Gale Warning 803, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
546 PM AKST Sun Dec 10 2023

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

Key Messages:

-A strong low pressure system will move up from Kodiak Island to
 the Kenai Peninsula tonight. Strong winds, heavy snow and
 whiteout conditions are expected or ongoing across the greater
 Prince William Sound and Kenai Peninsula Regions. Several inches
 to a few feet of snowfall will be possible for portions of these
 regions.

-Additional rounds of moderate to heavy precipitation are likely
 to continue into the remainder of the week as several additional
 low pressure systems ride up into Southcentral along a deep
 moisture axis streaming north into the coastline.

-There is still low confidence for timing, duration, type and
 amounts of additional precipitation potential through Wednesday.
 The highest chance for continued heavy snow/rain from Tuesday to
 Wednesday will be focused over the eastern Prince William Sound
 and southern Copper River Basin.

Discussion:

A winter storm is now underway for much of Southcentral as a
strong ~978 mb low now moving near Kodiak Island to the south of
Shelikof Strait. A massive plume of moisture and synoptic scale
lift now extends out ahead of this low near and along the low's
occluding warm front, visible in the mid to upper levels as a
large shield of dense, cold cloud tops arcing from Bristol Bay all
the way out to the southern Yukon on GOES West satellite imagery.
Gale to storm force winds are spreading into much of the gulf and
are beginning to encroach on much of the Gulf Coast, with gusts
near 70 kts recently recorded at Augustine Island near the mouth
of Kamishak Bay. Meanwhile, widespread moderate snowfall is now
beginning to work inland, including along much of the Kenai
Peninsula and out to Prince William Sound. Near whiteout
conditions have been observed where snow is combining with gusty
winds, including near Whittier, Homer and along the Seward Highway
between Portage Valley and Seward. While strong cross-barrier flow
across the Kenai and Chugach range is picking up ahead of the
front, there is still some snowfall trying to spill over the
terrain on to the lee side in places, including over the interior
Kenai Peninsula out to Sterling and Soldotna, as well as towards
the upper Hillside and far southeast Anchorage. Gusty winds have
begun to pick up out of the Matanuska Valley, so far mainly
resulting in minor blowing/drifting snow.

Looking to the forecast, several feet of snow remains expected
for the Kenai and Chugach Mountains. For the road system, the
heaviest snow is likely for Thompson Pass and Valdez, as well as
portions of the Seward Highway. An influx of warmer air is likely
to lead to rain/snow mix or rain for some coastal areas such as
Seward and Whittier Monday morning. The strong east wind through
the Turnagain Arm will lead to warmer temps in Portage and
Girdwood as well, which may lead to a brief changeover to rain
after several inches of snow has already fallen. Winter Storm
Warnings/Advisories going into effect now or later this evening
have been issued for areas covering the Kenai Peninsula and much
of the greater Prince William Sound & Chugach Range. Please see
information covered under the matching product for your location
for further details for timing/amounts of snow, winds and reduced
visibility.

For Monday night and beyond, much of the forecast remains highly
in flux. Models (with one exception) have mostly trended towards a
more east solution for the next low expected to lift northwest
into the Gulf around the nearly stalled upper trough centered over
the Alaska Peninsula, and this low should reach the coastline by
Tuesday morning as it follows along a wavering frontal zone
stretching over the Gulf. A near continuous stream of moisture
firing into the mainly eastern sections of the Gulf coastline
will result in near continuous snow (low-level coastal rain) for
mountainous areas, with blizzard conditions possible in
susceptible areas such as Thompson Pass. A big forecasting
challenge with this next system will be the degree of 'inland'
(including much of the Copper Basin) precipitation and the amount
of warm air trying to filter into valleys that could cause some
mixing with rain between Monday and Tuesday, including near
McCarthy and Chitina. Farther west out towards the Kenai
Peninsula and Mat-Su Valleys, the potential for additional
snowfall past Monday morning has down-trended, with the majority
of scenarios (now reflected in this forecast package) are keeping
this region mostly dry through Tuesday night. There is a low end
chance for parts of the Mat-Su Valleys and Anchorage to see
several inches of snowfall if the one model solution clinging to a
much farther west track for the low & atmospheric river pans out,
but there appear to otherwise be very little support for this
idea.

Yet another low pressure will join the parade of systems arcing
north and west from the North Pacific on Wednesday morning. While
details are again not clear-cut, it looks like there will be
better chances this time around for light snow to spread back over
to eastern parts of the outlook area, while another batch of
moderate snow (and again possible mixing with rain near the coast)
moves into the Prince William Sound and Copper River Basin. Stay
tuned for the forecast and for more updates to come as this very
active winter pattern continues.

-Brown/AS

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Tonight
through Wednesday)...

