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FXAK68 PAFC 191440
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
540 AM AKST Sun Jan 19 2025

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

Active weather is currently underway across Southcentral Alaska with
the primary weather concerns being 1) Strong Wind through Turnagain
Arm, Anchorage Hillside, and Upper Elevations of Eagle River and 2)
Heavy snow in Turnagain Pass and Thompson Pass.

GOES satellite imagery shows troughing extending from the eastern
Bering Sea southward into the Western Gulf of Alaska and into the
North Pacific. Strong upper-level support is evident, with a North-
South oriented jet streak exceeding 100kts at 250mb centered across
the Gulf of Alaska, promoting large-scale lift and surface
cyclogenesis along the effective triple point of the frontal
system off the coast of Kodiak Island. Moisture transport is
robust with this system, with uninterrupted deep southerly flow
stretching all the way from the Central Pacific, not too far from
Hawaii. In fact, the North American Ensemble Forecast System
(NAEFS) Mean Integrated Water Vapor Transport (IVT) shows values
near/at max percentiles across the Gulf of Alaska,
meeting/surpassing what`s been observed historically this time of
year. In other words, significant poleward atmospheric moisture
transport is currently underway, with the Chugach Mountains as the
recipient for this impressive atmospheric river.

Mosaic radar imagery shows precipitation covering much of the Gulf
of Alaska, as well as portions of the Cook Inlet into the Mat-Su
Valley. Snow levels appear to be in the 500-1000ft range across the
eastern Kenai Peninsula at the moment based on several observations
and webcams, with occasional heavier bands of precipitation dragging
the snow levels down closer to sea level, particularly evident
across Whittier-Portage at times. The Turnagain Pass SNOTEL site
which sits just a few hundred feet above the road level has averaged
nearly 1 inch an hour overnight, with temperatures hovering near 30
degrees. These snow rates will likely continue through the first
part of the morning before tapering down a bit as the atmospheric
river shifts east. A Winter Weather Advisory remains in place
through this evening, with the bulk of snowfall likely occurring
before noon.

Similarly, heavy snowfall is expected for Thompson Pass with 2 to 3
feet of snowfall expected. Uncertainty remains for Valdez, where low-
level warm air intrusion may result in a rain/freezing rain mix at
times. Can`t rule out mixed precipitation at times north through
the Edgerton Highway. Snow will be the primary type, though any
rain on cold and snow-covered surfaces will result in slick
conditions.

Moving farther north, 5 to 10 inches of snowfall can be expected
closer to Paxson and Isabel Pass, as well as from Talkeetna north
through Broad Pass. Like snowfall may linger into early Monday as
upslope southerly flow through these valleys is expected to
result in light upslope snowfall before dry air finally shuts
things down.

For Anchorage and the Mat Valley, while light precipitation has
already been observed, the best chance for accumulating snowfall
will be this afternoon into the evening with the passage of the
upper-level trough and the associated deformation band on the
backside of a eastern Kenai / western Prince William Sound low. The
most likely scenario is that of 0.5-1.0" inches of snowfall in a
quick period with temperatures hovering near or even above freezing,
limiting accumulation. However, quite a few outliers exist in
guidance suggesting 2-4 inches isn`t out of the realm of
possibility.

With regards to the wind threat, winds across the areas of concern
have been slow to materialize this morning with winds across Portage
Valley generally less than 40 mph and across the Anchorage Hillside
less than 30 mph. This may partially be attributed to the strong
down inlet gradient in place, as well as stable level air as
observed in the 12z ANC sounding. Will leave the high wind warning
hoisted as we often don`t see strong winds materialize until the
front or trough feature lifts north. Should winds begin to mix
down to the surface, it still remains possible for gusts in excess
of 70 mph to reach the Anchorage Hillside this morning.

Precipitation and winds abate Sunday night into Monday with upper-
level ridging shifting east, giving much of Southcentral AK a
reprieve in the active weather on Monday.

By Tuesday, yet another frontal system similar to today`s will begin
to pivot across the western Gulf of Alaska, bringing precipitation
first to Kodiak Island and then the eastern Kenai Peninsula. While
the details of the general synoptic wave pattern are a bit different
than the current system, expect generally similar sensible weather
with mild temps, low-level snow/rain, upper-level snow, and probably
no snow for Anchorage.

-Brown

&&


.SHORT TERM FORECAST SOUTHWEST ALASKA AND THE BERING
SEA/ALEUTIANS (Sunday morning to Wednesday morning)...

A compact surface low is lifting north across the Kuskokwim
Delta, creating areas of light precipitation across Southwest.
Light snow will fall at higher elevations, while areas of light
rain may be found across the rest of the region.

Meanwhile, a North Pacific low is bringing gale force winds and
rain to the western and central Aleutians today as its low center
tracks just south of the Aleutian Chain, sending its front into
the Bering Sea and Southwest mainland. Entrainment of colder mid-
level air from the central Bering will increase the potential for
snow in the Pribilof Islands, Nunivak Island and Kuskokwim Delta
this week. Wind gusts up to 45 mph and light to moderate snow
rates may cause periodic visibility reduction below one-half mile.
Expect snowfall totals up to 6 inches between Monday morning and
Tuesday morning for the Pribilof Islands.

This system crosses the AKPen and lifts toward the Kuskokwim
Delta Tuesday and Wednesday. The next North Pacific low approaches
the Western Aleutians on Wednesday morning. Current model
projections indicate high confidence that precipitation could
continue through the end of the week for Southwest Alaska, with
lower confidence concerning precipitation type and the exact
tracks beyond Wednesday.


&&


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

The active weather pattern over Alaska will continue as a number
of upper level shortwaves rotate across the region through the
forecast period. Late in the week, additional energy from a
transient North Pacific low will add to the movement.

Some lingering areas of rain moving across the southcentral
coasts Wednesday will have a brief reprieve Thursday. The next low
brings locally heavy rain, or a rain and snow mix spreading
across southwest Alaska, the northern AKPEN and Kodiak Island for
Friday, with rain and snow moving into the Kenai and south central
Alaska for Saturday.


Baker

&&

AVIATION...

PANC...


Northerly to northwesterly winds continue through this morning.
However, southeasterly to southerly winds ranging from 30 to 50
kt from 1500 ft to 2500 ft continuing the likelihood of low-
level wind shear into Sunday afternoon. Light precipitation
causing VFR to IFR conditions this morning and afternoon before
1500 ft to 2500 ft winds shift southerly to southeasterly wind
shift and IFR conditions return this evening.


&&

$$