Winter Weather Advisory 43, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
554 AM AKDT Sun Sep 19 2021

.ANALYSIS AND UPPER LEVELS...

Clear skies across the southern mainland overnight have allowed
for ample radiational cooling, resulting in many locations across
Southcentral dropping to near freezing temperatures or in some
cases, below freezing temperatures. The Copper River Basin had no
trouble dropping to the upper 20s overnight while population
centers in the Mat- Su Valleys and Kenai Peninsula hovered in the
mid 30s with some locations falling to right around the freezing
mark. Meanwhile, portions of East Anchorage fell just below
freezing.

As we analyze the pattern this morning, a transition is currently
underway as a departing shortwave trough continues to progress
eastward toward the Alaska- Yukon border. Weak upper level ridging
is keeping skies clear across much of the southern half of the
state. Farther west, a vertically stacked low centered south of
the Aleutian Chain continues to bring cloud cover, gusty winds and
rain to much of the Bering and southern Alaska Peninsula. Farther
north, an Arctic low centered over the Beaufort Sea will be the
next key feature as an associated short wave begins to drop
southward, introducing much colder air to southern Alaska.

&&

.MODEL DISCUSSION...

Model solutions are in good synoptic agreement and depict the low
currently south of the Aleutian Chain tracking eastward into the
Gulf for Monday. Model solutions are also keying in on an upper
level trough moving southward and introducing a much colder
airmass to the region. However, there are several differences in
the interaction of these two features. At this time models agree
that the upper level trough and inbound low will eventually merge,
however the GFS and NAM solutions paint a different picture with
an open wave over the northern Gulf Tuesday morning. Conversely,
the EC and Canadian solutions show a much more organized system
with the bulk of the upper level energy centered over the northern
Gulf coast. Overall, confidence remains low beyond 36 hours as
several key details still need to be ironed out. As was
previously mentioned, the Alaska Range tends to shield
Southcentral from the first few cold air masses of the season,
thus confidence remains somewhat low with how these two features
will interact and whether enough cold air will work in to produce
the snowfall that some model solutions are calling for.

&&

.AVIATION...

PANC...VFR conditions will continue under high pressure with
light northerly winds. As a Gulf low nears the northern Gulf
coast, gusty southeasterly winds through Turnagain Arm will pick
up overnight tonight. These winds are expected to remain over
Turnagain Arm and not bend over the terminal.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Today through Tue night)...

Clear skies and light winds overnight made for efficient
radiational cooling leading to the low temperatures being observed
this morning across Southcentral. Temperatures will rebound back
into the mid 50s today across much of the region, with a
relatively quiet and dry day in store. However, the next round of
impactful weather brews as a low currently located south of the
Aleutian Islands begins to move into the Gulf this afternoon and
evening. The first impacts from this next storm system will be
realized with the development of a weak coastal ridge which will
drive a Turnagain Arm wind which has the possibility of affecting
West Anchorage this evening, though winds are expected to be
weak and brief. Upslope precipitation will develop overnight
ahead of the low, with more considerable rainfall potential along
the north Gulf coast beginning Monday morning with the arrival of
the front.

Monday night, an Arctic trough, more potent than last night's,
will begin to phase with the Gulf low, making for a complex setup
over the eastern portions of Southcentral. As moisture rides north
on strong southerly flow over the Copper River Basin, cold air
seeps through the Alaska Range behind the Arctic trough, resulting
in rain turning to snow late Monday night in places like Gulkana.
Confidence is increasing that this setup is likely to result in
the first significant snow accumulation of the season in
Southcentral Monday night through Tuesday, although the strongest
impacts will be confined to the Copper Basin, where several
inches of snow may fall along the Glenn and Richardson Highways.
Some snow showers will be possible on the Anchorage Hillside with
the initial brunt of colder air aloft Monday night, though
accumulations, if any, are expected to be minimal.

Gap winds will quickly develop Tuesday morning behind the surface
low as it moves east, aided by strong synoptic forcing and cold
air advection brought by the Arctic trough. Strong gusty winds
should occur out of gaps from the Barren Islands eastward through
the Valdez area, with widespread gales picking up across the
Gulf on Tuesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

Mild, dry and cool conditions anticipated today. The models all
bring the polar jet into Southwest Alaska this forecast period.
Late Sunday, an Arctic front will push across Bethel and
Sleetmute. This system will usher colder temperatures and
northerly winds into the region. Additionally, brief light showers
are possible overnight. Monday will be breezy and brisk.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

On satellite imagery, there is an occluded front pushing across
the central and eastern Aleutians this morning. There is 988 mb
surface low south of Dutch Harbor that will move eastward towards
the Gulf Of Alaska. More gale force winds are expected today. On
another channel of satellite imagery there is a discernible fetch
of modified tropical air entrained into the occlusion. But there
is also deep tropical moisture associated with this system which
is further south of the Aleutian chain. There is a second low that
will approach the Western Aleutians Monday. Between these 2 lows,
an area of high pressure that will extend from the north Pacific
and into the Bering.

&&

.MARINE (Days Monday through Wednesday)...

Gulf of Alaska...A gale-force front will be draped along the
north Gulf coast, stretching east of Kodiak Island. As the
responsible low pressure system sits in area, a cold arctic air
mass will filter through the coastal mountains on Monday night
into Tuesday which will drive gusty gale-force sustained winds
through the gaps, with storm-force gusts, especially Kamishak Gap
and Barren Islands regions. The next gale-force front rolls in
from the Pacific quickly on Wednesday afternoon and evening.

