Gale Warning 812, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
532 AM AKST Fri Dec 9 2022

.ANALYSIS AND UPPER LEVELS...

A surface low continues to weaken and move southward into the
eastern Gulf, while a ridge of high pressure builds in from the
north and west. Cold air advection along with tightening pressure
gradients between the Gulf low and high pressure is maintaining
strong winds across much of the Gulf and Kodiak Island, and
through bays and passes. Skies are slowly clearing across western
portions of Southcentral with overnight temperatures in the teens
to single digits. Out west, benign conditions prevail across the
eastern Bering Sea and Southwest Alaska under a ridge of high
pressure and broad northwesterly flow. A frontal system extending
ahead of a strong North Pacific low is bringing rain and gale
force winds over the western and central Aleutians.

&&

.MODEL DISCUSSION...

The latest operational model runs are in decent agreement with
the placement and intensity of synoptic scale features at both the
surface and in the upper levels. The frontal system over the
western Bering Sea will move east across Southwest Alaska this
weekend, bringing snow and southerly winds. As the frontal system
enters the western Gulf on Saturday, snow returns across
Southcentral beginning Saturday night or early Sunday morning, and
continuing through early next week. The main challenge is the
timing and where the heaviest snowfall or precipitation will fall.

&&

.AVIATION...

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

&&

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

There have been some changes to the synoptic scale forecast over
the past 24 hours which will affect the timing of peak winds along
the coast. Essentially, the upper level ridge is building in
faster from the west today, while Arctic short-waves which looked
like they would track southward along the leading edge of the
ridge will now remain over the northern tier of the state. There
is still a weak short-wave tracking southeast across the Copper
River Basin and northeast Gulf this morning. Subsidence will
increase behind this while the overall strength of northerly flow
increases and pressure and thermal gradients tighten. As a result,
expect ridge top and gap winds to peak this morning through early
afternoon for eastern Kenai Peninsula/western Prince William
Sound areas (including Resurrection Bay and Passage Canal) then
peak this afternoon into the evening for eastern Prince William
Sound (including the Valdez area and the Copper River Delta).
Winds will diminish from west to east across the Gulf and
Southcentral tonight through Saturday morning as the upper ridge
moves overhead.

There will be some large disparities in temperatures today, with
windy areas remaining quite a bit warmer than sheltered areas. As
winds begin to die down tonight, temperatures will fall across the
region. By Saturday morning, expect the coldest temperatures of
the week, with sub-zero to single digit temperatures for most of
the region. The exception will be the Gulf coast where temperatures
will remain in the teens.

The upper ridge will quickly flatten across mainland Alaska
Saturday through Saturday night as a deep low tracks to the Bering
Strait. A frontal system trailing southward from this low will
move into the western Gulf Saturday afternoon and evening, with an
atmospheric river feeding into it from the north central Pacific.
The airmass across the region will be quite cold, so expect snow
as precipitation reaches Kodiak Island Saturday afternoon followed
by a transition to rain as a warm front lifts up from the
Pacific. Light overrunning snow will then overspread much of
Southcentral Saturday evening, producing light accumulations.

More impactful weather will follow this, with an upper trough and
surface low tracking out of the Pacific and into the Gulf Sunday
through Sunday night. There are some key model differences in the
evolution of this system which are leading to low forecast
confidence for this time period. It general looks favorable for a
deepening low tracking across the Gulf, with heavy precipitation
along the coastal zone. The low intensity and track will be key to
the snow forecast for inland areas and whether the coastal areas
change to rain or not. Stay tuned, as there is potential for heavy
snow inland with this storm system.

&&

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

The relatively calm, quiet conditions will soon be coming to an
end, with multiple strong systems now on the horizon. Today will
be the last calm and dry before the onset of the upcoming active
period, with a shortwave ridge expected to pass overhead as it
shifts easts towards Southcentral. Gusty north to northwest winds
across Bristol Bay will diminish later in the day today as the
upper ridge axis approaches and cold, northwest flow shuts off.

