Heavy Freezing Spray Warning 227, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
544 PM AKST Mon Dec 13 2021

.ANALYSIS AND UPPER LEVELS...

A sprawling front extends from eastern Russia across the northern
and eastern Bering. A low is forming near Sand Point at the
southern end of the front. Arctic high pressure centered over the
northern part of the state is keeping temperatures as much as 20
to 30 degrees below normal across much of the mainland. Cloud
cover associated with the front has overspread all of Southwest
Alaska and much of Southcentral. A weak front is stalled over the
eastern Prince William Sound, resulting in light snow over
Cordova, and is slowly spreading north towards Valdez. Gap winds
continue through the Matanuska Valley and Thompson Pass.

The upper level pattern is very progressive this afternoon. A
pair of ridges are over the western Bering and the western Gulf,
respectively, with a shortwave trough in between. There's also a
large upper level trough over the northern half of the state.

&&

.MODEL DISCUSSION...

The models are in very good agreement on the large scale synoptic
features across southern Alaska this afternoon. However, there is
still a fair bit of inconsistency on the smaller features embedded
in the large scale pattern. The areas of lower confidence include
how much snow will fall over the next 2 days over eastern Prince
William Sound up through the Copper River Basin, how far north
impactful snow will get as the front moving into Southwest Alaska
reaches the Cook Inlet region this evening, and how impactful the
expected snow and blowing snow will be over Kodiak Island.
Forecast confidence is average.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist to start the
evening. Snow may develop around midnight, but if it develops at
all, it will be light. The snow will end around 3am. MVFR ceilings
are likely both during and following the snow, with visibilities
falling to MVFR during the snowfall. Ceilings will improve Tuesday
morning back to VFR. Confidence in snow is moderate, but is high
that any accumulations will be a half inch or less.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 to 3: tonight through
Thursday)...

Two main features in the Gulf are set to produce active weather
across Southcentral through Wednesday. The first is a surface low
passing south of Kodiak Island this evening, expected to transit
the southern Gulf and exit into the North Pacific by Tuesday
afternoon. From this system, Kodiak Island is expected to see 3
to 5 inches of snow through early Tuesday morning, with the
potential for some blowing snow to reduce visibilities at times.
Snow accumulations decrease further north, with the main impact
from the system being cloud cover and warmer temperatures. While
Anchorage is expected to see some precipitation, it is likely to
be in the form of flurries, with only minimal accumulations, as
dry air flowing in on the northern periphery of the system is
likely to evaporate much of the snow on its way down.

The second significant feature is a weak surface trough lingering
in the northern Gulf through Wednesday morning. Moist southerly
flow, aided by the low skirting the southern Gulf will bring
several inches of snow to the Eastern Prince William Sound area.
Most significant snowfall totals will occur along the coast near
Cordova, however, a Winter Weather Advisory was issued for
Thompson Pass, where 4 to 7 inches of snow is expected to combine
with gusty winds to produce blowing snow with low visibilities
dropping as low as one half mile. Tuesday night, a northwesterly
push of cold, dry air is expected to cut off the moisture and push
the trough off into the eastern Gulf, resulting in snow
diminishing through early Wednesday.

Gusty gap winds will also persist through Wednesday night as cold
air from the Interior continues to trickle out across the coastal
mountains. By Thursday, upper-level ridging and broad southerly
flow ahead of a low entering the western Gulf relaxes temperature
and pressure gradients, leading to diminishing gap flow.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Day 1 through 3...Tonight
through Thursday afternoon)...

An area of low pressure that formed along a frontal boundary near
Sand Point will continue to bring near blizzard conditions to the
Bristol Bay coastline through this evening. Very cold temperatures
in the single digits below zero with gusty winds upwards of 35 mph
will make way for very cold wind chills as low as 35 below zero as
well while it is snowing. Therefore, a Blizzard Warning remains
in effect through 9 PM this evening due to the gusty winds and
reduced visibility. Additional snow accumulations of 2 to 4
inches are expected through this evening especially around the
Port Heiden to Pilot Point area with a general 1 to 2 inches of
additional snow accumulation for King Salmon to Dillingham and
points north and west. Conditions will gradually improve this
evening for the Bristol Bay area as the snow tapers off and
pressure gradient relaxes diminishing the winds.

