Heavy Freezing Spray Warning 121, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
542 AM AKDT Tue Mar 16 2021

.ANALYSIS AND UPPER LEVELS...

A deep longwave trough is closing off at the highest levels into
a complex upper level low centered over the southern mainland.
Several smaller shortwaves and mesoscale lows are rotating within
the broader cyclonic flow, with one of such lows located south of
the AKPen and another over the Prince William Sound. The latter of
these compact lows produced a long-duration round of snowfall
across the Chugach Mountains, across parts of the Susitna Valley,
and over Anchorage since yesterday morning. Very light snow is
still falling in places along an axis from the northern Kenai
Peninsula to the Anchorage area early this morning. Scattered snow
showers are continuing north and east of the sound near Cordova
and Valdez near the slowly filling, stacked low. All of this
lingering snowfall should continue to end and decrease in
coverage as the supporting low continues to weaken today.

In the west, an upper ridge continues to move into the western
Bering, with northerly flow and scattered convective snow showers
continuing to the east along and north of the Aleutian Chain. Low
stratus continues to drift across much of the Southwest, with fog
also persisting at times around mainly coastal sections of
Bristol Bay.

&&

.MODEL DISCUSSION...

Models remain in generally good agreement for the synoptic level,
and forecast confidence thus remains well on track for the next
few days. There are some minor differences for where a pair of
lows will develop in the far western Bering along a front moving
into the region, but the general conditions (rain/snow with up to
gale force winds near the front and developing lows) across the
Western Bering/Aleutians over the next several days will be the
same regardless of the exact low center tracks and pattern
evolution. There is also some concern as to whether fog or low
stratus will linger or develop due to the residual moisture in
the boundary layer in the wake of the snowfall by later today into
tonight across parts of the Mat-Su Valleys and Kenai Peninsula.

&&

.AVIATION...

PANC...Light snow will gradually end early this morning, with
generally MVFR to occasional periods of IFR conditions remaining
possible until the snow fully comes to an end. VFR conditions
will return for the rest of the morning through this evening. By
tonight, cooling temperatures combined with lingering moisture in
the low levels may lead to the development of fog or low stratus
over or near the terminal, bringing a returning threat for
MVFR/IFR conditions at times through Thursday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Today
through Wednesday Night)...

Light snow continues across the Anchorage area this morning, as
low level moisture continues to be drawn into Anchorage on the
western side of a departing upper level low. Instability has
increased on this morning's Anchorage sounding, while the
atmosphere is almost entirely saturated right up to near the
tropopause, and is completely saturated from 800 mb down. Thus,
it stands to reason that until the upper level low moves far
enough away to the northeast, at least flurries will continue in
the area. This should occur by midday, though daytime heating may
only further increase instability. Either way, any accumulations
still to go are very likely to hold to under a half inch. The 4.1
inches of snow recorded here at the forecast office yesterday
makes it the snowiest day of 2021 so far, despite most of
Anchorage getting far more snow during the March 10-11 snowfall
event.

A strong low tracking due north across the eastern Gulf tonight
along with an approaching weak upper level disturbance will usher
in a renewed push of Arctic air into Southcentral. The gap winds
will increase once again tonight and will ramp up through
Wednesday afternoon. For most areas this will maintain the
remarkable streak of below average temperatures, along with
clearing skies (though some areas will clear much faster than
others). For those in the gaps, such as Seward, Whittier, Thompson
Pass, and the Copper River Delta, another prolonged period of
gusty winds, below zero wind chills at night, and relatively
warmer temperatures relative to the surrounding areas is expected.

Another weak Gulf low will develop south of Prince William Sound
late Wednesday night as an upper level low and associated
negatively tilted trough build over the Cook Inlet region. As
opposed to developing west of the area, the trough/low building
directly over the Cook Inlet region should prevent any significant
snow from forming through Thursday, but upslope flurries and snow
showers cannot be completely ruled out. Further east, however,
this will be the next snow event for Prince William Sound and into
the southern Copper River Basin. Depending on how these features
evolve, it could lead to another period of blowing snow through
Thompson Pass, and another several inches of snow in Cordova, the
neighboring mountains, and along the Edgerton Highway into
McCarthy.

Looking ahead, the next chance of more widespread snow will be on
Saturday, but plenty of details remain to be ironed out.

