Brisk Wind Advisory 51, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
603 AM AKST Sun Dec 19 2021

.ANALYSIS AND UPPER LEVELS...

A broad upper-level ridge has its axis moving through southwest
Alaska this morning. Out west, a strong 300 mb jet spreads from
southwest to northeast across the Bering, which is located in
tandem with the upstream region of the ridge. An associated front
is pushing eastward through the western and central Aleutians, and
southerly flow is bringing warm, moist air from the North
Pacific. Thus, rain and gusty gale force winds have been the
predominant weather conditions for the Bering and Aleutians.

Out east, subsidence ahead of the ridge has been creating low
stratus for much of Southcentral. However, a warm front pushing
eastward across Southwest and toward Southcentral is briefly
lifting the low ceilings at times. This front is also moving in
concert with an upper-level jet. Because there is a deep layer of
warm air behind the front, there has been a relatively clean
transition from snow to rain as it moves eastward. So far, this
transition is evident over Southwest as the front is still pushing
through this region this morning.

&&

.MODEL DISCUSSION...

Overall, the models continue to have good synoptic agreement
through the middle of next week. However, disagreement remains in
regards to the track and amplitude of a shortwave trough moving
over a ridge (over Southcentral) on Tuesday. Currently, the NAM is
very progressive and shows the highest wave amplitude and
greatest QPF amounts, whereas the GFS has the flattest wave
amplitude and lowest QPF amounts. The other model solutions are
somewhere between the NAM/GFS solutions. Nonetheless, the track
and amplitude of this shortwave will determine which Southcentral
areas will end up seeing the most precipitation. The details of
this feature will be closely monitored in the subsequent forecast
packages.

&&

.AVIATION...
PANC...Expect ceilings to hover on the low end MVFR range today
with the possibility for ceilings to drop to IFR by tonight. Low
ceilings may briefly lift at times through the day as a warm front
passes through. Winds are expected to be light and primarily out
of the N to NE.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Today through Wed morning)...

A transient upper-level ridge is building across Southcentral this
morning. Given the relatively weak flow both aloft and near the
surface, low stratus continues to linger from Cook Inlet to the
Mat-Su Valleys and across the Copper River Basin. Although some
additional clearing is expected, the bulk of these clouds will be
tough to scour out of the lower elevations completely. The
challenge today will be whether we see any precipitation squeeze
out of this low cloud deck. Guidance is all showing a weak mid-
level trough moving over top the ridge from west to east through
the day. A developing southwesterly flow ahead of this feature
will begin to advect low-level moisture and warmer air aloft over
the region, especially across Cook Inlet and the Anchorage Bowl.
As the wave passes overhead, it may induce enough lift to allow
for flurries, or perhaps some areas of freezing drizzle, from
Kenai north to Talkeetna, from late Sunday morning into the
evening hours. In fact, the 00z and 12z soundings from Anchorage
do reflect a freezing drizzle profile. In addition, forecast
soundings from Kenai to Talkeetna also indicate this possibility.
Right now, flurries is in the forecast for these locations.
However, the possibility of freezing drizzle will have to be
closely monitored through the day as it is tough to forecast in
advance.

The ridge axis crests over the region Sunday night as a vigorous
shortwave begins to move over Southwest Alaska. The flow aloft
again turns to the west-southwest, allowing more moisture to move
over the area from the North Pacific. Temperatures aloft will
also warm, with areas along Cook Inlet possible climbing above
zero degrees Celsius just as precipitation begins to develop over
the area for Monday morning. Given this, locations in the Western
Kenai Peninsula, mainly along the Sterling Highway from Anchor
Point to Kenai, could see a brief period of a wintry mix of
snow/sleet/freezing rain at the onset of precipitation before
colder air advects in and the precipitation changes to all snow.
For Homer, enough warmer air is expected to advect in the lower
levels of atmosphere to push temperatures above freezing and allow
for a quick transition to rain.

Farther north, the precipitation is expected to be all snow for
the Anchorage Bowl and Mat-Su Valleys. The snow, however, is
expected to begin as wet snow Monday morning then become more
powdery through the afternoon and evening hours as the
southwesterly flow transitions to a northwesterly flow and
temperatures cool both aloft and at the surface.

The challenge to this forecast is snow amount. Usually, low
amplitude waves moving over the region from west to east are
rather moisture starved due to the blocking nature of the Alaska
Range to our west. This shortwave, however, is tapping into a
deep fetch of Pacific moisture that is well over the
climatological norm (150% to 300% of normal precipitable water)
for this time of year. Furthermore, there is more of a
southwesterly than westerly component to the flow aloft, which
should allow this moisture to move up Cook Inlet and not be
inhibited by the Alaska Range to the west.

As the upper-level flow becomes northwesterly Monday afternoon and
colder air is advected in from the north, snow ratios will
increase. This is expected to occur just as omega values, a signal
of lift in the atmosphere, over the Anchorage Bowl and Matanuska
Valley increase. Thus, snowfall rates are expected to increase
from Palmer and along the Glenn Highway to east Anchorage by
Monday evening with snow likely continuing through the overnight
hours. Snow should eventually come to an end by Tuesday morning as
the trough moves east of the area. Snowfall totals are expected
to range from 1 to 3 inches across the Western Kenai Peninsula, 2
to 5 inches for the Susitna Valley, and 4 to 8 inches for the
Matanuska Valley and Anchorage Bowl.

