Small Craft Advisory 8, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
655 AM AKST Thu Jan 2 2025

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

Key Messages:

- Cold temperatures of 30 to 40 below zero are expected for
  lower valleys of the Copper River Basin this morning through
  Saturday morning. The combination of cold temperatures and light
  winds of 5 to 10 mph will result in very cold apparent
  temperatures (wind chills) of 30 to 50 below zero each morning
  through Saturday.

- Strong north-to-south gap flow winds expected for the Matanuska,
  Valdez area, Thompson Pass/Keystone Canyon, and Copper River
  Delta tonight through Saturday.

- Cold temperatures and strong winds gusting up to 50 mph in
  Thompson Pass/Keystone Canyon will allow apparent temperatures
  (wind chills) to drop to 40 to 50 below zero early Friday and
  into the weekend.

Discussion:

Much of interior Alaska, including most of Southcentral, once
again remained cloud-free. Optimal radiational cooling, light
winds, and an arctic airmass starting to move in made for a cold
night over the Copper River Basin where the lower valleys saw
temperatures drop to 25 to 35 below zero. Portions of the Copper
River Valley saw light freezing fog or areas of low stratus
develop again overnight which helped to keep overnight
temperatures in check. These cold temperatures will stick around
for the next few days with temperatures likely dropping several
more degrees Friday and Saturday mornings as arctic air continues
to spread across Alaska. The rest of Southcentral will also see
colder temperatures over the next several days...possibly the
coldest temperatures yet this cold season by Saturday.

North-to-south gap flow winds are expected to start increasing
later today into tonight as an elongated low moves into the Copper
Basin, eventually stretching all the way towards Southwest
Alaska. The pressure gradient will steepen over the Chugach
Mountains and the combination of the cold, dense airmass and
increasing gradient will make for strong gap flow winds through at
least Saturday. Thompson Pass could see wind gusts of up to 50
mph, but Valdez and the associated gap regions also seeing winds
increase later today into Friday. The Copper River Delta and
Matanuska Valley will also see increasing winds, and may see some
dust being lofted depending on the extent and depth of existing
snow cover.

By Saturday, a weak trough is expected to move north into the
central Gulf towards the Southcentral coastline and Kodiak Island.
Light rain/snow will accompany the trough as it lifts northward,
however, models continue to struggle with this system. What does
look certain is that most of the Gulf coastal areas will likely
see precipitation.

- PP

&&

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

The forecast remains on track with mostly cold, dry, and breezy
conditions for much of Southwest Alaska and windier, warmer, and
wetter conditions for the Alaska Peninsula, Aleutians, and Bering
Sea.

Potentially hazardous weather we are monitoring through the end
of the week:

* Heavy to extreme freezing spray off the ice edge from tomorrow
  night through the weekend

* Wind chills in the -20s to -30s for Southwest Alaska, and
  perhaps as low as -40 for parts of the Lower Kuskokwim Valley,
  near Stoney River and Sleetmute tonight through Saturday morning

* Peak winds up to 65 mph from the east-southeast for the central
  Aleutians, south and western Bering Sea. Prolonged precipitation
  is also expected across the Aleutians.

Diving into the details... A high pressure ridge over Mainland is
continuing to bring persistent cool, and light north to
northeasterly across Southwest Alaska. The low south of the Alaska
Peninsula yesterday has greatly dissipated, allowing for a steady
pressure gradient to develop between the mainland and the bent
back occluded low over the North Pacific, south of the western
Aleutians. Stray showers may persist across the Alaska Peninsula.
Its front is nearly aligned with the western Aleutians this
morning and an area of prevailing easterly winds of 30 to 40 knots
appearing on surface observations and scatterometer.

As the front continues to sweep north and east through the end of
the work week, expect gales to become much more widespread and
persistent rain across the Bering and Aleutians. Models indicate
the ridge strengthening over the northern Bering Sea, suppressing
the progression of the front. As a result, the Pribilofs
precipitation chances have decreased through the rest of the week.
There is still enough uncertainty around that latitude of the
Bering for the temperature, and precipitation forecast to change.
Simultaneously, as the front lingers across the Aleutian Islands
for a prolonged period of time, rain amounts through the weekend
could be as high as 1 to 2 inches total. Models are also
continuing to hint at the development of a compact low along the
front. Though there are timing differences, confidence is
increasing that this compact low will develop west of the Eastern
Aleutians, with the potential for localized winds to storm force
to graze the Central Aleutians.

Shifting back to Southwest Alaska, the main story will be high
pressure and cold air advection in northerly flow leading to wind
chills and apparent temperatures down to -30 along the Kuskokwim
River with pockets of -40 during the morning hours. By late Friday
through the weekend, the arctic ridge hedges toward Russia,
allowing for weak troughing to spill into Southwest Alaska. Models
point toward this feature, but how it develops continues to
change. Reduced visibilities in blowing snow would be possible
during this period should snowfall occur while winds remain gusty,
especially near Dillingham, Koliganek, and coastal Southwest
Alaska.

-Rux/Chen

&&

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

An expansive upper level low covers the Aleutians and Bering with
several low centers and troughs rotating through the region. The
main energy core will relocate from the Aleutians into a more
Central Bering position through the forecast period. This gyre is
bounded by a ridge over the Russian Arctic and a second ridge
building over Interior Alaska from Western Canada. Coldest air is
expected to remain in the Eastern Interior through midweek, with
the more active weather occurring across the Southern half of the
state. Southerly winds sustain moisture inflow across Southcentral
Alaska with transient lows and fronts supplying the rest. A blend
of GFS and ECMWF carry the active weather features although they
jockey with the Canadian model for strengths and tracks throughout
the forecast period.

A well developed North Pacific surface low South of Kodiak Island
and its front spread locally heavy rains over the Alaska
Peninsula and Kodiak Island and across to the Copper River Delta
starting Sunday. Locally heavy snow should occur inland over the
Aleutian and Chugach Ranges. A second well developed North Pacific
low follows for Monday with heavy rains and gale force winds
along the Southern end of Kodiak Island into the AKPEN. A third
North Pacific low brings a repeat performance for Wednesday to the
AKPEN and Kodiak Island. Widespread rain and gale force winds
will extend over most of the Gulf of Alaska for midweek.

In the West, a moderately strong low in the North Pacific South
of the Aleutians brings rains to the Western and Central Aleutians
with gale force winds and snow over the Bering Sunday. This North
Pacific low slips away from Alaskan waters South of the Central
Aleutians through Wednesday. Rain or rain and snow mixed continue
in the Northeasterly gales that slowly diminish in areal coverage
over the Northern Bering through Wednesday.

&&

.AVIATION...

PANC...VFR conditions and light northerly winds will persist.

&&