Small Craft Advisory 68, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
507 PM AKST Mon Jan 20 2025

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

Active weather persists as several storms move across Southcentral
in the coming days, with the strongest being the front that will
move through from tonight into tomorrow. Expect prolonged
precipitation along the coast, with very little snow for locations
along Cook Inlet and the interior. Confidence is lower with
precipitation types as temperatures hover around freezing, but
generally expect rain for communities along the immediate Gulf
coast and Prince William Sound as snow levels rise to 1000-1500 ft
in these locations. The biggest hazards we'll be monitoring are
the potential for (a) snowfall rates approaching 1 inch per hour
in Turnagain Pass, and (b) gusts as high as 50-65 mph along parts
of Turnagain Arm and Turnagain Pass, both with the front moving in
tomorrow.

Diving into the details... a gale force front sweeps across the
Gulf from tonight through late tomorrow night, bringing strong
winds and moderate to heavy precipitation. This front will bring
about many of the same impacts as the last, though perhaps a bit
weaker and quicker moving. Ridgetop winds will once again be
rather strong, with gusts in excess of 60 mph not out of the
question of the Anchorage Hillside and other higher elevation
areas. Precipitation will largely be confined to the typical
windward locations of the eastern Kenai Peninsula and south slopes
of the Prince William Sound and Chugach Mountains. With good model
agreement, forecast confidence is fairly high with this front
though, as mentioned above, there remains uncertainty with
precipitation types. This may affect forecast snow amounts.

After the front moves through Tuesday night, several weaker storms
will follow. This will prolong precipitation along the coast, and
bring the potential for precipitation to make it into interior
Southcentral. Weaker winds, as well as flow that is more
southwesterly or westerly, will minimize any downsloping that
tends to keep precipitation out of Western Kenai Peninsula,
Anchorage, and Mat Valley. However, weaker systems tend to be more
difficult for models to hone in on, so there remains uncertainty
with the timing of potential snowfall. Still, any precipitation
that does make it into the interior is expected to be fairly
minimal for all but northern Susitna Valley.

-Brown/Chen

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA AND THE BERING
SEA/ALEUTIANS (Monday afternoon to Thursday afternoon)...

The Winter Weather advisory remains in effect in the Pribilof
Islands until Tuesday morning as snow and gusty winds continue to
impact the islands. This is due to an occluded front that is
impacting the Bering from a low south of the Aleutian Islands. In
the Aleutian chain itself, temperatures are too warm for snow at
lower elevations, so precipitation falling there is mostly in the
form of rain. Over the course of the day, the low will split into
two and the new one will move into the mainland by Tuesday
evening. This will set the stage for another round of
precipitation for the southwest mainland. However, due to the
orientation of the winds (southeasterly), parts of the Bristol Bay
region (a triangle from King Salmon, Dillingham, to Iliamna) will
be downsloped which means lower chances for precipitation and
less accumulations. Furthermore, the southerly flow will allow
warmer air to continue filtering into the southwest, meaning
precipitation will be mostly in the form of rain at lower
elevations. Gusty winds will also impact the southwest mainland as
the low moves through. Areas of precipitation and gusty winds
will continue until Wednesday afternoon when a small ridge moves
in. Cooler air will accompany the ridge, so high temperatures will
drop below freezing mainly north of Dillingham by Wednesday.

Out in the Bering, a Kamchatka low moves into the western Bering
by Wednesday morning. A trough attached to the low will dip into
the Aleutian Islands as the low travels along the Kamchatka
coastline. This will bring precipitation and high-end gale force
winds to the Bering as the trough tracks eastward. Cold air will
wrap around the back of the system, so snow showers may follow
behind the trough. Come Wednesday night/Thursday morning, a
compact low within an atmospheric river will barrel into the
Bering, quickly pushing the Kamchatka low northwards and driving
the trough into the mainland. The mainland could see some snowfall
before warmer air filters in behind the trough. Wind speeds will
soar into storm force category in the central Aleutians by
Thursday afternoon. Heavy precipitation mostly in the form of rain
will also accompany this low. Looking a bit ahead reveals that
this core of storm force winds will track into the Kuskokwim
coast, which could drive coastal flooding into communities such as
Kwigillingok and Kongiganak by Thursday night into Friday. Behind
this low and atmospheric river is a cold air mass that will move
into the western Aleutians from Siberia by Thursday evening. This
much colder air could have implications for the near future.

