Brisk Wind Advisory 84, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
441 PM AKST Mon Dec 26 2022

.ANALYSIS AND UPPER LEVELS...

A trough encompasses nearly all of Alaska, with a ridge to the
north extending from eastern Russia to the North Slope of Alaska.
A large upper level low centered over the western Aleutians and
southern Bering Sea anchors the upper trough. There are some
snow showers associated with this low affecting the western to
central Aleutians. In general, weather across the Bering Sea and
Aleutians is quite benign.

A trough axis extends east from the upper low center across the
southern Bering Sea and then southeastward across the southern
Gulf of Alaska. This entire trough is lifting northward toward
southern Alaska. Just ahead of the trough, a vertically stacked
low is centered over the northwestern Gulf. The upper center is
being pulled westward toward Bristol Bay while the surface center
is drifting northeastward toward the eastern Kenai Peninsula.
The original occluded front pushed well onshore of southern
Alaska, with warmer air aloft advecting across all of Southcentral
and interior Southwest Alaska. Most of the precipitation in
Southcentral is along the coast where temperatures have warmed
above freezing. However, thanks to a strong 850mb jet riding over
top of a cold dome sitting in the lee of the coastal mountains,
light precipitation has been making it over the mountains, falling
in the form of freezing rain over portions of the western Kenai
Peninsula and Anchorage. Meanwhile, snow and blowing snow continue
through Thompson Pass, with significant thermal and pressure
gradients along the bordering Copper River Basin mountains helping
to maintain strong winds. These gradients are also leading to
continued gusty winds over portions of the Matanuska Valley. A
trough lifting northward to the east of the remnant surface low
center is also maintaining gale force easterlies across much of
the northern Gulf coastal waters. On the back side of this system,
light snow is falling across much of the Bristol Bay region. Weak
cold air advection with the upper trough lifting toward southern
Alaska is combining with a low level jet to produce some gusty
westerly winds through bays and passes along the Alaska Peninsula
and into Shelikof Strait and Kodiak Island.

&&

.MODEL DISCUSSION...
Models are generally struggling with handling of weak short-waves
embedded within the long-wave trough. This creates an especially
challenging precipitation forecast for southern Alaska tonight
through Tuesday. Models vary in their precipitation fields,
especially with how much precipitation will make it over the
coastal mountains to the western Kenai Peninsula and Anchorage.
Add in the precipitation type challenges and that leads to below
normal forecast confidence.

There is also some model spread with the track of a low lifting
out of the North Pacific on Wednesday, with the NAM/GFS tracking
this low into the southern Bering Sea and the ECMWF/Canadian
solutions keeping it just south of the Alaska Peninsula. This
will affect the wind and precipitation distribution across
Southwest Alaska and the eastern Bering Sea Wednesday/Thursday.
Additional differences develop downstream with formation of a
new low over the Gulf on Thursday. Bottom line, there is quite
a bit of forecast uncertainty for the first three days of the
forecast, so check back for updates.

&&

.AVIATION...
PANC...VFR conditions and north winds will persist. Weak upper
level waves passing overhead this afternoon and evening could
bring a brief period of light freezing rain but expect dry
conditions to dominate for most of the TAF period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: This
afternoon through Thursday afternoon)...

A low pressure system centered just east of the Barren Islands
continues to be the main weather-maker for Southcentral Alaska in
the near term. Freezing rain is the main concern with this system
as warm air has worked in aloft while surface temperatures remain
below freezing in interior locations. Waves of moisture will continue
to lift up Cook Inlet tonight through the day Tuesday as a negatively
tilted upper trough lifts northward. Temperatures are expected to
hover around freezing tonight for the Western Kenai Peninsula as
rain re-develops and lasts through overnight hours before turning
over to snow as colder air works back in. In response, a Winter
Weather Advisory remains out until Tuesday morning highlighting
the freezing rain threat. Ice accumulations are only expected to
be around a glaze; but if it takes longer to cool temperatures, a
few hundredths to around a tenth of an inch could occur. A Winter
Weather Advisory also remains out for Anchorage as light freezing
rain/freezing drizzle will continue into the overnight hours
tonight, especially for the Hillside areas. As the occluded low's
front continues to lash the coast of Southcentral, coastal
locations continue to see rain while interior locations see snow.
Blowing snow continues to be an issue for Thompson Pass and a
Winter Weather Advisory remains out for that area until Tuesday
morning to highlight the reduced visibilities and additional light
snow accumulation. Temperatures will cool down aloft over the
coastal locations by Wednesday morning as cold air works in on the
backside of the northward advancing upper trough. Coastal areas
will see off and on rain/snow mix as southerly and southeasterly
upslope flow combines with additional shortwaves Wednesday
afternoon through Thursday morning.

