Gale Warning 904, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
540 AM AKST Mon Dec 20 2021

.ANALYSIS AND UPPER LEVELS...

A ridge of high pressure is shifting its axis east toward the Al-
Can border. An impressive 140 to 160 kt jet streak at the 250 mb
level continues to run up and over the northern part of the ridge
and then turns back toward the southeast. Subsidence underneath
the ridge is helping to trap widespread stratus over Cook Inlet
through the Susitna Valley.

A strong surface low just east of Kamchatka is interacting with
the surface ridge over Southwest Alaska to bring in a strong
southwesterly flow for both the surface and aloft over most of the
Bering Sea which is increasing the amount of warm air flowing int
the region. Based on the CIMSS MIMIC imagery, a steady stream of
tropical moisture of 1-2" of precipitable water extends
northeastward to southern Alaska.

&&

.MODEL DISCUSSION...

Models are in good synoptic agreement overall as we head into the
middle of the week. The main issue is with some important
differences in temperatures aloft which will impact precipitation-
type, especially for the southern Kenai Peninsula. For the Bering
Sea and Aleutians models are coming into a better consensus with
the track and intensity of the low moving into the southwest
Bering Sea from the North Pacific. Therefore, forecast confidence
remains average.

&&

.AVIATION...

PANC...The main challenge this morning is the precipitation type.
Current thinking is that the terminal may see snow mixed with ice
pellets as the dominant precipitation. Colder air moves late this
afternoon and evening to change any mix to all snow. Expect MVFR
to IFR conditions to perish with light winds through the TAF
package.

&&

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

The well-advertised storm system expected for much of Southcentral
today is on track. A period of light snow was already reported
overnight for locations along Cook Inlet and area radars are
beginning to show precipitation develop across the Susitna Valley
and southern Cook Inlet as of 5am. The precipitation is in advance
of a vigorous shortwave that is moving toward Southwest Alaska
this morning. As it does, and the ridge downstream slides farther
east, a developing southwesterly flow will advect more moisture as
will as warmer temperatures aloft.

The warm nose of air aloft looks to extend up Cook Inlet this
morning, with the potential for mixed precipitation (snow, sleet,
freezing rain) for parts of the western Kenai Peninsula through
Monday morning. There is still some uncertainty as to whether this
warm nose will make it over the Anchorage Bowl. Latest guidance
is trending colder, and that would lend to all snow across
Anchorage with perhaps an slight increase in snow totals,
especially for west Anchorage where an initial mix of
precipitation was favored. However, given the uncertainty, a
snow/sleet mix is in the forecast this morning changing quickly to
all snow by early afternoon.

Farther south, it still looks like enough warm air near the
surface works into the southern Kenai Peninsula for places like
Homer to see mainly rain from this event. Farther north,
precipitation will likely be all snow for the Mat-Su Valleys,
with the highest totals expected across the eastern half of the
Matanuska Valley ( up to 10 inches of snow) given the
southwesterly to northwesterly flow aloft through the duration of
the event.

The transition of winds aloft to a colder, more northwesterly flow
later this afternoon will also mean a cooling of the atmospheric
column and higher snowfall rates, especially for favored upslope
locations such as along the Glenn Highway around Eagle River and
the Anchorage Hillside. Again, upwards of 10 inches of snow is
expected for these locations through early Tuesday morning.
Elsewhere, 1 to 4 inches is possible for the western Kenai
Peninsula; 2 to 5 inches for the Susitna Valley; 4 to 8 for the
remainder of the Matanuska Valley and Anchorage Bowl.

By Monday afternoon, the snow spreads east as a surface low spins
up in the northern Gulf. Recent model runs are more bullish with
snow amounts for the Copper River Basin and around Valdez given
the moist southerly flow in the lower levels and the colder air
advecting from the north through the Monday evening and nighttime
hours. 4 to 8 inches of snow is now expected for the Copper River
Basin with 2 to 6 around Valdez and Thompson Pass.

This system exits the region by Tuesday morning. However, decent
cold air advection, trailing upper-level waves, and a stout jet
over the region will all work to produce gusty gap winds through
passes and along the coast. These winds will likely cause area of
blowing snow, with the potential for blizzard conditions through
Thompson Pass overnight Monday into Tuesday as the already fallen
snow gets resuspended by the wind. Blowing snow may also greatly
reduce visibility through Keystone Canyon and around Valdez at
times. Although these winds are expected to peak in intensity
by Tuesday afternoon, wind gusts will persist into Wednesday as
temperatures trend downward in the wake of the cold air
advection.

&&

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

High pressure ridge axis previously over the Bering has shifted
east of the Alaska Peninsula this morning as a shortwave trough
rides over the top of the ridge. Satellite and radar reveal
morning areas of light rain and mist occurring over much of
Southwest, though this activity is forecast to gradually shift
east and into Southcentral over the course of the day. Of greater
interest is the development of a strong upper low over the Bering,
which is forecast to bring a shortwave trough into Southwest late
Tuesday. Despite temperatures having warmed to above freezing at
the surface with yesterday's warm front, the arrival of a cold
front will bring temperatures back below freezing tonight into
Tuesday morning.

