Winter Weather Advisory 62, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
533 PM AKST Fri Dec 3 2021

.ANALYSIS AND UPPER LEVELS...

An upper level ridge axis extends across the state from the
Beaufort Sea south toward the Alaska Peninsula. Meanwhile, an upper
level trough is in place over the northern Bering while a second
upper level trough axis extends from the Alaska-Yukon border
southward into the eastern Gulf. A front extends from north to
south over the western coast of the state. This has produced
blizzard conditions across the Kuskokwim Delta Coast with snow and
blowing snow observed. Farther east, a persistent stratus layer
has remained over the Copper River Basin. Fog and low level
stratus also developed along Knik Arm this morning with some fog
drifting into the Anchorage Bowl. Farther west, westerly gale
force winds persist across the Bering.

&&

.MODEL DISCUSSION...

Model solutions are in good agreement through the short term and
have been consistent with the arrival of a broad low over the
mainland this weekend. Initially, the 12Z NAM painted a very
different picture of the 300 mb jet pattern, showing more of a
zonal orientation with the jet streak positioned farther eastward
over Southcentral. Now with the 18Z NAM run, the solution is more
in line with the GFS which has been depicting the jet streak
oriented from southwest to northeast positioned over the Alaska
Peninsula and not extending over Southcentral as much. Another
difference previously observed between the NAM and GFS solution
was the location of a triple point low spinning up along the
frontal boundary. The GFS has consistently advertised this low
developing in the vicinity of Kodiak Island while the NAM placed
this low over Prince William Sound. However now that the NAM has
fallen in line with the GFS, this has led to increased confidence
in the GFS solution. Overall, we remain confident in an active
winter weather pattern across the southern half of the state this
weekend, however the finer details including snowfall amounts and
potential for rain/snow mix in places are still being ironed out.
Please stay tuned as we continue to closely monitor this
developing situation.

&&

.AVIATION...

PANC...Fog and low stratus remain the biggest challenge. Expect
low ceilings around the terminal with persistent MVFR to IFR
conditions. However, there is potential for fog to develop over
the terminal again tonight, reducing conditions to LIFR.

&&

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

Light snow showers are developing near western Prince William
Sound in response to an upper level wave and associated mesolow
moving southeastward across Southcentral. Another mesolow will
form near Seward, keeping showers around into the weekend along
the northern Gulf coast as well as the offshore marine areas. A
cold, dry airmass combined with clear skies will allow overnight
temperatures to drop down in the single digits to below zero
across the lower and higher elevation regions for tonight.
Elsewhere, temperatures will be moderated due to cloud cover
sticking around through Saturday morning.

Otherwise, benign conditions will prevail as a ridge of high
pressure dominates at least through Sunday. As the ridge axis
moves east, a broad longwave trough over the Bering Sea will shift
over the eastern half of the interior state and bring unsettled
weather conditions into early next week. A triple point low will
develop along a frontal boundary near the western Gulf and Kodiak
Island, with strong winds and moderate to heavy snow for all
areas. With strong dynamics and deep moisture fetch, this could be
a significant snow event for Southcentral, but specific details
in the forecast are still being ironed out. So stay tuned for the
latest updates.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Tonight
through Monday morning)...

A weakening frontal system will continue to bring a band of snow
across parts of the Kuskokwim Delta/Valley and Bristol Bay as it
progresses eastward, with snowfall generally tapering off by
tomorrow morning as the front disintegrates. Expect moderating
temperatures behind the frontal system as well, with the entire
area finally coming out of the deep freeze, with warming to at
least single digits above 0F nearly everywhere by tomorrow
afternoon.

Attention immediately shifts to what will likely be the most
significant winter storm system for the entire season thus far. An
expansive and intense low moving across the far northern Bering
will send a very potent attendant front and supporting 160 kt jet
streak aloft directly across the whole forecast area late this
weekend into early next week. A plethora of weather hazards will
be possible as the front tracks across the Southwest from west to
east, particularly from Saturday night to Monday morning. Strong
southerly winds gusting as high as 55 mph will develop along the
coastline Saturday evening, then spread inland over the Y-K Delta
and Bristol Bay during the overnight. Moderate to heavy snowfall
rates fueled by an influx of tropical moisture will follow in
tandem with the winds, resulting in widespread areas of blowing
snow that could produce near blizzard conditions near the peak of
the event. The worst conditions are currently on track to peak
earlier near the coast as the front arrives, or Sunday morning,
with the worst of the wind/snow arriving later on Sunday into
early Monday for further inland areas.

