Storm Warning 168, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
431 AM AKST Tue Dec 12 2023

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

A high amplitude, longwave trough remains in place over western
Alaska, extending south of Kodiak Island where it is anchored by a
510 dm upper-level low. Stout, southerly flow downstream of these
features is drawing both copious amounts of moisture and a
surface low north toward Prince William Sound this morning. The
surface low is moving north-northwest along a stalled frontal
boundary that is has been the focus for the heavy precipitation
stretching from the central Gulf over the Central Chugach and
Copper River Basin for the past 24 to 36 hours. To the west,
conditions are quieter; however, residual moisture over Cook Inlet
and a weak trough beginning to usher in colder air aloft has
resulted in the redevelopment of scattered showers from Homer
north to Anchorage.

Snow levels did drop slightly overnight with rain changing to snow
for Valdez, Cordova, and around the junction of the Edgerton and
Richardson Highways in the southern Copper River Basin. Snow
levels will likely remain steady through the day as the low
tracks inland by mid morning and precipitation intensity during
the afternoon and evening hours wanes. The exception to this is
likely Cordova, where enough warmer air will work back in this
morning to change snow, or mixed rain/snow, back to all rain for a
time before colder air works back in later today. Farther west,
the main challenge will not only be the extent of snow shower
activity this morning across Cook Inlet, but whether the western
edge of the precipitation shield makes it far enough over the
Chugach to allow for some light snow for places like Anchorage and
Palmer. The best chance for any light snow through the afternoon
hours is over the Matanuska Valley.

The parade of surface lows continues this evening as the next
follows a similar track to the previous and moves toward Prince
William Sound. Another round of heavy precipitation will move over
the Central Chugach and across the Copper River Basin through
Wednesday morning. Again, the biggest challenge will be the
western extent of the precipitation. This system appears more
likely to bring a round of steady, light snow to the Mat-Su
Valleys, Anchorage Bowl and parts of the Kenai Peninsula Wednesday
morning as the anchoring upper-level low to the west dips south
and becomes more negatively tilted, drawing the surface feature
and attendant shortwave back to the west. Snowfall amounts for
these locations should be light, on the order of 1 to 3 inches.

The southerly flow across the Gulf persists into Thursday with
showers rotating around the lows. Much colder air will also advect
in, bringing snow levels down to sea level across the Southcentral
coast by Wednesday afternoon.

TM

&&

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

Conditions across the Bering Sea and Southwest Alaska remain
largely driven by a large low pressure system in the Gulf of
Alaska and a ridge set-up in the Western Bering Sea. The influence
from these two systems is driving the strong influx of cold air
from the northern Arctic down into the the Southwest coastline,
Alaska Peninsula, and eastern Aleutians. This strong plunge of
cold air will continue to produce gusty winds in the Eastern
Bering, and across the the Alaska Peninsula through Wednesday.
Storm-force gusts through gaps and passes along the Alaska
Peninsula and Eastern Aleutians are expected during this time.
Areas of blowing snow reducing visibility will be likely along the
coastline from Bethel to Cold Bay. Similarly, this area will
likely see new snowfall from persistent snow showers with the
strong northerly winds diminishing only by Wednesday evening. The
easing wind flow will weaken the presence of blowing snow across
the aforementioned areas, however not out of the realm of
possibility until about Thursday afternoon.

The system that will eventually break the hold of this cold air
flow will be a new low pressure system pushing in from the western
Bering sea. This low pressure system will be initially noted by a
leading frontal passage that will bring warmer southerly air
arriving to the western-most Aleutians by Wednesday morning. The
front will bring with it another round of precipitation, however
this will likely come in the form of liquid rain due to the warmer
temperatures. The front will be producing Gale-force southerly
winds with some Storm-force winds possible further north into the
Central Bering sea, however will likely be less "gusty" in nature
due to the temperature of the air coming in being warmer than
whats currently in place. The front will advance steadily
eastward, reaching Unalaska by Thursday morning. As the front
approaches, precipitation may initially begin as snowfall, but is
expected to transition within a few hours to plain rain. By this
time, the tight pressure gradient driving strong winds through the
Eastern Aleutians will have relaxed, and the strong winds eased
significantly. By Thursday morning, the parental low pressure
center will enter the Western Bering sea, re-initializing the
precipitation over the western- most Aleutians, however this time
likely in the form of snowfall due to more colder air entrained in
the system.

CL

&&

.LONG TERM FORECAST (Days 4 through 7)...Friday through Monday...

The long-term forecast continues to feature general agreement on
the continuation of the active pattern Friday through at least
Monday. For Friday, a new deep upper trough drops into the
western/central Bering Sea while a shortwave ridge moves over the
eastern Bering and Southwest Alaska. The upper low that provided
Southcentral Alaska with very active weather in the short-term
continues to move eastward towards Southeast Alaska as the
shortwave ridge builds over Southcentral. However, any break in
weather associated with the ridge will not last long. Later on
Friday and into Saturday, the deep trough in the Bering moves
eastward and puts all of mainland Southern Alaska back in moist
southerly to southwesterly flow. A front over then central and
eastern Bering moves to the coast of Southwest later in the day on
Friday and eventually to Southcentral on Saturday. There is lots
of uncertainty exactly where a secondary low will spin up along
the front in the Gulf on Saturday. All ensemble and deterministic
guidance all show different solutions. However, generally
speaking, coastal Southcentral chances for moderate to heavy
precipitation are average. Southwest flow in the middle to upper
levels continues for Monday as additional shortwaves lift
northward through the Gulf and into Southcentral. The Bering Sea
and Aleutians stay active Saturday through Monday as shortwaves
pivot around the main trough. An additional low coming from
Kamchatka looks to send its gale-force front from the western
Aleutians Sunday afternoon to the central Aleutians by Monday
morning. This will spread gusty winds and precipitation across the
Aleutian Chain early next week.

&&

.AVIATION...

PANC...Expect scattered snow showers to continue through mid to
late Tuesday morning. These snow showers could cause temporary
reductions to ceilings and vis, especially between 12z and 15z.
MVFR conditions are most likely in snow showers, but conditions
could briefly drop as low as IFR. Conditions will improve during
the morning hours on Tuesday, with dry conditions and VFR ceilings
likely until snow with the next storm arrives sometime late
Tuesday night or early Wednesday morning.

Expect northerly winds to continue throughout the day, with the
strongest winds, around 10 kt, through 18z.

&&