&&

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

Broad northwesterly flow aloft exists between a building ridge
over the Bering and a trough over the interior. A few lingering
convective snow showers are possible today over the Kusko Delta
and Middle Kuskokwim Valley. Otherwise, low stratus and patchy fog
will be prevalent throughout the region. On Wednesday a fast
moving, polar shortwave will bring a quick shot of snow and
blowing snow to the Kuskokwim Delta. Please see the Special
Weather Statement for more details. Some light snow is also likely
for the Middle Kuskokwim and Bristol Bay, with snow totals up to
3 inches expected. Areas to the south/southeast of the Kilbuck
Mountains will likely be downsloped out of precipitation but a few
flurries are possible. Areas along the Western Alaska Range and
Aleutian Chain are likely to see snow through early Thursday
morning due to orographic lifting. The weather will quiet down on
Thursday as cold, dry air moves in from the north.

&&

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

A west to east pattern change will take place through the forecast
period as an upper level ridge builds in from the western Bering.
Broad northwesterly flow and convective snow showers will linger
around for one more day as a frontal system moves in from the
west. Winds up to gale force and rain are expected over the
western Aleutians this afternoon as the front moves in. Due to
the broad, disorganized nature of the parent low, as well as
little baroclinic energy, the front will quickly weaken as it
moves across the Aleutian Chain through Thursday. Precipitation
type will likely start off as snow and quickly transition to rain
as the warm air erodes the remaining cold air in place.

&&

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

Gulf: A low moves across the Northern Gulf and dissipates Sat.
Forecast confidence is good. Widespread small craft winds and
waves south of the low move across the Gulf, dissipating Fri. Gale
force gap winds exit into the Gulf from the Barren Islands and
Sitkinak areas, and dissipate Fri as the low moves east. A new
front approaches the Southwest Gulf late Fri into Sat.

Bering: A Western Bering low moves to the Central Bering by Sat.
Its front extends through the Eastern Bering and Alaska Peninsula
through Sat. Forecast confidence is good. Widespread high end
small craft winds and waves with areas of gale force winds
wrapping the low and front across the Bering through Sat. Winds
dissipate south of the low and front, but remain across the
Northern Bering through Sat. Seas to 22 feet over the Western
Bering, 18 feet on the Pacific side, subsiding through Sat.

&&

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

The long term forecast begins late Thursday with a persistent
broad trough over much of Mainland Alaska. Meanwhile, a ridge
situated over the eastern Bering and Southwest AK coast begins to
breakdown as a low moves east across the Aleutian Chain. By
Sunday, models diverge greatly as to how far eastward this system
progresses and how it interacts with the aforementioned ridge. The
GFS pushes this system into the southern Gulf by Sunday while the
EC and GEM keep the low farther west, with some semblance of a
ridge remaining in place. The GEM goes so far as to establish an
omega block, with an amplified ridge extending to the Chukchi
Sea. The latter solutions would allow a second shortwave to move
underneath the aforementioned Aleutian low and steer it north into
the Gulf by Monday. The GFS collapses the ridge by dropping a
shortwave southeast from the northern Bering. The interesting
thing of note, is that while these solutions are different in the
evolution of the shortwaves through the weekend, they are
convergent on the solution of the upper-level trough re-
establishing itself over the state with an amplified ridge over
the Bering by the start of next week. In terms of sensible
weather, the EC and GEM would keep conditions cooler and drier
over the Southern Mainland through the weekend, while the GFS
would allow for some precipitation to develop from the AKPEN east
across Bristol Bay and perhaps the immediate Southcentral coast
beginning Saturday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 121,130,177,178.
Heavy Freezing Spray: 121,139,160,180,185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AS
SOUTHCENTRAL ALASKA...JPW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CJ
MARINE/LONG TERM...MK/TM

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
317 PM AKDT Tue Mar 16 2021

AKZ155-171230-
Kuskokwim Delta-
Including the cities of Bethel, Hooper Bay, and Nunivak Island
317 PM AKDT Tue Mar 16 2021

...BRIEF PERIOD OF SNOW AND BLOWING SNOW WEDNESDAY MORNING...

A fast moving front is expected to move south from the Norton
Sound early Wednesday morning. Westerly winds up to 25 mph with
gusts to 40 mph combined with snow will reduce visibility to one
half mile at times. These conditions are not expected to last more
than 2-3 hours at any one location. The front is expected to
reach Hooper Bay around 2 am and move south of Quinhagak by 10 am.
Snow accumulation up to 2 inches is expected. Northerly winds up
20 mph behind the front may cause blowing snow through the
afternoon.