By late tonight, attention shifts to a strong frontal system
associated with a low driving up into the northern Bering Sea.
Winds will pick up out of the southeast as the front approaches
from the west, with snow then spreading inland from west to east
across the Kuskokwim Delta and Bristol Bay on Saturday morning.
Sustained winds around 34 to 45 mph with gusts to near 60 mph will
be possible along namely the Kuskokwim Delta Coast, causing
significant blowing and drifting snow. Somewhat lighter but still
strong and gusty winds will pick up across western portions of
Bristol Bay as snow begins, with areas of significant blowing snow
again expected. By Saturday afternoon, persistent onshore and
southerly winds will advect warmer air into these areas as the
front also begins to shear apart. This should abruptly end the
threat for blowing snow as winds die down and any falling snow
near the coastline changes over to mainly rain through Saturday
night. Winter Weather Advisories remain in effect for both Bristol
Bay and the Kuskokwim Delta through Saturday afternoon due to the
expected blowing snow prior to temperatures warming above
freezing.

There will be very little break, if any, between this first round
of wintry weather and the next one. By Sunday morning, a large North
Pacific low will approach the southern Alaska Peninsula on its way
towards the western Gulf. A deformation precipitation band will
develop north of this low along a southwest to northeast axis,
mainly between Bristol Bay and the Lower Kuskokwim Valley.
Significant snowfall is possible across this corridor, with
perhaps as much at 6 to 12 inches of snowfall accumulation during
an extended period from Sunday morning to Monday morning. There is
still some uncertainty for the track of this next system, so
check back for further developments on timing, amounts and
location for the heaviest snowfall with this next system as
forecast confidence improves.

&&

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

A strong low will continue to track northeast across the Bering
Sea today into tonight, with a leading gale to storm force front
shoving across the Aleutian Chain, southern Alaska Peninsula and
the Pribilofs. Moderate rainfall will progress across the
Aleutians and past the Pribilofs along with southerly gusts to
storm force as the front moves through, with winds then becoming
southwesterly and weakening by Saturday morning. The break in the
weather will not last long, however, as two new systems move into
the picture from different angles by Sunday morning. Out towards
the eastern Aleutians and Alaska Peninsula, a North Pacific low
will approach from the southwest, driving northeasterly winds
back into gale force through at least Sunday evening as areas of
rain and snow move north of the center and across mainly the
southern Alaska Peninsula. Meanwhile, a separate low will drive
another likely gale to storm force front across the western
Bering/Aleutians, with warmer temperatures and mainly rain
expected for this separate system as it advances into the Bering
through early Monday. There is still quite a bit of uncertainty
for the track and strength of both of these lows, so stay tuned
for further forecast updates.

&&

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

The long term begins Sunday night into Monday with a highly-
amplified pattern in place with an potent surface low and
associated upper-level wave situated over the western Gulf. These
features will be tapping into an impressive moisture plume that is
expected to lend to significant snowfall amounts across much of
Southcentral for Monday. There is still some uncertainty as to the
position of the low and its associated front, with the GFS pushing
the front right to the immediate coast. If the front does make it
onshore, there will likely be a changeover of snow to rain for
coastal locations like Seward and Whittier. However, with cold air
in place initially, if warmer air does not make over the region,
as other models are suggesting, the precipitation should remain
all snow for along the coast.

This system departs on Tuesday as another highly amplified upper-
level ridge moves from the Bering east to the western half of the
state. Both the GFS and EC show this ridge quickly moving east as
yet another upper-level trough moves across the Bering. A surface
low is expected to lift north across the eastern coast of Russia,
with its front moving into the eastern Bering by Tuesday
afternoon. As the front occludes, a shortwave lifting north from
the Pacific looks to spin up a new surface low along the front and
drive the low northward. This solution, although slightly varying
between the GFS and EC, would likely bring warmer air aloft
northward across the state, and potentially introduce
precipitation type issues across the Southern Mainland for
midweek.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 155 161.
MARINE...Gale Warning: 119-121 127-129 131 132 136-138 150 155 160 165
					   171-176 178-181 185.
		 Storm Warning: 130 170 177 412-414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MF
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AS
LONG TERM...TM