Gale Warning 829, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
434 PM AKST Wed Dec 20 2023

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

A longwave trough, with its axis over western Alaska and the
eastern Bering Sea this evening, will shift east across the state
of Alaska through Friday. A train of shortwave troughs will
traverse through the flow, tracking northeastward across
Southcentral Alaska. One such shortwave is lifting north across
the Copper River Basin this evening, having produced light snow in
Gulkana earlier today. Light snow showers in the area will wane
this evening as the trough exits to the north. In its wake,
southerly low-level flow will continue to produce upslope
rain/snow showers in eastern Prince William Sound. Further south,
a mature low centered around 125 nm east of Kodiak City will track
northeast across the Gulf of Alaska to the north Gulf coast for
Thursday morning. Uncertainty in the track of the low remains
similar to the previous forecast, with the most likely scenario
continuing to be a low track south of Cordova, which will bring
its front ashore within eastern Prince William Sound tonight.
However, GFS has held onto a further west solution, which would
suggest a further west extent of precipitation into western Prince
William Sound. While there is not much support for this track,
have held onto a period of chance precipitation over portions of
the eastern Kenai Peninsula for tonight.

Regardless of track, expect much of the precipitation to fall as
snow along the coast, with some rain mixing in near Cordova, where
the warmest air is currently situated. Further inland, now
expecting a deformation band to set up ahead of the low, sweeping
north and west through the Copper River Basin beginning around
midnight, with low-level northerly flow, and weak cross-barrier
flow in general, allowing precipitation to spread inland. The
deformation band sweeps over the central and western portions of
the Copper Basin through the morning, bringing around one to two
inches of snow to the low elevations. The low occludes on
Thursday, with the deformation band weakening through the day,
causing snow to taper off. For the southern portion of the basin,
near McCarthy, snow showers could continue into Thursday night on
the east side of the weakening low for an additional inch or two.

For much of the Kenai Peninsula, Anchorage, and the Mat-Su
Valleys, expect little precipitation under dry, cold
northwesterly flow through Thursday night. What will come along
with this calmer weather, though, is lingering stratus over Cook
Inlet, which will contribute to possible fog in the area through
at least tonight.

Continuing through the work week, a low tracking south of the
Aleutian Chain Thursday and the Alaska Peninsula Friday begins to
push a strong front into the Gulf on Friday. Expect this front to
track north and east across the Gulf, with moderate to heavy
precipitation spreading from Kodiak Island to the Southcentral
coast. Upper level amplification will lead to rising snow levels,
especially for the Kenai Peninsula. Based on the current model
guidance, it does look favorable for downslope drying for much of
the western Kenai Peninsula northward to Anchorage and the
Matanuska Valley. There is some indication that a strong shortwave
tracking north across Southcentral Friday night may be able to
provide enough dynamic lift for some precipitation to push past
the mountains. However, any precipitation would likely be light
and intermittent. With much of the deterministic guidance keeping
temperatures below freezing through the atmospheric column at
inland locations, there is only a low potential for freezing rain
with the push of warmer air provided by the front through
Saturday.

CQ

&&

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

An upper trough positioned over the eastern Bering and Southwest
Alaska is gradually scooting eastward and into the Gulf of Alaska
this evening. Multiple meso-lows within the trough are producing
light scattered snow showers over Southwest Alaska, with the most
notable area of precipitation occurring over the Kuskokwim Delta,
from Bethel south and east. Over the next 24 hours these smaller
areas of low pressure will weaken before eventually consolidating
south of Prince William Sound through tomorrow. An arctic air mass
located over Siberia and the Chukchi Sea has been waiting
patiently for the trough to move out of the way. Tomorrow into
Friday will see the first opportunity for some of the colder
temperatures associated with the arctic air mass to spill
southward, following the Kuskokwim coast on Friday, and eventually
reaching Bristol Bay on Saturday. While heavy freezing spray is
possible along the west coast of Nunivak Island tonight, the
greater threat for heavy freezing spray won't develop until
Friday night into Saturday, from east of Saint Matthew Island all
the way down to offshore of Cape Newenham.

The main weather feature over the next couple of days will be a
North Pacific low that deepens over the Western Aleutians on
Thursday morning. This low will bring easterly gales over the
Aleutians Thursday while the center of the low tracks just south
of the Aleutian Chain. Warmer Pacific temperatures associated with
the low may see a mix of rain/snow along the low's front. While
blowing snow will be less of a concern due to the warmer
temperatures, reduced visibility due to occasionally heavy
precipitation will be possible. Winds across the eastern Bering
then increase substantially late Friday into Saturday as the axis
of the low shifts south of the Alaska Peninsula. Gale to storm
force northerly winds will become common through the region. The
combination of continued snow showers will also promote a threat
for blowing snow and advisories may be needed as Friday gets
closer. The other concern will be for potential blizzard
conditions over Greater Bristol Bay. Cooler temperatures,
increasing winds, and the potential for several inches of snow to
fall from Togiak and Dillingham, northward, could promote quarter
mile visibilities due to blowing snow on Saturday.

BL

&&

.LONG TERM FORECAST (Days 4 through 7: Sunday through Wednesday)...

A broad upper level longwave trough extends across most of the
Bering into Western Alaska through the period. A number of
shortwaves rotate through the pattern and distort the trough
through midweek. Most of the active weather will be occurring over
the Gulf of Alaska with areas of precipitation, locally heavy
across coastal Southcentral Alaska. Precipitation starts from the
Southern end of the Eastern Kenai Peninsula across Prince William
Sound into the Copper River Valley. Locally heavy snow or mixed
rain, with rain along the coast. Snow should occur further inland.
While there is some risk of downslope drying, the troughs and
weak fronts pushing over the Southcentral portions of the state
may have some warm air aloft and bring local areas of freezing
precipitation Sunday, ending Christmas Day. A second front moving
into the coastal zones repeat the cycle Tuesday into Wednesday.
Gusty winds up to gale force are on tap for the marine areas
through the entire forecast period.

Most of the Bering Sea and Aleutian Chain will be within the
broad trough through the long-term period. Unsettled conditions
are expected throughout the Bering Sea and Aleutian Chain through
Wednesday. However, precipitation should be more showery in
nature. Where the uncertainty here is exactly where North Pacific
lows track before they enter the Gulf. Depending on how far north
of south they track in proximity to the Aleutian Chain and Alaska
Peninsula will determine if the Aleutians could see enhanced
precipitation from fronts or if they remain showery. As for
Southwest Alaska and the AKPEN, precipitation from two fronts
lifting into the Gulf Sunday and Tuesday especially for the AKPEN.
Conditions look to remain showery for Monday as northwest flow
kicks in along with cold air advection behind the fronts.

Kutz

&&

.AVIATION...

PANC...IFR and MVFR ceilings are likely through tomorrow morning.
The potential exists for fog development, but it looks more likely
for it to remain with just lower ceilings with minimal fog. Wind
over the terminal to remain fairly light and out of the north.

&&