Gale Warning 741, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
527 AM AKDT Sun Oct 27 2024

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

Key Points:

* Sunny skies once again for Southcentral today. Gap winds along
  the coast will diminish this afternoon and evening.

* A snowstorm is on track for much of Southcentral Monday through
  Tuesday. The heaviest snow will be Monday night through Tuesday
  morning. While there is some uncertainty in exact amounts,
  expect enough to accumulation to affect travel along area
  roadways.

A deep low and high amplitude trough are exiting eastward across
Southeast Alaska and British Columbia. A surface trough extends
from the low along the British Columbia coast up to the north Gulf
coast, with high pressure over interior Alaska. A separate short-
wave trough crossed mainland Alaska yesterday through last night
and is now exiting eastward across the ALCAN border. This led to
weak cold air advection across Southcentral and into the northern
Gulf. The end result of the persistent tight pressure gradients
and cold air advection is a restrengthening of gap winds along the
coast. However, winds are not as strong as they were yesterday
morning. Otherwise, mostly clear skies prevail across Southcentral
and Kodiak Island with temperatures in the single digits in the
Copper River Valley to teens and 20s elsewhere.

Expect another sunny day across the region a a broad upper level
ridge builds in from the west. This will serve as the calm before
the storm, with progressive upper level flow and multiple storms
to impact the region early this week, resulting in the first
significant snowfall for many areas. First off, a trough over the
western Bering is currently interacting with a weak low from the
Pacific. This low will be steered up the western Alaska coast
tonight while zonal flow drives the frontal system eastward. The
speed of the frontal system has sped up in recent model runs.
As a result, now expect light precipitation to begin overspreading
Southcentral beginning late tonight, in time for the morning
commute for the Kenai Peninsula, Anchorage, and the Mat- Su.
Upper level forcing looks weak, with just some weak vorticity
maxima embedded within the fast moving flow. However, there is
some entrainment of high moisture content air from the Pacific.
Precipitation will mostly be driven by low level processes,
specifically vertical motion induced by warm air overriding
colder air in place. While the airmass over Southcentral is
initially cold enough for all snow, warm air will advect
northward across the Kenai Peninsula during the day Monday,
causing precipitation to mix with or change to rain, especially
over the southern peninsula. Precipitation from Anchorage
northward will likely be all snow, though a brief mix with rain is
not completely out of the question due to the presence of a
Turnagain Arm jet and a shallow layer of southeast winds across
the front range Chugach. Also, though the thermal profile will be
below zero, the dendritic growth zone will be quite high (above
10,000 feet) and there could be quite a bit if supercooled water
droplets in the lower levels. In any case, precipitation with this
first phase of the event will be light.

As precipitation is overspreading Southcentral on Monday, a
short-wave will dig down the back side of the Bering Sea trough
and steer a deeper Pacific low northward into the Gulf. Looking
at operational and ensemble model trends over the past couple
days, they have been quite consistent in the general synoptic
scale evolution, but the details of placement of individual
short-waves and track of the surface low across the Gulf have
fluctuated. The latest runs do seem to be converging on a common
low track across the central Gulf offshore area. Precipitation
will intensify out ahead of the low and digging trough beginning
Monday evening. As the trough takes on a negative tilt, a
deformation band will set up somewhere across the Kenai Peninsula
up toward Anchorage, leading to locally heavier precipitation.
Generally expect a transition back to all snow for most areas
Monday night due to both the increase in precipitation intensity
and cold air advection on the back side of the low. The
complexities of precipitation type transitions and uncertainty in
exact placement of individual troughs and the deformation band
leads to lower confidence in storm total snow accumulation.
However, there is high confidence that most areas will see on the
order of 3 to 6 inches, with potential for 6 to 10 inches wherever
the heavier snow sets up. Thus, expect widespread impacts to area
travel Monday night through Tuesday.

The low will deepen over the Gulf on Tuesday and begin to exit
southeastward as a strong jet on the back side of the trough
causes it to aggressively dig toward British Columbia and the
Northwest U.S. Precipitation will taper off across most of
Southcentral Tuesday afternoon through Tuesday night while gale
to storm force winds develop over the Gulf coastal waters, with
locally stronger gap winds.

-SEB

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA AND THE BERING
SEA/ALEUTIANS (This afternoon through Tuesday evening)...

* Winter Weather Advisory in effect from 7 PM this evening until 6
  AM Monday for interior Bristol Bay from Dillingham north,
  including Koliganek and New Stuyahok.

Conditions remain relatively quiet across mainland Southwest
Alaska through this afternoon. Low stratus is noted on satellite
this morning across portions of the Bristol Bay area northward
into the Middle Kuskokwim Valley.

