Winter Weather Advisory 63, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
716 PM AKST Sun Dec 5 2021

.ANALYSIS AND UPPER LEVELS...

The strong low pressure system over the Bering Sea is continuing
its trek toward the Bering Strait. This system's occluded low is
located in the left exit region of a 170+ kt upper level jet,
which is spread across the southern Bering and along the Aleutian
Chain. The intense front ahead of this system has created moderate
rain and gale to storm force winds for the western and central
Aleutians and Pribilof Islands. The front continues to push
quickly toward Southcentral, and it has produced widespread
snowfall, gusty winds, and reduced visibilities across the eastern
Aleutians, southwest Alaska, and the southern Kenai Peninsula. In
fact, Bethel recorded wind gusts up to 65 mph, most of the
Southwest has been seeing down to 1/4 mile visibility, and snow
has started across Southcentral with several snow accumulation
reports already rolling in. Amplified upper level flow and strong
south-southeast low level flow across Southcentral is beginning to
increase temperatures for the region which, in concert with
increasing downslope flow, is ultimately going to start
restricting snow potential for the area.

&&

.MODEL DISCUSSION...

Models remain in overall good synoptic agreement with the track
of the low pressure system and upper-level jet streak moving
across the Bering. Confidence has increased with the track of the
surface low that is developing along the system's front today.
Models generally agree that the low will track along the Alaska
Peninsula through Monday and into Tuesday. This means stronger
winds and warmer temperatures are expected to spread northward
across the western Gulf and toward the Kenai Peninsula.

Starting Tuesday morning, most of the models track this low into
Prince William Sound, but the GFS is the outlier again by keeping
the low out of the Sound and instead over the northern Gulf. The
models all show this weather pattern being very progressive, and
so the low is not likely to stall out in the Sound. Per usual, the
details will sort out better as time continues and the forecast
is refined.

&&

.AVIATION...

PANC...Predominantly MVFR cig/vsby are expected through the TAF
period. Strong northeasterly winds near the surface and
southeasterly winds aloft have maintained a nearly 35 kt wind
shear from the surface. The onset of southeasterly surface winds
can be expected this afternoon with gusts of 25+ kt likely. This
will bring an end to the wind shear.

&&.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
tonight through Wednesday)...

Forecast is on track for a strong winter storm set to bring heavy
snow across Southcentral over the next couple of days. Heavy snow
warnings are in effect for Prince William Sound areas and the
Susitna Valley through Tuesday morning, as well as an advisory
for western Kenai Peninsula for areas of blowing snow. A highly
amplified trough moves into the western Gulf this afternoon and
will spend the next few days traveling towards the North Gulf
Coast. By Monday, a developing surface low at the base of the
trough moves over the AKPen and will track lee side of the Western
Alaska Range before pushing into the Gulf. Southwesterly winds
ahead of the low will start the warming trend across Southcentral.
Temperatures have already started warming up, already rising just
above freezing in Homer, Soldotna, and Kodiak City, and as the
front progresses, warmer air will reach into Anchorage (above
freezing) and the Mat-Su Valleys (above/approaching freezing). As
the surface low tracks into the Gulf on Monday, winds will
pick up and reorient to southeasterly. Increased southeasterly
winds may end up causing northwestern portions of the Kenai
Peninsula and western Anchorage to downslope out, drastically
decreasing the chance for any precipitation tomorrow. If there
does happen to be any falling precip, it will likely be a light,
rain/snow mix reaching as far north as Wasilla. Warm air
encroaches the North Gulf Coast tonight, pushing over Cordova and
Whittier by Monday afternoon and then up to Valdez by late Monday
night. Precipitation intensity will still be moderate-to-heavy,
but expect a transition to rain/snow or all rain Monday afternoon
into Tuesday morning. Note, the transition is for falling
precipitation near sea level, the higher elevations will continue
to see snow. Cold arctic air wraps in behind the low (again) and
will gradually drop temperatures back to a near-normal degree
Tuesday and Wednesday and the transition back to all snow.
Additionally, instability creating lift on the backside of the low
can generate showery (snow) conditions with enough residual
moisture lingering across Southcentral, likely for Anchorage, Mat
Valley, and northern Kenai.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 to 3: Today through
Wednesday)...

The major winter storm has materialized about as expected, with
many areas across the Southwest now/earlier observing strong
winds, heavy snowfall rates, and near blizzard to blizzard
conditions as a powerful front continues to work its way inland.
Most notably, Bethel recorded winds gusting to as high as 70 mph
this morning in the midst of whiteout conditions, with preliminary
tidal data farther south on the Kuskokwim Delta Coast near
Kwigillingok indicating a possible 6 foot storm surge may have
just occurred near the peak of the onshore winds. Warm air
advection has won out over the Y-K Delta for the time being, with
everywhere now warming to or above the freezing mark and remaining
precipitation switching over to rain. Much more inclement
conditions are still occurring over interior portions of Bristol
Bay and over the Middle Kuskokwim Valley, where Winter Storm
Warnings remain in effect this evening. Even so, the worst of the
conditions should begin to improve overnight as the front becomes
increasingly sheared out and winds begin to come down.

