should only drop
conditions to MVFR at worst at any particular time.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2..Sun
afternoon through Tue afternoon)...

An upper level trough over the Gulf will keep precipitation and
gale force easterly winds in place along the northern Gulf coast
this evening. The corresponding surface low over Kodiak Island
will weaken and lift northward over the Alaska Peninsula through
Monday morning. While most interior Southcentral areas will remain
dry, rain is expected along the coast overnight before
precipitation lightens up throughout the day Monday.

Attention quickly shifts to the next event that has the potential
to bring light snow to Southcentral Monday night into Tuesday. As
the upper level trough remains over the Alaska Peninsula, several
waves of energy will propagate northward. Precipitation along the
coast is likely to fall as a rain/snow mix, while snow is more
likely farther inland. At this time, accumulations are expected to
be up to 2 inches across the Anchorage Bowl and northward into
the Mat-Su Valleys. There is still a fair amount of uncertainty
with this system as model guidance continues to fluctuate. By
midweek, a much colder airmass looks to arrive from the north,
particularly over the Copper River Basin as an upper level trough
noses in across the Alcan border.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

Southwest Alaska will continue to see snow this afternoon into
Monday afternoon as a front associated with a low near Kodiak
stalls over the region. Downsloping from Iliamna to King Salmon
will keep these areas with little to no snow accumulations through
early Monday. The main focus will be for the Dillingham area, as
well as areas north and east, which could see a few additional
inches of new snow accumulation. Gusty winds with snow could cause
periods of reduced visibility in blowing snow, but should not last
long enough to meet advisory level criteria. Snow showers, with
light snowfall accumulations, will persists for the Kuskokwim
Delta and Valley through Monday afternoon when the snow begins to
taper off.

A surface low developing over the eastern Bering on Monday
afternoon will bring another shot of snow to the Bristol Bay
region and Kuskokwim Delta, spreading into the Kuskokwim Valley
for Tuesday afternoon. This should be a weaker system than what
is happening today, allowing for a few new inches of snow, once
again favoring the Dillingham region and Bristol Bay coast.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

Winds and precipitation over the Alaska Peninsula will weaken
through Monday as a low near Kodiak shifts out of the region. Out
west, a front briefly moves over the western Aleutians and Bering
this evening through Monday afternoon, before lifting northward.

The biggest forecast change has been to the compact North Pacific
low that moves near the central Aleutians on Tuesday. Models have
all moved this system much farther south than yesterday's runs,
which means that winds over the southern Bering and over the
Aleutians have been reduced. The storm force winds that were in
the previous forecast have shifted south out of our forecast area,
keeping only a small area of gale force winds south of the Chain.
However, if the track does change again, these stronger winds
could move back into the forecast area throughout the day on
Tuesday. Another North Pacific low will move over toward the
eastern Aleutians and AKPEN, bringing around round of strong winds
and precipitation to the area.

&&

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

Gulf of Alaska: Confidence is high early on in the forecast period
for winds less than 30 kts and seas less than 15 feet for
Wednesday through Wednesday evening. Beyond Wednesday evening, a
gale force storm system is anticipated to move into the Gulf.
Confidence has increased some on this system since yesterday
though exact placement still remains uncertain. Confidence beyond
Thursday is low as there is the risk for another strong low to
impact the Gulf.

Bering Sea: Confidence has increased for another gale force
system to affect the marine waters south of the eastern Aleutian
Chain and AKPEN Wednesday with uncertainty remaining for the north
side. For the western and central Bering, confidence remains low
with the potential for a compact gale force low or relatively
benign conditions Thursday and Friday.

&&

.LONG TERM FORECAST (Days 4 through 7/Thursday through Sunday)...

The weather pattern from late week through next weekend continues
to look quite dynamic. The subtropical jet stream is expected to
remain strong over the North Pacific with a deep Arctic low/trough
anchored somewhere over the Yukon and Mainland Alaska. More often
than not, the subtropical jet will phase with the northern polar
jet stream energy. The combination of these two will continue to
spawn storm systems anywhere from the western Bering/North Pacific
to the Gulf of Alaska with the details of each of these being
entirely uncertain (intensity/track of each one). Bottom line for
this period is expect periods of sensible weather impacts with
occasionally quiet conditions between any systems.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 119 120 127 130 131 138 139.
FIRE WEATHER...NONE.

&&

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