A surface low will develop off the coast of Kamchatka early this
afternoon, with a triple-point low also forming near the Pribilof
Islands. This low, with its associated front and moisture, will
reach the Southwest Alaska coast late this afternoon and progress
quickly eastward, reaching the Alaska Range by Monday morning.
This system will bring wintry weather hazards across the region.

In Bristol Bay, precipitation will begin as snow this evening but
could shift to a period of freezing rain overnight as warmer air
moves in aloft. The freezing rain threat, primarily affecting
interior areas from Dillingham north, including Koliganek and New
Stuyahok, has prompted a Winter Weather Advisory in effect from 7
PM this evening until 6 AM Monday. Surface temperatures are
expected to warm above freezing by early Monday, with
precipitation then transitioning to rain.

A brief period of light snow is expected across the Kuskokwim
Delta through this evening, with minor accumulations near the
coast and up to 1 to 3 inches further inland, especially toward
the Alaska Range, with even higher amounts over elevated terrain.
Most precipitation here will be snow, though western areas like
Aniak could see a short period of freezing rain Sunday evening
before precipitation transitions back to snow overnight as colder
air moves in. A Special Weather Statement highlights moderate snow
accumulations from Crooked Creek to Lime Village, with the
highest totals likely near the Western Alaska Range.

Wind gusts across the Kuskokwim Delta may reach up to 35 mph tonight,
while northwesterly gap winds along the Alaska Peninsula are
expected to strengthen to gale force through Tuesday morning.
Following the initial low, cold air will spread into Southwest
Alaska as the system exits on Monday, bringing light snow showers
or a rain/snow mix across coastal Southwest and the Bering side of
the Alaska Peninsula through Tuesday.

Another stronger system is forecast to organize in the North
Pacific by late Monday, sending a storm-force front across the
western and central Aleutians by Tuesday evening. This front will
bring the potential for hurricane-force gusts and moderate to
heavy precipitation as it tracks eastward.

-ME

&&

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

A stormy pattern continues across the Alaska Weather map into the
weekend. Dynamic and machine-learning guidance trends towards a
better clustering and good certainty for the Eastern system
through the end of the week. A well developed upper level low in
the Eastern Gulf of Alaska continues its exit from Alaskan waters
by early Thursday. On the surface, a moderately strong closed low
in the low 980s spreads gale force winds and heavy rains across
the Southern Gulf through Thursday.

In the West, a second well developed low moving out of Eastern
Asia, with potential central pressures briefly below 960 mb over
the Southern Bering before weakening and turning into Bristol Bay
by Saturday. Heavy rains spread over the Aleutians and Bering into
the Alaska Peninsula for Wednesday. High-end gales from the South
accompany the leading front up to the Western Alaska coasts,
before diminishing over the Central and Eastern Aleutians
Thursday. The gale force winds with the low center become Westerly
along the Southern periphery of the Bering low, diminishing
Thursday.

Gaining a greater notice is a former extra-tropical low that
crosses the Central Aleutians for Friday. Central pressures are
expected to be in the lower 950s, and perhaps briefly lower.
Another round of strong winds and heavy rains are expected to
accompany this low on its trek over the Central and Eastern
Bering, Southwest Alaska and the Alaska Peninsula for the weekend.

&&

.AVIATION...

PANC...A MVFR cloud deck at around 2500 feet is banked up against
the Chugach this morning. With northwest low level flow this deck
will remain along the mountains and should burn off with full sun
today. VFR conditions and light winds will persist at the terminal
through 12Z tomorrow morning. Conditions will then rapidly
deteriorate after 12Z as snow arrives. With deep southwest flow,
ceilings will quickly drop with the arrival of snow.

&&

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
134 PM AKDT Sun Oct 27 2024

AKZ152-281400-
Lower Kuskokwim Valley-
Including the cities of Aniak and Sleetmute
134 PM AKDT Sun Oct 27 2024

...MODERATE SNOW LIKELY ACROSS THE LOWER KUSKOKWIM VALLEY THIS
EVENING THROUGH MONDAY MORNING...

A Bering Sea front will move ashore of Southwest Alaska coast
this afternoon. Snow will spread from west to east across the
Lower Kuskokwim Valley starting this evening and pick up in
intensity overnight tonight. Snow will become lighter from west
to east Monday morning while moderate snow will persist for
locations near the Western Alaska Range. In general, expect most
snow to fall from Crooked Creek southeastward to Lime Village
where 3 to 6 inches are possible. Higher amounts are likely near
the Western Alaska Range.

There is some mixed precipitation/freezing rain potential across
the western most areas of the region, including Aniak with warmer
air trying to nudge in aloft. The best chance at a period of mixed
precipitation for western areas would be this evening before
cooler air aloft works back in late tonight into Monday morning.