Gale Warning 856, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
602 PM AKST Sat Dec 4 2021

.ANALYSIS AND UPPER LEVELS...

A weak meso-low in the northwestern Gulf continues to produce
convective snow showers for the marine areas surrounding Middleton
Island. Fog and low stratus from earlier this morning persisted
into the early afternoon for parts of Kenai Peninsula stretching
northeastward to Palmer.

A sharp upper level trough continues to move over the western
part of the state, including the eastern Aleutians, toward
Southcentral. Some weak overrunning ahead of the trough continues
to produce short periods of light snow along the Alaska Peninsula
through parts of inland Bristol Bay. Precipitation associated
with this trough has begun to taper, however, as the trough tracks
farther inland and weakens.

Out west, a deep low near Russia is tracking eastward toward the
western Bering. Its front pushes through the Bering Sea today,
spreading southerly gale-force winds across the Bering and
Aleutian Chain. A nearly 200kt upper level jet streak is moving
eastward along the southern Bering/Aleutian Chain and pushes this
system into southern Alaska through today. There is deep
subtropical moisture associated with this system visible on
satellite.

&&

.MODEL DISCUSSION...

Model solutions remain in good agreement taking the upper level
trough eastward across the state, with it weakening as it moves.
In addition, all models agree that the meso-low currently in the
northwestern Gulf will remain nearly stationary throughout the
forecast period.

Models are relatively consistent with the overall track of the
system moving across the Bering. However, there is some
disagreement with the track of a surface low that develops along
the system's front Sunday. For Monday, the GFS is initially
tracking the surface low farther north over the Alaska Peninsula,
while the other models bring the low south of the Alaska Peninsula
and closer to Kodiak Island. Depending on where the low tracks
will determine whether certain locations will see southerly vs
southeasterly flow, where warm air advection will occur, which
precipitation type will occur and where, and how much snow
accumulation there will be for a given location. For now, this
forecast leans more towards the NAM and the GEM as the GFS is an
outlier with how it handles the next low.

&&

.AVIATION...

PANC...Low stratus, courtesy of the Knik Arm, is keeping
conditions IFR. There is low confidence in exact timing of how
long these conditions will persist, but expect a transition to
VFR later this evening. Mostly VFR conditions are then expected to
continue through early Sunday morning, thereafter predominantly
MVFR conditions are expected due to light snow reducing
visibility. Winds are also expected to shift to southeasterly
around this time, but not be overly strong.

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

Light snow showers are developing near Prince William Sound in
response to an upper level wave and associated mesolow currently
situated south of Seward. Meanwhile, a shortwave trough is
crossing the western Alaska Range this evening. This will bring a
chance of light snow into Anchorage Bowl northward into the
Susitna Valley for tonight through Sunday morning, though not much
snow accumulation is expected.

A major pattern shift will be getting underway as a broad
longwave trough over the Bering Sea will shift over the eastern
half of the interior state. Beginning Sunday afternoon, a strong
frontal boundary ahead of the trough will provide widespread
snowfall and gusty winds in the Cook Inlet region. As the front
progresses eastward, snow and winds will spread north and east
across Southcentral. With upper level dynamic support and a deep
moisture fetch out in the Pacific, this will be a long duration
snow event expected for all areas, especially along the coastal
mountain ranges. Additionally, strengthening low level south to
southeasterly winds ahead of the front will lead to areas to
blowing snow and reduced visibility at times. Please see our
webpage that has several products out including Winter Storm
Warnings for the Susitna Valley and Prince William Sound.

Thereafter, a deep surface low will form south of the Aleutians
on Monday morning. Model guidance vary on the track of the low,
but the general trend is to have it migrate along the AKPEN and
into the western Gulf sometime Monday night into Tuesday morning.
Depending on future model runs and consistency, expect a
continuation of snow and winds to persist through much of next
week. Stay tuned for the latest updates.

&&.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 to 2: Today through
Monday)...

It is now the eve of the well-advertised and likely quite
impactful winter storm system, with weather headlines in effect
for all zones covering the Southwest forecast area. Winter Storm
Warnings remain in effect for the Kuskokwim Delta, Middle
Kuskokwim Valley, and Bristol Bay, with impacts expected to begin
late tonight into the very early morning hours of Sunday. A storm
force front will move onshore overnight tonight, bringing intense
snowfall and fierce winds gusting as high as 65 mph for much of
the Y-K Delta and inland areas of the Kuskokwim Valley. Winds will
be slightly lower over Bristol Bay, with gusts to 50 mph
remaining possible. The unusual combination of strong southerly
winds with surface temperatures initially cold enough to support
heavy precipitation in the form of snow could lead to near
whiteout conditions coupled with the strongest winds, especially
during the day on Sunday. There continues to be potential for snow
to mix with or even changeover to rain with strong warm advection
and mixing down of warm air from aloft, mainly close to the coast
over Bristol Bay and the Y-K Delta. This combined with a somewhat
faster progression for the front should result in somewhat lower
snowfall totals then earlier anticipated. Still, snowfall rates
remain on track to be very intense until temperatures warm to or
slightly above freezing for areas where mixing becomes an issue.

