Small Craft Advisory 1122, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
443 PM AKST Fri Dec 13 2024

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

Discussion:

Dense freezing fog continues to persist in Anchorage this
afternoon leading to low visibilities and frosty surfaces. Given
the light winds, the fog is expected to persist at least into this
evening and possibly through tonight before lifting tomorrow
morning. Elsewhere, much of Southcentral is seeing relatively
clear skies this afternoon ahead of returning cloud cover this
weekend associated with an upper level low dropping southwestward
into Interior Alaska. This feature will bring much colder air
southwards with it with the potential for light snowfall across
Southcentral. The initial push of this system on Saturday is
expected to bring light snowfall to the Copper River Basin while
strong cold air advection brings the possibility for gusty
northerly gradient driven winds through favorable gaps Saturday
night into Sunday morning. As the upper low interacts with a ridge
over Southwest Alaska, it will begin to flatten and head westward
towards the Seward Peninsula Sunday and should make it no further
south than the Alaska Range, limiting the upper level forcing
available for heavy snowfall throughout Southcentral.

By Sunday morning, a weak trough in the northern Gulf forms
bringing the heaviest snowfall to Prince William Sound and a
slight chance for a band of snow to set up over the Mat Valley and
Anchorage Bowl. As the trough swings into the Gulf coast Sunday
into Monday, a weak upper level shortwave will move through the
region providing additional forcing for snow to fill in over the
MatSu Valleys as well as the Anchorage Bowl although confidence
remains low at this time. Given the weak forcing, heavy snowfall
is not expected but a longer duration light snowfall event is not
out of the question for these areas. By Tuesday morning, another
North Pacific low makes its way into the southern Gulf, bringing
the return of strong southeasterly flow, warmer temperatures, and
another round of precipitation for Kodiak Island and the eastern
Kenai Peninsula.

-JH

&&

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

Quite the complex weather pattern across Southwest Alaska and the
Aleutians/Bering through the next several days. For Southwest
Alaska, the region in sandwiched between two frontal boundaries.
The first is dropping down from the interior, providing an
increase in clouds and widespread light snow across the middle
Kuskokwim Valley. The second is advancing across the eastern
Bering with clouds on the increase just off the coast and the
potential for snow returning overnight into Saturday for the
Southwest coast. Between these two system, a weak upper-level
ridge is keeping low stratus and fog over interior and coastal
Bristol Bay as well as parts of the Kuskokwim Delta. Some
locations have seen brief periods of very light freezing drizzle
and light snow/ flurries as the fog tries to dissipate.

The forecast for the Southwest coast is quite tricky, if not
impactful. The incoming front looks to weaken; however, guidance
is suggesting the development of a compact surface low somewhere
along the front. Guidance is also hinting that the weakening front
and surface low will interact to some degree with an inverted
trough approaching the AKPen from the North Pacific. The degree
of interaction will determine the extent of moisture advection and
snowfall along the immediate coast for Saturday. This feature
then looks to stall and elongate just off the coast for Sunday.
Again, just how close to the coast this feature lingers will
determine snow amounts and intensity of wind. Although it appears
that locations from Togiak to Toksook Bay will see 1 to 3 inches
of snow through Sunday, any increase in wind along the coast could
also result in periods of blowing snow and reduced visibility.

For the Aleutians and Bering, the the aforementioned inverted
trough will bring a round of mixed rain and snow to the southern
AKPen and parts of the eastern Aleutians through Sunday morning.
Any prolonged snow in colder locations should not amount to much
given colder locations will still see temperatures in the mid 30s.
This trough will help to increase winds for places like Cold Bay
and Dutch Harbor.

The bigger weather feature will be a large storm-force low moving
toward the Western Aleutians late Saturday through Sunday. This
will be a rather impressive system, with central pressure dropping
as low as 940 mb as the system approaches the Western Aleutians.
Given the strength of this system, confidence is very high that
there will be widespread storm force winds both along the front
and wrapping around the low. There is moderate confidence of
hurricane-force wind gusts for the coastal and open waters of the
Western Bering and for the Western Aleutians, including Shemya.
While there is high confidence for gusts to 70 mph for Adak and
Atka on Sunday, confidence wanes for gusts of 75 mph or greater.
The occluded front associated with the low continues its eastward
progression, reaching the Pribilofs and Eastern Aleutians by
Monday morning. Gusts of 45 to 60 mph are possible from Saint Paul
to Unalaska to Cold Bay late Sunday through Monday.

-TM

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

Our small taste of more seasonal air over Southcentral will be
short lived as temperatures begin to warm again already on
Tuesday. A low developing along the front of a strong low pressure
system over the Aleutians will bring warmer southeasterly flow to
the Gulf of Alaska. Terrain enhanced rain at low elevations and
mountain snows are possible for coastal regions, while downsloping
should keep lee-side locations drier. Unsettled weather will
persist across the Bering Sea and Aleutians as the system stalls
out there.

There could be a brief break in unsettled weather on Wednesday,
after which poor model agreement gives us less confidence in the
exact placement of features. However, it is looking increasingly
likely another system will spin up from the parent Bering low and
bring enhanced winds and precipitation to the Gulf Coast on
Thursday. There are hints of especially strong easterly winds from
Kodiak Island up to the southern Kenai Peninsula, although this
too will be dependent on where features set up.

&&.AVIATION...

PANC...As of 02Z some of the dense fog has lifted and/or moved
over the Inlet. However, dense freezing fog may continue to
plague the PANC this evening with sub 1/2mi visibility. With few
mechanisms available to mix the fog out, there remains a
possibility that fog continues through the evening and overnight
hours. The wind field should increase just enough in response to
an approaching trough such that any fog should scour out tomorrow
morning. The trough / cold front may bring some light snow through
ANC from late Saturday through Sunday.

&&