A strengthening low centered over Shelikof Strait is bringing
heavy snow and blizzard conditions to locations around Bristol
Bay this evening. With models in much better agreement today,
there is increased confidence in the continued evolution of this
storm system. Northerly winds gusting to 45 mph will continue to
combining with widespread snowfall across much of Bristol Bay to
produce low visibilities through Monday morning. During this time
frame, an additional 3 to 9 inches of snowfall is likely, with the
highest amounts for interior Bristol Bay. While Iliamna is
currently not seeing any precipitation, easterly cross- barrier
flow is forecast to decrease and allow for 6 to 9 inches of
snowfall there tonight, though winds will diminish, meaning
blizzard conditions are not expected there and will remain limited
to areas further south and west. The threat of blizzard
conditions end with diminishing winds and lighter snowfall on
Monday morning.

Further north in the Lower Kuskokwim Valley, slightly less snow
is now expected, with weaker winds forecast as well. As a result,
there is a reduced likelihood for visibility reductions to one
half mile or less due to blowing snow, but there is still some
potential. Winds and snowfall intensity begin to diminish Monday
morning, which will decrease the threat of very low visibilities.
For the Alaska Peninsula and Aleutian Islands, cold air streaming
south across the Bering Sea is providing enough instability over
the relatively warm sea surface to produce persistent snow showers
as far west as Adak.

For Monday and Tuesday, snowfall will trend down across Southwest
Alaska. Light snowfall shifts westward over western portions of
the Lower Kuskokwim Valley and nudging into the eastern half of
the Kuskokwim Delta as a broad low remains centered over
Southcentral Alaska. However, with winds forecast to diminish,
blowing snow is no longer expected to reduce visibility in these
areas. Further west, strong northerly flow to gale force is
expected over the eastern Bering Sea, with gusty northerly winds
affecting the Kuskokwim Delta coast. Transportable snow at the
surface will likely be lofted, even without new snowfall, leading
to potential reductions in visibility through Wednesday, at which
point winds begin to diminish.

Another area of potential concern is the Alaska Peninsula and
Aleutian Islands. The broad low sitting over southern Alaska will
cause a continued stream of cold air to be drawn south between it
and high pressure in the western Bering Sea. This will lead to
persistent snow shower activity along the north side of the Alaska
Peninsula and along the Aleutian Chain. Shower activity is
forecast to become more vigorous, which may lead to more
significant reductions in visibility as gusty winds continue to
blow across the region. Northerly flow across Southwest Alaska
will also lead to decreasing temperatures through mid-week. By
Tuesday night, widespread lows in the negative single digits can
be expected. Combined with winds in the Kuskokwim Delta, wind
chills will continue to hover around -30F.

By Wednesday, a storm force low enters the western Bering,
bringing much warmer air to the western and central Aleutians.
This will bring the only rain out west during the short term
forecast period.

CQ

&&

.LONG TERM FORECAST (Days 4 through 7)...Thursday through Sunday...

The long-term forecast remains on track for the continued
unsettled and active pattern for Southern Alaska, the Bering Sea,
and the Gulf of Alaska. For Thursday and Friday, an upper level
low in the Gulf opens up into trough as it progresses eastward to
Southeast Alaska. Any snow showers should be confined to the
coastal mountains through Friday. Another deep trough moves
eastward to the central Bering Sea and becomes stationary by
Sunday. A gale to storm-force low pushes its front eastward from
the Central Aleutians to the coast of Southwest Alaska through
Friday. There will be a warm push of air with this front. Thus,
while precipitation could start as snow ahead of the front,
precipitation will quickly change to rain as the warm air works
in. Precipitation type is a little more uncertain for Southwest
Alaska Friday due to timing differences in when the parent low in
the western Bering occludes as well as when a secondary low forms
along the front. Regardless, precipitation overspreads Southwest
during the second half of Friday and makes it into Southcentral
Friday evening and Saturday. Coastal locations of Southcentral
look to again see the most precipitation with more uncertainty
for inland locations due to downslope drying flow. Cold air
continues to spill into the Bering Sea Saturday and Sunday as the
occluded low remains in the Bering with snow showers a good bet
for Aleutian Chain. Sunday for Southcentral features a lot of
uncertainty with regard to the track of a potential Gulf low.

&&

.AVIATION...

PANC...Gusty northerly winds and blowing and drifting snow will
be the main impact late this afternoon through the evening hours.
As loose snow becomes scoured out, it becomes difficult to say
whether there will be enough snow to loft in the air and reduce
the visibility above ground level. Northerly winds will then
diminish overnight. There is a small chance of low level wind
shear sometime between 06Z and 12Z as easterly flow strengthens
along Turnagain Arm and the front range Chugach. Low level flow
will rapidly diminish between 12Z and 15Z as an occluded front
passes by and an upper level short-wave nears. Expect a brief
period of snow, with MVFR conditions most likely. Showers will
then persist in the vicinity through the day Monday. Most likely
ceiling/vis will bounce up and down as snow showers move over the
terminal.

&&