Gale Warning 38, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
541 AM AKST Fri Jan 10 2025

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

Key Points:

* A strong upper level jet stream will steer two strong storm
  systems from the North Pacific to southern Alaska tonight
  through Sunday night, leading to widespread impacts to travel
  from wind and precipitation.

* There is potential for a damaging Chinook windstorm with the
  second storm on Sunday. The most likely areas to be impacted are
  Anchorage up to Eagle River and Chugiak as well as in the
  Matanuska Valley. Stay tuned as the storm track and location at
  which the storm is at its strongest is key to whether a damaging
  windstorm will occur.

* No matter what, both storms will bring some gusty winds to most
  of the region, leading to widespread above freezing
  temperatures.

* Coastal areas will see periods of heavy rain at lower elevations
  and heavy snow in the mountains. Inland areas will see much less
  precipitation, though there will be brief periods of
  precipitation spreading inland.

* Colder air will move in behind the second storm system Sunday
  night into Monday. All signs point to a shift in the overall
  pattern, with a return to more seasonable temperatures and
  chances of snow for Southcentral Alaska next week.

The upper level flow is highly amplified across the North Pacific,
Alaska and North America, with a strong and wavy jet stream.
Models indicate a 200kt+ subtropical jet streak across the North
Pacific, with a downstream ridge over the Northeast Pacific up to
the Gulf of Alaska. This is a great set-up for cyclogenesis over
the North-central Pacific, with a storm track northward to
southern Alaska. With the table set, today's weather looks quite
tame, with a weak short-wave trough tracking rapidly northward
across the Gulf and Southcentral. This will produce areas of light
precipitation, mostly in the form of snow, generally of very
short duration for any one location in Southcentral. Thus, expect
little in the way of snow accumulation.

Not far behind this, a deepening low over the North Pacific will
track northward toward the Alaska Peninsula and western Gulf/Kodiak
Island today. As has been the case with recent storm systems, the
strong upper jet will drive the leading frontal system well out
ahead of the low center. This will lead to development of a triple
point low north of the parent low and ultimately result in a more
complex storm system (and more difficult forecast). The upper low
center will then elongate and open up into a large negatively
tilted trough tonight. This evolution leads to a much larger storm
system with widespread winds and precipitation affecting all of
southern AK.

Expect moderate to heavy precipitation to spread to Kodiak Island
and the Southcentral coast tonight. For the coastal waters this
will bring another round of widespread gale to storm force winds.
The leading occluded front will push onshore late tonight/early
Saturday, leading to strong gap winds and winds spreading inland.
This in turn will cause temperatures to rise back above freezing
across most of the region. Inland areas will be drier due to
strong downslope flow across the coastal mountains. The one
notable exception is the western Susitna Valley, which will see
steady precipitation in strong upslope flow. For lower elevations,
look for precipitation to start as snow, but likely mix with rain
as winds and warmer air progress inland late tonight.

Meanwhile, the opening up of the upper low center into a trough
will cause the triple point low to track into Bristol Bay while
the parent low crosses the western Gulf. Prior days model runs
indicated a low track up Cook Inlet, but there has been an
eastward trend in recent model runs - with some guidance now
tracking the low inland across the eastern Kenai Peninsula. This
results in some uncertainty in how strong the winds will get in
Southcentral. A track up Cook Inlet and the western Kenai would
lead to stronger winds for Turnagain Arm, Anchorage, and the
Matanuska Valley while a further east track would not be nearly
as strong. For now, have a compromise forecast which allows us to
pivot either direction.

More concerning is the next storm system moving in for Saturday
night and Sunday. Everything about this system looks more
impressive: a larger and deeper storm system with more amplified
flow (and a 100 kt southerly jet pointed at Southcentral). The
track is also concerning, with a fairly fast northward storm
motion across the southern Alaska Peninsula Saturday night and up
to the Yukon Delta by Sunday afternoon. This set-up is favorable
for downslope "Chinook" windstorms in Anchorage. Some of the keys
to weather a windstorm occurs include the timing of the low
crossing 60N latitude compared with timing of the occluded front
pushing onshore of Southcentral; whether the low is weakening or
strengthening as it tracks northward across Southwest AK; and the
stability profile near and above mountain- top in the Front Range
Chugach. Right now, most of the factors look favorable for a
windstorm. However, as anyone who follows the weather knows, this
can easily change. So we will closely monitor this potential and
hoist High Wind Watches soon if nothing fundamentally changes.

Aside from the wind threat, precipitation looks very heavy along
coastal areas Saturday night through Sunday. This could lead to
localized ponding of water or even minor flooding as rain falls
in the lower elevations. With such a dynamic set-up with high
atmospheric moisture content, expect precipitation to spread
inland Sunday, falling primarily in the form of rain. This will
soak into and remaining snow/ice covered roadways. As the storm
continues northward across western AK Sunday night and Monday,
Arctic air will wrap around the south side of the low across
southern AK, likely bringing a quick shot of snow before
precipitation ends. This looks like it will lead to larger shift
in the pattern, bringing an end to the prolonged period of warm
and windy conditions with little snowfall.

-SEB

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days
1 through 3: Today through Sunday night)...

A weakening gale-force low is centered just south of the Pribilof
Islands this morning. Its front extends across the Kuskokwim Delta
and Bristol Bay, producing widespread light snowfall. The front
lifts north over the next several hours, pushing into the Lower
Kuskokwim Valley and bringing snow accumulations up to around an
inch through this afternoon. Behind the front, limited
precipitation is expected for Bristol Bay, so expect snow to
quickly taper off with a mostly dry day ahead. The Kuskokwim
Delta, however, will continue to see snow showers through the day
as the low center approaches. Easterly winds shift to
southeasterly and allow temperatures to warm to just above
freezing today, and this may limit snowfall accumulations in some
places and allow rain to mix in with snow. To the southeast of the
low center, gusts as high as 65 mph are possible early this
morning in the eastern Aleutians, though this quickly diminishes
through the day. The strongest winds shift into the Kuskokwim
Delta where winds pick up this afternoon. Along the coast,
southeasterly onshore winds gust up to 50 mph this afternoon and
evening, leading to potential for storm surge.

