Gale Warning 975, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
653 AM AKST Fri Dec 27 2024

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

A return to seasonable weather with a few passing chances of
light snow are back underway across Southcentral Alaska. Satellite
imagery shows a well-defined, though low-amplitude, shortwave
trough gradually shifting eastward and lifting up through
Southcentral Alaska. Kenai Radar (PAHG) shows snow showers have
developed across portions of the Susitna Valley and Cook Inlet in
response to this wave. Light snow is expected to continue in this
area for much of the day, though snow rates / total snow
accumulation is expected to be limited without stronger upper-
level support. Farther south and east, precipitation in the
Northern Gulf / Prince William Sound area has been been focused
along a weak surface trough axis as it propagates westward. As it
does so, interaction with the aforementioned upper-level shortwave
may help enhance snowfall briefly later today across the Eastern
Turnagain Arm and western Chugach. Between these features, there's
a non-zero chance of some snowfall for Anchorage today. In fact,
latest model trends have ticked up ever so slightly, though any
accumulation should be minimal.

Farther east, webcams show Valdez up through and just north of
Thompson Pass continue to have snowfall. Another 1 to 2 inches for
the pass is likely by noon, with lesser accumulation for Valdez.
The Copper River Basin is likely to have snow as well, though
accumulation is likely minimal outside of the higher elevations
and up towards Isabel Pass, where snow is likely to persist
through the day with another inch or so of snowfall.

By this evening and through the overnight hours, another area of
low pressure drifting north through the Gulf of Alaska will renew
precipitation for much of the areas in the Prince William Sound
such as Valdez and Thompson Pass. Snow levels will be at or very
close to sea level in most locations with approximately 1 to 3
inches of snow forecast with this round.

Colder air will gradually filter into much of the area this
weekend. Cold high pressure building across the Copper River Basin
coincident with low in the Gulf of Alaska will result in
outflow/gap winds into the Prince William Sound to increase on
Sunday. Thompson Pass and/or any other locales with transportable
snow in these gaps and passes may experience times of drifting and
blowing snow. Otherwise, cooler and quiet weather is anticipated
for much of the area.

-Brown

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days
1 through 3: Friday through Monday Morning)...

Northerly flow that has affected much of Southwest Alaska and the
Bering Sea continues to diminish in strength overall as the
pressure gradient within the flow lessens and the flow transitions
further southeast with a more continental source. This means that
while winds have diminished overall, precipitation has continued
to become more showery in nature, but that cold air will continue
to seep south into the southwest mainland, bringing high
temperatures near or just below zero to a a large swath of the
Kuskokwim Delta, from Kipnuk to Lime Village, into next week. Also
worth noting is the large swath of freezing spray that persists
across the Bering, with areas of heavy freezing spray and extreme
freezing spray within 50 miles of the ice edge in and around
Nunivak Island and the Kuskokwim Delta over the next several
days. The region of freezing spray focuses towards the Alaska
coastline by Sunday as northerly winds turn southerly as the next
system effects the Bering.

This next low moves north out of the North Pacific, bringing
southerly gale-force winds to the Western Aleutians as early as
today as the low propagates a front out ahead of it. Along the
southerly edge of this front, and just south of the Aleutian
Chain, a 965-975 low will try to form as it swings north and
across the Central Aleutians on Saturday. Model agreement has been
questionable and ensemble guidance hasn't offered much
assistance, but south to southeasterly winds of at least gale-
strength seems to be likely on Saturday for areas east of this
progged low, including much of the Central Aleutians. Further east
towards Unalaska, winds maintain some of gusty nature, but the
low diminishes in strength as it transitions north into the Bering
Saturday afternoon, beginning a Fujiwara dance with the parent
low and eventually becoming absorbed into a larger and more
disorganized Bering Sea low, while upper-level support wanes.
Easterly flow over the Southwest Mainland however, will lead to
clearing skies for the weekend and warming temperatures by the
beginning of next week.

By Sunday, the large low will cover the entirety of the Bering,
as shortwaves embedded within the flow propagate counter-
clockwise around it. This means gale-force winds will continue to
affect a majority of the Bering and the Aleutians. This pattern
will invite another low to form along the southern edge of an
elongated trough, that may again propagate north into the Bering,
this time across the Eastern Aleutian Chain / Southern AKPen. This
low looks to be a bit weaker at this time, though will continue
gale-force winds along the low's eastern edge, through Bristol Bay
and north along Alaska'a coast.

AB

&&

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

A broad upper level trough extending down from the Arctic and
into the central Gulf will remain entrenched until it starts to
break down Wednesday. The ridge will migrate north and westward. A
broad surface low over the Gulf will provide the opportunity for
coastal precipitation across Southcentral until Saturday evening
as the low weakens and moved up into central Alaska by Wednesday.
Conditions improve over the SouthCentral region through the middle
of the week. Over interior Southcentral and Southwest, a
considerable cool down in temperatures will take place as the
trough axis shifts eastward over the weekend, with lows dropping
below zero into early next week.

Behind the weakening ridge in the Bering is a front of a
Kamchatka low moving over the western Aleutians. The low center
will likely remain to the west of the forecast area, however model
guidance suggests the formation of a strong triple point low
forming in the North Pacific and moving over the Central Aleutians
late Sunday and into Monday afternoon. A core of gale-to-storm
force winds is possible near its center as it moves northward into
the western Bering. As this takes place, the trough axis
originating from the parent Kamchatka low will extend south and
east towards the North Pacific and western Gulf as the previously
mentioned Alaska/Canada ridge extends to the north and west over
mainland Alaska through the remainder of the extended forecast
period. Several surface lows will form along the trough axis
through the end of the period, though the placement and intensity
of them is still uncertain.

DD

&&

.AVIATION...

PANC...The Anchorage area will remain under the influence of weak
troughing through the TAF package. Low-level moisture and weak
flow at and near the surface will allow for low clouds and very
light snow to redevelop today. Much like yesterday, there may be
occasional breaks in both clouds and snowfall as these weak
upper-level waves move through then exit the region. Thus, while
the pattern overall will support redevelopment of occasional very
light snow, often with ceilings and visibility dropping to low
MVFR or IFR, there is low confidence on the exact timing of each
wave, each period of snow, and when current VFR ceilings and vis
reduce down to MVFR or IFR.

Much like the overnight hours, the redevelopment of a low stratus
deck and/or patchy fog is possible overnight tonight into early
Saturday morning. Very light snow and/or freezing drizzle is
possible from any low stratus deck developing overnight. However,
amount and type of precipitation is low confidence.

&&