The other primary impact from this system will be limited to
immediate coastal areas. The strong onshore winds up to storm
force will potentially enough to cause major disruptions to the
existing sea ice pack, with water likely to well up through cracks
in the ice and possibly pool over shorefast ice close to the
coastline. There remains a high amount of uncertainty as to just
exactly how the ice in place will respond to these winds, but
there is enough confidence for potential impacts to issue a
Coastal Flood Warning for the Kuskokwim Delta Coast and a Coastal
Flood Advisory for the Bristol Bay Coast with this package. Again,
uncertainty for the outlined impacts in these products remains
somewhat higher than anticipated impacts from inclement winter
weather. Interested parties in and around coastal communities
should take appropriate precautions and stay away from the ice at
least until the winds subside early on Monday morning.

Looking further into Monday, another change in the forecast is
that a triple-point low is now expected to form along the front
near the Aleutians and Alaska Peninsula, then track northeast
towards the Cook Inlet and perhaps even as far north as Bristol
Bay by Monday afternoon. This could bring renewed light to
moderate snowfall back across much of the area, with several more
inches of snow possible for parts of the Middle Kuskokwim Valley
and Bristol Bay interior depending on the low track. Check back
for forecast updates on this potential for additional snowfall on
Monday as the details for the track and timing of the second
system become better refined.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS/AKPEN (Days 1 to 2:
Tonight through Monday)...

An intense, storm force front is well on its way across the Bering
this evening, with moderate rain and wind gusts as high as 55 kt
already observed for parts of the western Aleutians earlier in the
day. These strong and gusty winds are on track to impact the
Pribilof Islands during the next several hours, with gusts up to
60 mph possible and moderate to heavy rainfall rates likely. The
front will move at quite a fast clip across the Bering, reaching
the Alaska Peninsula and Southwest coastal areas by late tomorrow
morning. Meanwhile, the parent low will send a surge of cold air
back across the northern Bering behind the front, with storm force
winds and gusts near Hurricane force possible across mainly the
western Bering through tomorrow morning. Westerly winds will
weaken throughout the day on Sunday as snow showers spread back
across the Bering with the colder airmass, with periods of snow
passing over the Pribilof Islands at times through Monday.
Additionally, another low is expected to form somewhere along the
front as it stalls out along or just south of the Aleutians. This
could bring renewed periods of rain/snow to the eastern Aleutians
and southern Alaska Peninsula on Monday as the low moves
northeast towards the Cook Inlet region.

&&

.MARINE (Days 3 through 5: Tuesday through Thursday)...

Gulf of Alaska: Southerly flow over the gulf will transition to a
westerly direction on Tuesday as an aggressive shortwave trough
digs eastward toward the Panhandle. Small craft to gale force
winds are forecast to expand in coverage across much of the gulf
and linger through Thursday as a series of smaller disturbances
pass through the region. Westerly storm force gusts may become an
increasing concern across the Kamishak Bay Gap, the Barren Islands
and east, and points east of southern Kodiak Island over the
latter half of the day on Tuesday. Later next week a large and
potentially significant area of low pressure will move into the
southern Bering with enhanced south to southwest gulf winds
returning to the gulf late Thursday into Friday.

Bering Sea: A break from enhanced weather early next week will
likely be short-lived across the Bering. Guidance is suggesting
that a shortwave trough will dig across the Bering and into the
Kuskokwim Delta Tuesday into Wednesday. Gale west to northwest
winds will spread over much of the Bering, clipping the Western
Aleutians and expanding southeastward through the Alaska
Peninsula. Gusts through gaps in the Alaska Peninsula should be
near or above storm force. These conditions will be followed by
the arrival of yet a larger and stronger upper low into the Bering
Wednesday into Thursday. Guidance hints at an area of low
pressure lifting across the Western Aleutians late Wednesday and
undergoing significant amplification over the southern Bering on
Thursday. While still a ways off, this second low would promote
another round of widespread gale to storm force winds over the
Bering and the Aleutian Chain on Thursday and Friday of next week.

&&

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

Weather is expected to remain very active in the long term with
the jet remaining between 50 and 60 north. This will keep things
very progressive with the storm track from the Bering Sea into
Southcentral. To start things off on Wednesday another surge of
warm air and moisture is expected to move northeast from the coast
of Japan, rapidly tapping into cold air from Siberia to generate
yet another deep low in the Bering. Bombogenesis appears likely
once again with storm to hurricane force winds over the marine
areas of the Bering Sea. There is some uncertainty on the exact
track as the system moves east on Thursday, making landfall
somewhere in Southwest Alaska and then forming a triple point low
in the Gulf on Friday night. Rain or freezing rain will be
possible on Thursday for the Southwest coast, with heavy snow
likely in the interior. As the front moves across the Alaska
Peninsula another large push of moisture is expected to impact
Southcentral. Some coastal areas along the Gulf may see a period
of rain Thursday afternoon but overall the cold air is expected to
wrap around the low rapidly, switching everything back to snow by
Friday afternoon. With the overall progressive nature of this
pattern, the storm will move out into Southeast Alaska on
Saturday. Uncertainty remains fairly high, so stay tuned.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Warning 125 131 145 152 155 161.
		 Coastal Flood Warning 155.
		 Winter Weather Advisory 121.
		 Coastal Flood Advisory 161.
		 Winter Storm Watch 135.
MARINE...Storm Warning 165 170 172-176 179-181 185.
		 Gale Warning 130-132 138 150 155 160 171 177 178.
		 Heavy Freezing Spray Warning 139 140 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MSO/AF
SOUTHCENTRAL ALASKA...MF
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AS
MARINE/LONG TERM...BL/CJ