Gale Warning 957, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
536 AM AKST Sat Dec 21 2024

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

The leading shortwave to the north of a broad low south of Kodiak
Island this morning recently pushed through the area, bringing a
period of moderate precipitation to the western Prince William
Sound and a 4 minute period of sleet here in west Anchorage.
Otherwise, most areas have remained dry overnight. Winds have
already started to warm temperatures, with conditions already in
the mid to upper 30s on the western Kenai Peninsula and Matanuska
Valley. Anchorage has remained mostly calm with a slight northwest
wind, which has helped to maintain the pool of colder air.
Out east, temperatures are warming along the coastal mountains,
but the Copper Basin has and will likely remain colder, but still
above normal for this time of year.

Later this afternoon, an upper level shortwave and a weak inverted
surface low will pivot into the coast. This will enhance
precipitation rates over the Sound and eastern Turnagain
Arm/Portage area, where rain will be heavy. With this storm track,
ample warm air has moved in, resulting in snow levels climbing as
high as 3000 ft later today and tonight. This same shortwave may
move some light precipitation into the lee of the Chugach and
Kenai Mountains tonight, resulting in rain or freezing rain, but
it's difficult to say whether or not downsloping will completely
mitigate the moisture. If it is to fall, the closer one is to the
mountains, the more likely the precipitation will be. The
precipitation type forecast for the Richardson Highway north of
Thompson Pass is difficult, but current thinking is that warm air
will move in aloft from the southeast, while mostly calm winds
near the surface keep temperatures cold. This should result in
snow, freezing rain and rain along the road from Tonsina south to
Thompson Pass. See the advisory for more details. For Sunday and
Monday, yet another low will move from the north Pacific, across
Kodiak and into Bristol Bay. Coastal rain will continue with
slowly falling snow levels, but they will remain well above sea
level.

-CJ

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS
(Days 1 through 3: This afternoon through Monday evening)...

The pattern featuring a train of lows moving across the region
from mainly east to west continues this morning, with a new low of
interest now moving south of Kodiak Island and the AKPen. Gusty
north to northeast winds have picked up north of this low near
Bristol Bay and along much of the AKPen, with sustained gale force
winds now picking up out of Bristol Bay, as well as between the
northern AKPen and Kodiak Island. A leading inverted shortwave
trough north of this low has spurred off a fast-moving band of
light sprinkles/flurries showing up on the King Salmon radar on
the northeastern portion of Bristol Bay, but most of this
precipitation is succumbing to dry air before reaching the
surface.

A very uncertain forecast remains in terms of precipitation type
across parts of Southwest for later today into tonight as the low
moves south of the Shumagin Islands. Several shortwaves will
continue to rotate west around the northern side of the low,
helping support a band of precipitation that is expected to
develop over Bristol Bay this afternoon, then lift northwest and
brush past the Kuskokwim Delta as it heads into the eastern Bering
Sea. Strong easterly flow aloft in this region will help a nose
of warm air above the surface develop as warmer air originating
from the Gulf streams west. This will set the stage for what
could be a very messy mix of snow, freezing rain and sleet for
parts of Bristol Bay and the Kuskokwim Delta this afternoon into
tonight, where Winter Weather Advisories remain in effect.
However, this warmer air aloft does not look particularly
pronounced, meaning it could be just barely warm enough to melt
snow as it falls into this layer. The best chance to see any
freezing rain continues to be roughly between Dillingham and
Koliganek from late this afternoon into the early evening. Farther
north across the Kuskokwim Delta, the air mass at the surface
will remain a good bit colder, possibly allowing any melting snow
aloft to refreeze and fall as mostly sleet as the band of
precipitation lifts through tonight.

By Sunday morning, the low will cross over into the eastern Bering
and begin to stall out and meander near the southern tip of the
AKPen. Snow will overspread the central Bering and move into the
Pribilofs, where a couple inches or so of accumulation will be
possible with temperatures hovering at or just above freezing
throughout the day on Sunday. Yet another low will approach
Bristol Bay from the Gulf between Sunday and Monday, where again
there will be a chance for freezing rain to mix with areas of
rain/snow across parts of the area through at least Monday
morning. Meanwhile, a strong front will approach the Aleutian
Chain from Monday into Monday night, sending a band of easterly
gale force winds and rain across much of the central/western
portions of the Chain.

-AS

&&

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

Gale force winds will be present across much of the Bering Sea on
Tuesday due to the influence of cold air advection and a low
south of the Shemya region. Southcentral will be much calmer in
comparison with southerly winds continuing to keep temperatures
warmer than average. Light precipitation is relegated to the
Southcentral coasts and Kodiak Island as weak troughing will
linger in the region. Wednesday swings a low from the south into
Southcentral. This low will move northward and despite some
uncertainty in exactly what track it will take, guidance is
indicating that it will be somewhere near Kodiak Island by
Thursday. Impacts include gale to storm force winds moving with
the low and a potential for a barrier jet to set up once its
pressure gradients move up to the Southcentral coast. Other
impacts are heavy precipitation along the coasts and Kodiak
Island. The positioning of the low will initially prevent
precipitation from making it more inland due to the mountains
blocking moisture flow.

The west sees a different picture for Wednesday and Thursday. An
Arctic trough will sweep down and continue to cause cold air
advection to take place all across the western domain. Convective
but light snow showers will be widespread all across the Bering. A
line of snow will stretch from the North Slope all the way down
to the Kuskokwim Delta through Thursday. Temperatures will plummet
for the southwest mainland through Saturday and the GFS is
indicating that some locations in the Lower Kuskokwim Valley could
see temperatures as low as -40F by the end of the forecast
period. Looking back to Southcentral for Friday has the
aforementioned low linger through the end of the forecast period.
This, combined with the cold temperatures to the west could allow
for a nice setup for snowfall in the Anchorage Bowl and the MatSu
Valley regions as the low shifts and allows moisture to flow in.
This is quite uncertain and changes in the low's positioning and
cold air to the west could prevent this scenario from taking
place.

-JAR

&&.AVIATION...

PANC...VFR conditions and northerly surface winds will persist.
Two fronts moving across the Gulf will bring brisk easterly to
southeasterly winds aloft, with wind speeds of around 30 kt. This
will lead to low level wind shear through Sunday morning. Guidance
is trending towards a lower chance of freezing rain and rain for
today. If any rain or freezing rain occurs, it will likely happen
later this afternoon and evening as a trough moves through.

&&