On top of everything else, there is some chance the southerly and
onshore winds will be strong enough to disrupt the newly formed
ice pack, with potential for sea water to well up along/over
shorefast ice. This could result in minor coastal flooding if
enough water is able to make it through or past the existing sea
ice near shore. The chance for this happening is highest where
there are preexisting weaknesses in the ice, including near parts
of Kuskokwim Bay. A Winter Storm Watch has been issued for the
entire Southwest outlook area to cover the potential for all of
the possible hazards mentioned above. Please see the product
corresponding to your geographic area for further details on
timing of impacts and expected snowfall amounts.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS/AKPEN (Days 1 through
3: Today through Monday morning)...

A very powerful low and trailing frontal system will impact the
entire forecast area during the course of the next two to three
days. A North Pacific low will enter the far southwestern Bering
this evening, then track northeast near the Kamchatka Peninsula as
it rapidly deepens to near 950 mb by the time it approaches the
Bering Strait on Sunday morning. To the south, the associated
frontal system will bring a broad swath of gale to storm force
winds that will affect nearly the entire Bering/Aleutians at one
point or another, with impacts from wind and precip mainly in the
form of heavy rain beginning earliest over the western
Bering/Aleutians tomorrow morning. The strong winds and rain with
the front will shift quickly eastward on Saturday across the
southern Bering, easter Aleutians and Pribilofs, with storm force
winds reaching the Southwest Coast and Alaska Peninsula by Sunday
morning. The supporting low pressure center will stall in the
northern Bering and support another area of westerly winds up to
storm force, mainly over the northern and western Bering from
Sunday to early Monday.

&&

.MARINE (Days 3 through 5: Monday through Wednesday)...

Gulf of Alaska: Flow becomes southerly and reaches gale force as a
front enters the Western Gulf on Monday. Cyclonic development
within the front near Prince William Sound shifts wind directions
to west-northwest across the Gulf by Tuesday afternoon. Gales are
likely initially through the Barren Island, becoming widespread
across the Gulf throughout the rest of the day Tuesday. The GFS is
hinting at a brief period of low end storm force winds through
and east of the Barren Islands on Tuesday night.

Bering Sea: A series of lows move in and out of the Bering through
the early week. Two Russian lows skim the northern edge of the
Bering Sea before drifting over Western Alaska, both of with look
to produce widespread north-northwesterly gale force winds. The
second low may bring a period of storm force winds to the northern
Bering on Wednesday, per the GFS. A third, stronger, system out
of the North Pacific enters the southern Bering on Wednesday
night. The GFS solution is quite a bit more aggressive, but it is
likely there will at least be areas of storm force winds Wednesday
night into Thursday.

&&

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

Teleconnections support the amplified Rossby Wave pattern to
continue across the North Pacific during the extended period. The
Alaskan mainland will primarily remain under an upper trough which
will allow for these cold temperatures to continue. Thursday
appears to have the greatest chance of sunshine as the moderately
positive Eastern Pacific Oscillation will weaken which supports a
greater likelihood of upper level ridging to move over
Southcentral. There is good ensemble agreement on this upper
level ridge moving over Southcentral on Thursday, though it will
be very short lived. Temperatures will remain cold as the upper
ridging will help decrease cloud cover while the surface pressure
set up will allow for a dry offshore flow pattern. This pattern
can be conducive to nocturnal radiational cooling (clear skies
and light winds) which allows for very cold temperatures
overnight.

With respect to snowfall, there is good deterministic agreement
that a weak surface low pressure system will linger along the
northern Gulf south of the Kenai Peninsula on Tuesday which will
allow for snow showers to move from the northern Gulf into Prince
William Sound. There is a high degree of uncertainty with respect
to how much snow is expected. This surface low will then move
southeastward and allow for cold air advection across Southcentral
on Wednesday.

Farther out west, there is the potential for a strong low pressure
system to move across the Bering/Aleutians Wednesday through
Friday. There is extremely low forecast confidence at this time
due to poor agreement between deterministic and ensemble guidance.
Both the GFS and GEM ensemble guidance show differences of 1.5
standard deviation or more between ensemble members which is a
good indicator of a large variation of possible solutions.
However, one common denominator expected with this low is a large
area of gale force winds with the possibility of a smaller area of
storm force winds within the aforementioned area of gale force
winds.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Watch 152 155 161.
MARINE...Storm Warning 165 170 172-177 179 180 181 185 411-414.
		 Gale Warning 155 171 178.
		 Heavy Freezing Spray Warning 127 128 139.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...KO
SOUTHCENTRAL ALASKA...MF
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AS
MARINE/LONG TERM...KM/ED