This will set the stage for more snow and potential freezing rain
for the Kuskokwim Delta and Lower Kuskokwim Valley Tuesday night
into Wednesday. Precipitation is expected to begin as snow and
then transition to a snow/freezing rain mix as temperatures aloft
begin to increase due to strong warm air advection. While much
uncertainty remains in the finer details, the highest snow amounts
may remain confined to the coast from Hooper Bay to Kipnuk where
6 inches are possible. Further inland, amounts of 3 to 5 inches
will be possible for locations from Bethel to Togiak. Freezing
rain may mix in with the snow at times promoting slick roads and
surfaces. As the shortwave trough continues east lighter snow
accumulations will be possible across the Lower Kuskokwim Valley
into Wednesday.

Overall confidence in precipitation type for the snow/freezing
rain Tuesday night into Wednesday, decreases with southward
extent. The fly in the ointment remains the question of how much
warm air is pushed northward, which could easily make or break the
forecast. Therefore, have decided to wait on the issuance of any
advisory/watch products until confidence is higher.

&&

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

A weak cold front is draped from the Aleutians northeastward
across the Pribilofs this morning. Scattered showers are noted
along the front though most locations across the Aleutians are
currently seeing more mist than rain along with widespread low
stratus. The cold front is forecast to push inland of the
Kuskokwim Coast later tonight. A shortwave trough is also noted
skirting east across the central Bering this morning. This feature
is forecast to diminish into the overnight hours as a surface
high makes its way just north of the Central Aleutians to Saint
Matthew Island by Tuesday morning. Before the shortwave weakens,
an area of gales and 20+ foot seas will accompany its movement
just north of the Pribilofs this afternoon.

On tap, a strong warm front is forecast to impact the Aleutian
Chain tonight into Tuesday. Gale to storm force easterly winds are
anticipated as the front lifts across the chain bringing heavy
rain to the region. East winds Tuesday morning will become
southerly during the day with elevated winds and gusts through the
gaps. The strength of the front and its parent low over the
western Bering will result in widespread small craft winds with a
large swath of gales over much of the Bering by Tuesday night.
Coverage of winds will continue to expand in area through
Wednesday, eventually reaching to Nunivak Island and the Alaska
Peninsula as winds change from a southerly to southwesterly
direction Wednesday afternoon.

&&

.MARINE (Days 3 through 5/Wednesday through Friday)...

Bering/Aleutians...

There is a moderate degree of confidence overall in the 3-5 day
period. A low pressure system moving northeastward along the
Kamchatka Peninsula will make for areas of gale force
southwesterly winds across the western Bering/Aleutians on
Wednesday. There are discrepancies between guidance with respect
to how strong this low center will be, though there is a moderate
to high degree of confidence with respect to the track of this
system. Forecast confidence is moderate to high for Thursday and
Friday as winds will remain below gale force.

Gulf of Alaska...

There is a moderate to high degree of forecast confidence overall
for the 3-5 day period. Low pressure will continue to move away
from the area. The only area of uncertainty are the exact strength
of northwesterly gap winds around the Barren Islands region. As of
now, these winds are expected to at least reach gale force.

&&

.LONG TERM FORECAST (Days 4 through 7/Thursday through Sunday)...

An Omega Blocking pattern will extend across the North Pacific for
the entirety of the extended period. The ridge-axis will remain
over Southwest Alaska which will make for warmer temperatures
across the area. Southcentral will be cooler due to broadscale
northwesterly flow from being east of the ridge-axis. This will
allow for mostly sunny skies during the day and clear skies with
light winds at night as a result of strong subsidence aloft.
Moreover, clear skies and light winds will allow for cold
nighttime low temperatures across Southcentral due to radiational
cooling. The predominant low track will be across the central
Bering/Aleutians through the Bering Strait/Seward Peninsula.
During the extended period there will be chances for precipitation
across Southwest as broad southwesterly flow will allow for
moisture to enter the region. The bulk of the precipitation will
be on Saturday and Sunday. One area of lower confidence is
precipitation type as ample amounts of warmer air will move in
aloft and result in a variation of precipitation types across
Southwest. For now, the greatest chance of rain will be along
coastal areas in the Kuskokwim Delta and Bristol Bay. The best
chance for snow will be in inland locations such as the lower Kuskokwim
Valley and in the Kuskokwim and Alaska Ranges.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory: 101, 111, 121, 141.
   	     Winter Storm Watch: 131.
MARINE...Storm Warning: 131, 178.
		 Gale Warning: 120, 121, 129, 130, 132, 138, 139, 141, 175-177,
						351, 352, 411-413.
FIRE WEATHER...NONE.

&&

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