Snow will also overspread the Copper River Basin and eastern
Prince William Sound Monday afternoon through Tuesday morning.
Snowfall amounts here are expected to range from 2 to 4 inches.

Strong cold air advection is expected in the wake of the trough
overnight Monday through the day Tuesday with strong gusty winds
expected through the typical gaps wind areas (i.e. Seward,
Whittier, Valdez, Thompson Pass, Barren Islands area, and Shelikof
Strait).

&&

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

A warm front continues to push eastward across Southwest this
morning. Ahead of the front light snow also continues across the
Lower Kuskokwim Valley from Sleetmute to Sparrevohn. Previous snow
across Aniak has since transitioned to rain while areas further
west have cleared with fog and reduced visibilities being observed
from Hooper Bay to Bethel to Dillingham. Through today,
precipitation will shift further east and into Southcentral while
Southwest experiences a brief period of drier conditions as a
shortwave ridge overspreads interior Alaska. A cold front is
forecast to move into the Kuskokwim Delta later this evening,
eventually spreading southeastward across the remainder of
Southwest through Monday. One of the main forecast challenges will
be the possibility of light freezing rain for portions of the
Lower Kuskokwim Valley. Numerical guidance is suggestive that much
of the precipitation associated with the upcoming cold frontal
passage tonight into Monday will precede the return of colder
temperatures behind the cold front. Thus, freezing rain chances
appear limited but not out of the question. The greatest potential
for snow will exist on the backside of an upper trough as it
translates southeastward from Central Alaska into the northern
Gulf coast. As northerly winds redevelop on Tuesday, upslope flow
over the west and north faces of the Kuskokwim Mountains will
likely experience light to moderate snow on Tuesday.

&&

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

A broad area of high pressure sits stationary to the south of the
Eastern Aleutians and Alaska Peninsula this morning with strong
southwesterly flow extending from the North Pacific into the
Western Bering. Numerical guidance is in good agreement with the
ridge remaining in place through next week with only occasional
and temporary breakdowns from passing shortwave disturbances into
the northern Bering. The end result will be a pattern change with
multiple western Bering lows forecast to impact the Aleutian Chain
through next week. For Sunday, widespread southwesterly gales are
forecast across the Bering with a chance for storm force winds
over zones 178 and 411. Seas over the western and central Bering
will exceed 20 feet in most areas. Elevated winds will briefly
diminish on Monday as one low pressure system weakens offshore of
the Kuskokwim Delta. Meanwhile, the next upper low will be
garnering strength over the western Bering. By late Monday
night/early Tuesday an intense warm front is forecast to lift
across the Aleutians with gale to potential storm force easterly
winds. These winds will gradually expand across the western and
central Bering while transitioning to a southerly direction as low
pressure undergoes significant amplification west of the
Aleutians. Rain, heavy at times, can also be anticipated across
the Aleutians as ample moisture is drawn northward.

&&

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

Gulf of Alaska: Forecast confidence has greatly increased for
Tuesday, with a deeper surface low crossing the Gulf and moderate
to strong cold air advection behind it. This will lead to
widespread gale force winds across the Gulf waters, with strong
and gusty winds through bays and passes. There is some potential
for a period of storm force winds, especially in the Kamishak Bay
to Barren Islands corridor. Winds will diminish a bit on
Wednesday, but another shot of cold air advection is likely on
Thursday, so winds will strengthen again by Thursday - likely
reaching gale force in a lot of areas once again.

Bering Sea: Another deep low will track into the far western
bering Sea on Tuesday. While there is some uncertainty in the
track and depth of the low, it looks like a very large system
which will impact much of the Bering Sea and Aleutians. The
forecast largely indicates widespread gale force winds, but there
is good potential for smaller areas of storm force winds -
especially along the western Aleutians. Smaller and weaker systems
will follow quickly behind the first storm for Wednesday and
Thursday. It's too early to nail down the details for these
storms, but it looks like they will top out at gale force winds
and will affect a much smaller area.

&&

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

There is a high degree of ensemble agreement that an Omega Block
will extend across the North Pacific during the extended period.
The ridge-axis is expected to extend northward from the central
North Pacific to over Southwest Alaska. This upper level pattern
will then continue to persist for the remainder of the extended
period. This will lead to warmer temperatures across Southwest
Alaska and the Alaska Peninsula. Temperatures will be cooler
across Southcentral when compared to Southwest due to broadscale
northwesterly flow to the east of the ridge-axis. Mostly sunny
skies will continue across Southcentral due to strong subsidence.
This pattern will also allow for cold nights across Southcentral
due to clear skies and light winds which will allow for effective
radiational cooling.

The predominant storm track is expected to extend northeastward
across the Bering Sea and into the Bering Strait/Seward Peninsula.
This will allow a couple of warm fronts associated with lows to
move across Southwest Alaska during this period. The first front
will be on Wednesday, second will be on Friday and third will be
later on Saturday. There is low forecast confidence at this time
with respect to the specifics of weather with these fronts,
though temperatures will likely be warm enough across Southwest
to see a variety of precipitation-types. Chances for rain will be
higher in low-lying coastal areas while snow chances will be higher
across interior locations such as the lower Kuskokwim Valley and
the higher elevations of the Kuskokwim and Alaska ranges.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 170, 173-181, 185, 411-414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AF
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BL
MARINE/LONG TERM...ED/SEB