Bering Sea...Monday starts with high pressure encompassing much of
the Bering. The next system skirts the Aleutians to the south, so
winds along the front in the southern Bering should be sub-gale
force.

&&

.LONG TERM FORECAST (Monday through Friday)...
The big story with the extended forecast is an early Fall
"winter" storm (even though it is still Summer until Wednesday
morning). A cold front dropping southward will bring a very
winter like air mass to the area. Guidance has been very
consistent with digging the associated trough which will phase
with a north Pacific low pressure. The combination of these two
systems is needed to get the magnitude of Arctic air this far
south this time of year. The first trough brings the cold air
through the Interior and to the edge of the Alaska Range, while
the second system links up and pulls it through. Based on these
factors, details have yet to determined, but there is increasing
confidence of below normal temperatures, especially overnight
minimums which will be below freezing in many locations.

On the subject of precipitation, the big question will be how
much moisture is still in place when the cold air arrives. It will
be difficult to get snow down to sea level this time of year, but
for upper elevations of the Chugach it will be possible. Higher
confidence for snow exists in the Copper River Basin and Thompson
Pass where snow to the surface looks likely at this point along
with some accumulation. The next facet of this system will be the
first real "winter outflow" winds of the season. The Arctic air
mass being pulled through the gaps will make for gusty winds in
the usual favored locations along the north Gulf coast.

The cold air mass remains in place, though moderated as the Gulf
system exits quickly on Tuesday evening, but the next system comes
into the Gulf on Wednesday/Thursday keeping precipitation along
the coast and outflow wind in place. Definitely an active winter-
like pattern.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Special Weather Statement: 131, 141.
MARINE...Gales: 172, 174.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...KO
SOUTHCENTRAL ALASKA...CQ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
MARINE/LONG TERM...MTL

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
139 PM AKDT Sun Sep 19 2021

AKZ141-200030-
Copper River Basin-
Including the cities of Glennallen, Eureka, McCarthy, Paxson,
and Slana
139 PM AKDT Sun Sep 19 2021

...FIRST ACCUMULATING SNOW OF THE SEASON POSSIBLE FOR COPPER RIVER
BASIN...

A developing storm will bring winter-like conditions to the
Copper River Basin beginning late Monday night, continuing through
late Tuesday night. A low pressure system will approach from the
Gulf, bringing plentiful moisture into the region. Meanwhile, a
cold front from the north will bring in an Arctic air mass. The
combination of those ingredients will bring the potential for
accumulating snow with this early-season system. Precipitation
will start as rain on Monday before turning to snow overnight
Monday night. While warmer ground and road surface temperatures
will keep snow from accumulating initially, the combination of
precipitation intensity and a cooling air mass should eventually
win out, resulting in accumulating snow. While it is still too
early to nail down amounts, expect more accumulation in the
southern Basin than the northern Basin. The Glenn, Richardson, and
Edgerton Highways will all be impacted.

Stay tuned to the most current information at:
www.weather.gov/anchorage

AKZ131-200030-
Northeast Prince William Sound-
Including the cities of Valdez and Thompson Pass
139 PM AKDT Sun Sep 19 2021

...AN EARLY SEASON STORM TO BRING POSSIBLE SNOW AND BLOWING SNOW
TO THOMPSON PASS...

Monday afternoon and evening, a front will bring rain to the area.
As cold air filters southward on Tuesday morning there is
increasing likelihood that rain will turn to snow and begin to
accumulate. At the same time, increasing winds on Tuesday may begin
to cause visibility issues. Blowing snow should be mitigated by
warm road and surface temperatures, but the combination of falling
snow and wind are expected to be impactful.

Stay tuned to the most current information at:
www.weather.gov/anchorage

AKZ125-200030-
Western Prince William Sound-
Including the cities of Whittier, Seward, Girdwood,
and Moose Pass
139 PM AKDT Sun Sep 19 2021

...STRONG OUTFLOW WINDS EXPECTED OVER WHITTIER AND SEWARD MONDAY
NIGHT THROUGH WEDNESDAY...

A low moving into the Gulf will combine with incoming cold air
from the north to produce strong outflow winds through Seward and
Whittier Monday night through Wednesday. Sustained winds between
30 and 45 mph are expected for both areas with gusts between 45
and 55 mph. Winds will peak Tuesday morning and diminish slowly
through Wednesday. Time should be taken prior to the arrival of
the winds to secure loose items outdoors. With foliage still on
the trees there is a higher likelihood of damage to trees and
falling tree branches.

Stay tuned to the most current information at:
www.weather.gov/anchorage

AKZ171-200030-
Kodiak Island-
Including the city of Kodiak City
139 PM AKDT Sun Sep 19 2021

...RESUSPENDED ASH FROM NOVARUPTA LIKELY TO IMPACT SOUTHERN
KODIAK ISLAND MONDAY NIGHT THROUGH TUESDAY...

Strong northwesterly winds to 45 mph are expected to develop
across Shelikof Strait and the waters around southern Kodiak
Island Monday night through Tuesday. These winds are likely to
resuspend ash from the 1912 Novarupta eruption, causing haze and
reductions in air quality. Villages such as Karluk and Akhiok are
most likely to be impacted.

Those with respiratory ailments in these areas are encouraged to
avoid or reduce outdoor activity to avoid inhaling the ash.

The winds will subside by Tuesday night, which will cause the air
quality to improve.