The next notable weather system approaches from the south on
Wednesday night as an upper trough lifts northward from the North
Pacific into the Gulf of Alaska. Kodiak Island will be the first
to receive precipitation Wednesday night through the day on
Thursday as the associated deep surface low and front lift
northward to the west-central Gulf. The majority of coastal Kodiak
Island will see mostly rain, while mountainous areas will be
mostly snow. Temperatures will cool down on the backside of the
low as it moves northward through Thursday afternoon and coastal
locations could switch over to a brief period of rain/snow mix
before precipitation tapers off. This surface low and front will
then start affecting southern and eastern Kenai Peninsula and
western Prince William Sound Thursday morning into the afternoon
hours. Please pay attention to additional forecast updates as this
feature still has much uncertainty that surrounds it as far as
track, precipitation amounts, and precipitation type is concerned
for the rest of mainland Southcentral.

&&

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

Light snow will continue for the Bristol Bay region tonight into
Tuesday morning as a front lifts northward across Southwest
Alaska. Overrunning moisture from a low in the northwestern Gulf
of Alaska will feed a band of snow along the front, enhanced by a
shift to onshore and upslope flow this evening, which will bring 2
to 4 additional inches of snow accumulation to Bristol Bay from
King Salmon northward through Tuesday. The front will be
accompanied by weak upper-level dynamics, meaning very little snow
will spill over into the Kuskokwim Valley and Delta regions.
As the front pushes northward, onshore flow will allow
temperatures to warm slowly tonight across Southwest Alaska. A
shortwave ridge builds in overhead on Tuesday, bringing clearer
and drier conditions. Offshore flow returns Tuesday night as a
deep North Pacific low approaches the southern Alaska Peninsula.
This, in combination with clearer skies overnight will allow for
a brief dip back to colder temperatures. This looks to reverse on
Wednesday with the approach of the next front. Easterly flow
develops across Southwest Alaska on Wednesday morning ahead of the
next low and front moving across the Alaska Peninsula, causing a
quick rise of temperatures into the 20s and 30s for Bristol Bay
and in the lee of mountain ranges. There is some uncertainty with
how far north this front gets as it continues northward on
Thursday, which leads to uncertainty in the northerly extent of
stronger winds, heavier snow, and therefore whether blowing snow
will be an issue along the northern Bristol Bay coast. Stay tuned
for further refinements to the forecast.

&&

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

A strong upper-level low will track southward across the western
and central Aleutian Islands through Wednesday as cold air and
snow showers stream southward across the Bering Sea. To the east
of its center, the low will pull a series of meso-lows northward
into the Bering. A stronger low then approaches the eastern
Aleutians and southern Alaska Peninsula late Tuesday, bringing in
warmer air and moderate to strong easterly winds through
Wednesday. This system becomes the dominant low as the upper-
level trough elongates along the Aleutian Chain. Wednesday night,
uncertainty increases as to whether the low tracks across the
islands into the Bering Sea or remains over the North Pacific. The
positioning of this low will be important in determining blowing
snow potential for the Pribilof Islands, which could see
visibilities lower than one mile with a more northerly track
allowing for near gale force northeasterly winds to coincide with
snowfall on Thursday. The western Aleutians will continue to see
intermittent snow showers as cold air remains in place.

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

The latest operational models and ensemble means are in fairly
decent agreement with an upper closed low over the eastern
Aleutians and Alaska Peninsula. While the upper low sits nearly
stationary over the weekend, a strong North Pacific low rotates
around the low, moving northeastward past the Aleutians and into
the Gulf of Alaska. This will bring an unsettled pattern for all
of the coastal areas of Southcentral heading into early next week.
While specific details still need to ironed out, persistent
onshore flow will favor moderate to heavy precipitation for the
coast, while much of the mainland will be relatively dry during
the period. For Southwest AK, precipitation looks to be favorable
for the AKPEN, while much of the inland areas will be downsloped
due to the southeasterly flow. A general troughing pattern out
west will result in a wet pattern for much of the Bering Sea and
Aleutians. Cold, northerly flow over the relatively warm waters
will also help generate widespread showers over the Pribilof
Islands and along the Aleutian Chain.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 101 111 131.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...DN
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CQ
LONG TERM...MF