While there is more sea ice in place along the coast compared to
what we had for the last coastal flood event, there may still not
be enough stronger, shore-fast ice to protect coastal communities
from a possible storm surge with this low. Even where stronger ice
exists, water could pool up through cracks and cause localized
flooding for communities along the immediate coast near high on
Friday evening. Confidence on the degree of flooding is low, but
concern is high enough that a Coastal Flood Advisory remains in
effect for the Kuskokwim Delta Coast beginning at noon. The main
flooding threat occurs around high tide this evening, where water
levels could reach up to four feet above the normal highest tide
line. The low weakens and exits into the northern Bering Sea
Saturday morning, allowing the coastal flood threat to subside.

As the first low exits, another hurricane force low in the North
Pacific tracks toward Kodiak Island. A front from this second low
pushes across the Alaska Peninsula into Bristol Bay this evening,
bringing another round of light snow to the area. The front lifts
to the north and west, spreading snow into the Kuskokwim Delta
early Saturday morning. Some light showers, mostly in the form of
rain, are possible to the east of the front, particularly in
portions of Bristol Bay east of Dillingham. Warm air has the
potential to work in and turn snow to rain at times as
temperatures remain above freezing in Bristol Bay, though where
precipitation remains heavy it will likely remain snow. There is
potential for snow to persist for much of the night from
Dillingham west, which could result in snow totals of 3 to 4
inches, though most locations across Southwest Alaska are
expected to receive closer to 1 or 2 through Saturday. The front
lifts north across Southwest Alaska on Saturday as some colder air
works in behind the exiting low and temperatures in the Kuskokwim
Delta may drop back into the 20s.

A third, stronger and larger low pressure system enters the
eastern Bering Sea from Saturday into Saturday night. Strong
southeast winds will spread into Southwest Alaska through the
Kamishak Gap into the greater Bristol Bay region along with rain
and snow moving back across much of the area as well. Northerly
gale force winds set up across the Pribilof Islands and eastern
Aleutians, bringing in a colder air mass from the western Bering
Sea. Much of the model guidance situates the western periphery of
a front right along the Pribilof Islands, which leads to
significant uncertainty in the potential for low visibility and
blowing snow, with any westward shift in track favoring this and
an eastward shift making it virtually a non-issue. Moderate
snowfall, with around 6 inches of snow accumulation will also be
possible for Nunivak Island and the Kuskokwim Delta coast from
Kipnuk north. Stay tuned as we hone in on the forecast track and
resulting impacts for this third low.

Quesada

&&

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

A very active weather pattern continues across Southern Alaska
through the forecast period. The upper longwave trough over the
Bering gains energy from several shortwaves and reorients over
Mainland Alaska into the Gulf of Alaska by Thursday. These
shortwaves support a well developed surface low moving through the
Bering Strait into the Russian Arctic, with its associated front
extending across Western Alaska and along the Southcentral coast.
Deterministic GFS, ECMWF and Canadian models carry the features
and changes well through Tuesday before the ensemble means take
over through the week.

Cold air from the backside of the low keeps areas of snow and
gusty winds across the Bering into Southwest Alaska, Pribilofs and
the Northern Alaska Peninsula. The warm sector weather presses
locally heavy rains and gusty winds along the Southcentral coasts,
with areas of locally heavy snow over inland and higher
elevations through Monday. An Eastern North Pacific low tracks
into the Copper River Delta for Tuesday before slipping into
Prince William Sound and dissipating late Tuesday, prolonging the
heavier precipitation and gusty offshore winds for that area. A
new North Pacific low and front spreads areas of moderate rain or
rain and snow mixed over the Eastern Aleutians Tuesday, becoming
areas of heavy rain or rain/snow mixed over the AKPEN, Kodiak
Island and Southwest Alaska through Thursday. A second Eastern
North Pacific low brings snow to the Southcentral coasts before
changing to rain later on Thursday.

&&

.AVIATION...

PANC...A fast moving upper level short-wave will bring a quick
shot of light snow around 18Z this morning. At the same time, a
weak Turnagain Arm jet will begin to form, bringing some gusty
southeasterlies into the terminal. The snow may move through
before winds strengthen, in which case ceiling/vis will likely
be in the MVFR category. If wind moves in with the snow, then IFR
visibility is likely. In any case, the short-wave will move
really fast, so period of snowfall could be as short as one hour.

Winds will further strengthen this afternoon ahead of a storm
system moving into the southwestern Gulf, leading to VFR
conditions - possibly even a break in clouds for a while. As the
storm continues northward this evening, the Turnagain Arm jet will
strengthen further and begin to shift southward across the
northern Inlet. This should bring an end to gusty southeasterlies
at the terminal. However, surface winds will likely shift around
to WNW, leading to strong low level wind shear. There is some
uncertainty in the low track as it heads inland across
Southcentral Saturday morning, but no matter where it tracks
conditions look favorable to bring gusty southeasterlies back
into the terminal.

&&

Preliminary Local Storm Report
National Weather Service Anchorage AK
1053 AM AKST Fri Jan 10 2025

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0753 AM     Rain             6 SE Valdez             61.11N 146.21W
01/10/2025  M0.20 Inch       Chugach            AK   Official NWS Obs

            Temp 33.6 F.

&&

mb