-JAR

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

KEY POINTS:

- A highly amplified and potentially impactful weather pattern is
  expected to affect much of the forecast area from Friday into
  this weekend.

- Strong winds, heavy precipitation and very warm temperatures
  will be possible over much of southern Alaska from Friday to
  Monday as a strong frontal system moves across the region.

- Much colder air will move in from the northwest behind the heavy
  precipitation early next week.

An extremely active, highly unusual and potentially highly
impactful pattern will be well underway at the start of the period
on Friday morning. A very strong longwave ridge is expected to be
in place over the Northeast Pacific on Friday morning, with the
crest of this ridge extending well up into Mainland Alaska. To the
northwest, a deep longwave trough with an embedded closed low is
expected to be moving up into the far northern Bering Sea and up
into far northeastern Russia. Two dramatically contrasting air
masses will be building under the main upper trough and Pacific
ridge. A very warm air mass will be in place under and towards the
upstream side of the ridge, including much of the southern
Mainland out to the eastern Aleutians. Meanwhile, a much colder,
Arctic air mass will be building under the longwave trough across
much of the northern and western Bering up into Siberia.

The gradient in terms of temperature and pressure fields between
the two synoptic features outlined above, both near the surface
and the upper levels, will be quite impressive for even Alaska
standards. For example, the highest surface pressure under the
ridge in the northeast Pacific could be as high as 1045 mb,
contrasting with a surface low moving into the Bering that could
deepen to around 980 mb or lower - a difference of at least 65
mb. A strong baroclinic zone (an area of strong temperature and
pressure gradients) will exist between the main high pressure and
the low/trough in the Bering Sea, initially in place Friday
morning along an axis from the central Aleutians to the coastline
of western Alaska. This effective frontal zone will slowly advance
southeast with time throughout the weekend into early next week,
and this will largely be the focus of the most inclement
conditions with this pattern. The strong pressure gradient setting
up along this boundary, plus at least two distinct stronger areas
of low pressure that will move north along it, will support
intense southwest winds, initially over the eastern Aleutians and
Southwest. Stronger winds could also affect parts of Southcentral
on Saturday and Sunday as the second low moves up from the Pacific
into Southwest, but this potential for strong wind will be highly
dependent on a still very uncertain low track.

An anomalous moisture plume, or atmospheric river, is also likely
to stream north along and ahead of the main frontal zone,
initially expected to be impacting parts of Southwest and much of
the Alaska Peninsula and eastern Aleutians on Friday. This deep
moisture tap will help support rather intense precipitation rates
for much of Southwest and later Southcentral compared to what is
typically possible for late January. Temperatures in many areas of
Southwest close to sea level will likely initially be warm enough
for rain at lower elevations, and this will likely also be the
case as the atmospheric river and front shift into Southcentral
between Saturday and Monday. There could be a transition over to
snow across both Southwest and Southcentral as cooler air begins
to move in behind the front between Sunday and Monday.

Lastly, it looks increasingly likely for a significant, sudden
temperature drop to advance west to east across the southern
Mainland early next week as the front moves through and as the
Arctic air mass cascades south and east. There is still a fairly
large spread in terms of when it looks like the colder air will
arrive for any given location, but there is a strong signal in
both deterministic and ensemble guidance that this cold air mass
will eventually make it into our outlook area early next week.
This could lead to more weather impacts due to the sudden switch
from warm temperatures and rain/snow to bitter cold. Stay tuned as
we follow this very active pattern and resolve details for
timing, precipitation amounts and wind magnitude for this setup
in the coming days.

-AS

&&

.AVIATION...

PANC...Fog and low stratus apparently will continue to be an issue
through the night in the very moist airmass over the area. The
light and variable winds will probably turn light northerly late
in the evening which may also help keep some of the fog and
stratus in the vicinity. This has the look of a situation where it
is in and out throughout much of the night. Toward morning, the
winds aloft will increase from the southeast. That, and thickening
mid-level clouds may help to increase the mixing in the
atmosphere and decreasing the radiational cooling at the surface
so fog and stratus are expected to be less of an issue later
tonight and into tomorrow.

&&