The wintry weather does not look ready to end in the near future,
with a triple-point low and supporting shortwave expected to move
up along the front and either just east or west of the Alaska
Peninsula as it generally heads towards Southcentral. This
secondary low will bring a new band of light to moderate snow
across the southeastern 2/3 of the region, with up to 5 inches of
new snowfall possible over lower elevations of Bristol Bay and 8
inches over the Middle Kuskokwim Valley from Monday morning to
early Tuesday.

Looking even further ahead into Tuesday and Wednesday, the active
and progressive pattern shows no signs of stopping, with another
low and attendant front moving across the Bering and sending
another round of snow and gusty winds onshore Tuesday night and
spreading inland through Wednesday morning.

&&

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

The strong frontal system that brought widespread gale to storm
force winds to the Bering/Aleutians has now mostly moves south and
east of the forecast area, with the strong westerly winds closer
to the parent low to the north over the rest of the Bering left in
its wake beginning to subside. A triple-point low is expected to
soon form along the aforementioned front near or just south of the
Central Aleutians, then track northeast along the Alaska Peninsula
on Monday. At the same time, a transient high pressure ridge will
move into the western Bering, with cold northwest flow spreading
across much of the remainder of the Bering/Aleutians, helping to
support the formation of another round of convective snow showers
that will move across the Bering and impact much of the Aleutians
at times through Monday night. This could lead to some blowing
snow with the northwest winds, especially for the eastern
Aleutians and southern Alaska Peninsula.

Two more storm systems will be on the horizon up through midweek,
with the first expected to track across the central Bering from
west to east from Tuesday to Wednesday and bring more gale force
winds and rain/snow back across the Bering and Pribilofs. A much
more significant and rapidly intensifying North Pacific low will
approach the Western Aleutians by Wednesday, with potential for
heavy rainfall and very strong winds at least reaching storm force
for the southwestern portions of the outlook area.

&&

.MARINE (Days 3 through 5: Wed thru Fri)...
Gulf of Alaska: As a low near the Prince William Sound tracks
eastward through Wednesday, winds in the Gulf diminish to
widespread small crafts with pockets of gales. On Thursday,
another low near Bristol Bay pushes its front into the Gulf. Winds
become southwesterly Thursday morning, with speeds increasing to
gale-force as the front moves across the Gulf. As the low tracks
towards the Prince William Sound, gale-force westerly outflow
winds look to resume Friday. Forecast confidence is average, with
diminishing confidence Thursday onwards due to uncertainty with
the track of the low.

Bering Sea: A low near Russia will produce a broad swath of gales
early Wednesday morning that extends across the Bering, impacting
the seas around the Pribilofs, Eastern Aleutians, and AKPEN.
Another low enters the Bering from the North Pacific through
Wednesday, bringing at least gale-force winds in the waters around
the Western Aleutians and storm-force winds near the Central
Aleutians. As the low tracks towards Bristol Bay, storm- force
winds are expected in the Eastern Bering throughout Thursday
before diminishing to gales on Friday. In the Western Bering,
outflow from Russia will produce northwesterly gales before winds
diminish below small crafts with building high pressure.

&&

.LONG TERM FORECAST (Days 3 through 7...Wed through Sun)...

All models suggest an active storm pattern to continue across much
of the Great Land. The subtropical jet will remain largely
responsible for this pattern. It continues to drive through the
Aleutians at nearly 180 kts at 30,000' (300mb). To the north of
the jet, arctic air remains across much of the northern Bering. To
the south of it, warm air with abundant tropical moisture is
driving to the north. This is creating a breeding ground for
strong cyclogenesis. The next system will move into the Central
Aleutians late Wed into Thu. It will rapidly deepen as it
interacts with the jet and the colder air allowing it to fall over
24mb in 24 hours (bomb cyclone) with a minimum central pressure
progged around 950mb by Thu afternoon. This system looks will
track towards the west coast much farther south than the current
system. So even though it will bring warm air north with it, the
track should keep that warmer air farther south. This should allow
for a mostly snow event and create another possible round of
widespread winter weather hazards. It will plow from west to east
across all of southern AK through the early part of next weekend
(11-12 DEC).

The pattern does not look to change much by the end of the
weekend. The jet over the Bering will continue to allow for storms
to rapidly develop and track from west to east. Models are
currently advertising 2 distinct storms. One will be over the
Western Aleutians late Sat. The next one will follow on its heels
Mon and Tue. Meanwhile, the mainland will see a return to arctic
air and outflow winds near the coast. This set up will favor
strong (storm force) marine winds and heavy freezing spray as
well.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Warning 125 131 135 145 152 161.
		 Winter Weather Advisory 121.
		 Coastal Flood Warning 155.
		 Coastal Flood Advisory 161.
MARINE...Gale Warning 119 120 130-132 137-139 141 150 155 165
					  170-175 177-179 181 185 351 352 411-414.
	  	 Heavy Freezing Spray Warning 138 181 185.
FIRE WEATHER...NONE.

&&

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