Gale Warning 806, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
442 PM AKST Mon Dec 11 2023

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

Bimodal weather exists on either side of the Prince William Sound
this afternoon, and will remain that way through Wednesday morning.
The high amplitude trough remains over Southcentral, with the
focus of the atmospheric river and warm sector of the trough over
the Central Chugach and Copper Valley. Meanwhile, on the western
side of the Sound, cold air advection and some instability has
lead to continued showers from the Kenai P. north into the Mat-Su.
A warm nose left over from last nights strong cross-barrier flow
has lead to some freezing rain and sleet over the western Kenai
and Anchorage, but this threat should end quickly this evening as
colder air returns. Expect showers to continue overnight for
these areas, with snow accumulation around a half inch possible.

The main focus of this forecast package in the short term is from
the Valdez area north into the Copper Valley. Low level warm air
has intruded well north into the Valley with southerly winds. This
has risen snow levels to approximately 1500 ft today, causing
difficult travel in the southern basin due to a combination of wet
snow, slush and rain falling onto frozen roadways. These snow
levels will slowly drop overnight as cold air moves in from the
west, returning conditions to snow in Valdez later this evening,
and to Chitina Tuesday morning. Precipitation rates will also
increase overnight again, resulting in heavy snow for the Copper
Valley and Valdez as a surface low moves south of Cordova. Models
finally decreased in their spread in low track, providing higher
confidence that a band of snow will also develop over the central
Copper Valley, extending north to Paxson. As the low moves inland,
precipitation rates will decrease Tuesday afternoon.

For many, the weather may seem like one long duration storm, but
the past 3-5 days have been the combination of many low pressure
systems moving into the Gulf in rapid succession. To continue
that theme, yet another low will in the southeastern Gulf Tuesday
afternoon will pivot north and west along the coast into the
Sound on Wednesday. This will bring another round of snow to
coastal areas, and provide higher chances for more light snow from
the western Kenai north to Anchorage and the Mat-Su. This system
is occluded, and should be comfortably snow.

-Jones

&&

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

The low that brought blizzard conditions to the Bristol Bay region
yesterday has become elongated across the Western Alaska Range
this evening. Moisture wrapping around the back side of the low
will contributed to continued light snow across portions of
Bristol Bay and the Lower Kuskokwim Valley tonight as the low
slowly pushes into Southcentral Alaska. Light snow will hold on
longest in the Lower Kuskokwim Valley, but gradually tapers off
through Tuesday as northerly flow brings colder, drier air into
the region.

The continuous northerly flow will have multiple effects. First,
northerly winds gusting to 40 mph will continue to lead to
blowing/drifting snow in the Kuskokwim Delta, despite no new
snowfall from Bethel west. Second, the cold air streaming across
the relatively warmer waters of the Bering Sea will continue to
produce snow showers for the Pribilof Islands, the Alaska
Peninsula, and the eastern Aleutian Islands. This will lead to
intermittent reductions in visibility as low as one quarter mile.
There is a lack of strong shortwaves which would lead to an
enhancement to steady snow and more prolonged periods of
visibility less than one mile. Finally, temperatures across
Southwest Alaska, broadly in the single digits above zero will
drop into the single digits below zero beginning Tuesday night
through at least Thursday. Combined with northerly winds across
the area, wind chills will hover between -20 and -30F.

Meanwhile out west, a compact low along a front moving into the
western Bering Sea this evening brings rain and a rain/snow mix to
the western Aleutians tonight. The low produces gales through
Tuesday morning until it weakens and dips south into the Pacific.

By Tuesday night, a storm force low tracks over the far western
Bering, pushing its front to the central Aleutians/Bering by
Wednesday night. This will bring a wedge of warm air across the
Aleutians, along with a changeover from snow back to rain. As the
front pushes east, it erodes high pressure over the central Bering
which has been aiding in drawing colder air southward. As a
result, northerly flow begins to weaken and winds diminish over
Southwest Alaska and the eastern Bering. This will reduce the
intensity of snow showers along the Alaska Peninsula and eastern
Aleutians, and reduce wind in the Kuskokwim Delta, which will
allow visibility to improve in these regions on Wednesday.

For Thursday, there is increasing uncertainty with the front
moving into the Pribilof Islands. Strong southeasterly winds may
combine with snow falling initially with the front to
significantly reduce visibility. However, a southeasterly wind
direction does not typically favor blowing snow with visibilities
of one half mile or less, and precipitation type may eventually
turn over to rain. With warming temperatures on Thursday, this
threat currently appears unlikely.

CQ

&&

.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 a showery weather regime in Cook Inlet this evening
through Tuesday morning. There could be a rain/snow/Ice Pellets
shower at almost anytime, causing a brief reduction in ceiling/vis.
However, the most likely time to see snow showers will be between
06Z and 15Z out ahead of an upper low/trough lifting northward up
Cook Inlet. Mixed precipitation showers will transition to all snow
showers later this evening. 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 Tuesday night.

As for winds, southerly winds will prevail this evening. They
will mostly be less than 10 kts, but there could be a few gusts as
showers move through. Winds will lighten and shift around to
northerly Tuesday morning in response to a low moving into the
Gulf.

&&