Next, skies are expected to clear overnight as the front decays
over the area. The pressure gradient is expected to remain across
the Kuskokwim Delta keeping wind chills very cold around 35 below
zero. Therefore, with Arctic air being pulled back southward from
the interior of Alaska, expect another frigid morning on Tuesday
with lows in the teens to 20s below zero especially across the
interior Kusko Delta and Kuskokwim Valley. Where winds are light,
and sky clear, temperatures may approach 25 below zero in spots.
For Bristol Bay, temperatures will mainly be in the single digits
below zero with single numbers for around Pilot Point to Port
Heiden.

Tuesday and Tuesday night will remain quiet and dry for Southwest
but with Arctic cold once again in place. Temperatures expected
to be in the teens to single numbers below zero with near zero
for coastal areas during the day and then teens to 20s below zero
overnight with single digits below zero for coastal areas. Another
storm system is expected to approach the area for Wednesday
through Thursday with widespread snowfall, rising temperatures,
and gusty winds especially for the coast resulting in potential
blowing snow and visibility concerns. Stay tuned as we get closer
to the event and fine tune the forecast.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Day 1 through
3...Tonight through Thursday afternoon)...

Another Kamchatka low continues moving northward towards East
Russia tonight through Tuesday. As a result, expect an area of
storm force winds east of Shemya from now through this evening
with another area of storms in the marine waters west of Saint
Matthew Island for Tuesday morning through Tuesday evening as this
system reinvigorates the previous front. Expect widespread seas of
15 to 25 ft as well with the highest seas where the area of
strongest winds are. This system will have a great deal of warm
air with it resulting in mostly rain for the Aleutian Chain this
time around as places will be starting warmer with much less cold
air in place.

Then, on the backside of the low, much colder air will move back
across much of the Bering for Tuesday evening through Wednesday
night resulting in widespread rain and snow showers for the
Aleutian Island Chain.

Lastly, another compact but strong storm force low is expected to
move northward towards Shemya late Wednesday/early Thursday
bringing another round of warmer air, rain and snow, and strong
gusty winds.

&&

.MARINE (Days 3 through 5)...

Gulf of Alaska: Expect winds less than 35 kts and seas less than
20 ft for the Day 3 through 5 period.

Bering Sea: A series of lows track across the Bering Thursday
through Saturday. Widespread gales and seas approaching 20 ft are
likely with each system. Areas of northerly storm force winds or
wind gusts are possible on Thursday night behind the first low,
with 20-25 ft. The following Friday night is the front off a
Kamchatka low pushing into the western Bering. Another round of
storm force winds with seas 20-30 ft possible along the front.

&&

.LONG TERM FORECAST (Days 4 through 7/Friday through Monday)...

A pattern change is expected during the extended period as strong
upper level ridging builds across the area from the eastern Bering
Sea through Southcentral Alaska. There is a medium to high degree
of ensemble consistency the ridge-axis will remain over Southwest
Alaska during this time. The surface pattern is also expected to
change from an offshore to onshore flow pattern as the ridge over
the Alaska Interior will weaken while the ridging off shore
associated with the aforementioned upper level ridge will
strengthen. This will allow for warmer temperatures to enter the
region. This pattern change will also allow for a greater chance
of moisture entering the region due to the change from off shore
to onshore flow. There is currently a moderate degree of agreement
that a weakening low will move inland on Saturday and bring snow
to the Kuskokwim Delta and Bristol Bay. A warm front is expected
to move across Southwest Alaska on Sunday and Monday and bring a
mix of snow and rain throughout the region. Confidence is low with
the specifics of this front at this time due to differences
between guidance.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 161.
		 Winter Weather Advisory 131.
MARINE...Storm Warning 178 185.
		 Gale Warning 130 131 132 139 170 173-177 180 181.
		 Heavy Freezing Spray Warning 121 127 130 139 140 150 160
									  165 180 181 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JW
SOUTHCENTRAL ALASKA...CQ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MV
MARINE/LONG TERM...ED/KM