Winter Weather Advisory 15, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
421 PM AKST Sun Feb 21 2021

.ANALYSIS AND UPPER LEVELS...

Light snow is falling from the Soldotna area north through the
Anchorage Bowl and into the Mat-Su Valleys. This snow is falling
out of some low-level cloud cover that is slowly working its way
north. This minor event has developed from instability associated
with a weak shortwave trough moving through the area. Likewise,
on the Alaska Peninsula on-shore flow and weak instability is the
cause of light snow for areas around Bristol Bay. Otherwise, quiet
weather and cold temperatures due to high pressure continue for
mainland AK. The next round of low pressure is also entering the
Western Aleutians.

&&

.MODEL DISCUSSION...

Model performance on the synoptic scale through the short term is
good. However, models completely missed the light snowfall event
ongoing through Southcentral. This resulted in manual edits
through the morning. The closest was the GFS, which only had a
chance of precipitation today. Models also missed precipitation
near King Salmon in Southwest AK. Both appear to be from the
mishandling of a couple shortwave troughs over our forecast area.
From this, we can say model performance on the mesoscale has been
poor for much of our forecast area today.

&&

.AVIATION...

PANC... Light snow and MVFR conditions are expected to continue
through the afternoon. Some light snow showers and patchy fog may
develop at or near the airfield overnight, though there is some
uncertainty if these will develop at the airfield itself.
Therefore, included vicinity showers in the TAF with mist. We are
also expecting MVFR ceilings to redevelop overnight.

&&

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

Broad troughing in the upper levels persists through tomorrow,
followed quickly by a short-lived upper level ridge Tuesday and
Tuesday night. Ridging will help clear out the skies across much
of Southcentral by tomorrow afternoon, resulting in cooler
temperatures and drier conditions tomorrow through tomorrow night.
Light snow showers linger across the eastern portion of the
Copper River Basin as well as Cordova and areas east through
tomorrow morning. Cloudy skies and active weather returns Tuesday
afternoon when a shortwave trough pushes towards the North Gulf
Coast. Precipitation is expected across the North Gulf Coast as
well as the Susitna Valley Tuesday afternoon and evening, becoming
more widespread across the Southcentral region, including Kodiak
Island, by Tuesday night. There is still quite a bit of model
inconsistency, especially on Wednesday, with this shortwave with
regards to when it becomes a closed low and its strength. Thus,
specific details still need to be fine tuned.

A tight pressure gradient sets up along the Alaska Range Monday
afternoon, aiding in the strong, gusty flow of cold air across
southern Cook Inlet and through the Barren Islands, as well as
across Kodiak Island, through Monday night. A brief period of
storm force winds are likely west of the Barren Islands, with seas
between 20-30 ft Monday afternoon. Heavy freezing spray will also
be a concern tonight and tomorrow for southern Cook Inlet and
Shelikof Strait. This cold air flow over the warmer waters (think
lake-effect snow) is favorable to create a snow band across
Kachemak Bay and Homer Bluffs. While short-lived, this band would
bring a period of heavy snow and blowing snow, but expected
accumulations remain low. Though not as strong, another tight
pressure gradient exists over the Kenai Mountains, causing winds
to become gusty for both Seward and Resurrection Bay.

&&

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

An elongated shortwave trough stretched out across the Kuskokwim
Valley southward through west-central Bristol Bay will continue
moving eastward through this evening. Light snow continues to fall
out ahead of it especially for areas east and northeast of King
Salmon. Generally less than a half inch of total accumulation is
expected from this activity by the time it ends this evening.

Next, a more potent closed off upper level low will barrel its way
through Southwest overnight and into the day on Monday. This will
bring a reinforcing shot of Arctic air for Southwest with areas
across the Kuskokwim Delta falling possibly into the 20s below
zero. This will all depend on how much cloud cover actually moves
through as this upper level low passes through. Temperatures
for the Kuskokwim Valley will be in the teens below zero but could
be colder all depending on the amount of cloud cover in place. If
it stays clear, 20s below zero will be possible here as well.
Bristol Bay ranges from single digits below zero inland to single
digits to near zero along the coast.

Lastly, a frontal system currently moving through Shemya and the
western Aleutian Island Chain this afternoon will continue
across the Bering reaching Nunivak Island and the Southwest coast
by Monday evening. Expect the remainder of the forecast period
from Monday through Wednesday afternoon to remain unsettled.
Initially, when the front approaches, there may be a period of
blowing snow along the Kuskokwim Coast and for Nunivak Island. At
this time, it appears to be a brief window Monday evening/early Tuesday
as the front will be weakening on its approach. Snowfall amounts
from Monday night through Tuesday evening range anywhere from 3 to
5 inches across Southwest with a similar amount Tuesday evening
through Wednesday afternoon. Since the snow is not expected to all
come at once, an advisory is not warranted at this time.
Temperatures are also expected to be on the warming trend
beginning on Tuesday leading to the potential for precipitation
type issues mainly from King Salmon south down the Alaska
Peninsula. Right now, continuing to stay with a rain/snow mix for
King Salmon and areas nearby as there is still uncertainty as to
how fast this warm air moves in. Stay tuned for the latest
updates.

&&

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

Low stratus with embedded snow showers are occurring for the
Pribilofs and areas north and south near the ice edge. Cold air
flowing over the still relatively warm water of the Bering Sea is
creating an unstable atmospheric profile that is generating
clouds and snow showers as a result. Any of these snow showers
will continue to briefly reduce visibility as they pass through
the Pribilof Islands tonight and into early Monday.

The pattern change storm for much of Alaska is on its way with
the front currently moving through the Western Aleutians and the
western Bering Sea. This front will continue moving through the
Bering and into the Southwest Coast Monday evening. Expect
unsettled weather along the entire Aleutian Island Chain for the
forecast period. After the front moves through, the center of the
low will meander in the west-central Bering Sea through Tuesday
night before dissipating. The Pribilofs will remain the tough
precipitation type forecast as enough cold air may hang on to
keep much of any precipitation that moves through as snow.
However, rain may mix in at times. For Dutch Harbor, Cold Bay and
Sand Point, enough warmer air will move in to allow for mostly
rain.

Lastly, another frontal system will move into the Western
Aleutians and the western Bering Sea Monday night continuing the
active pattern and more unsettled weather through mid-week. Also,
heavy freezing spray will continue near the ice edge through mid-
week.

&&

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

Bering: The eastern Bering low moves inland for Thu. A new low
and front move across the western Bering through Fri. Models are
mixed, but confidence is good. Widespread small craft winds and
waves cover the Bering with the low on Wed, but an area of gale
force winds south of the Aleutians moves into the southern Bering
for Thu, then diminish Fri. Some areas of gale force winds over
the northern Bering linger for Fri. Waves build to 30 feet on the
Pacific side, 20 feet Bering side Thu and Fri.

Gulf of Alaska: A low tracks along the front across the Gulf,
then inland Thu. Widespread gale force winds are expected around
the low through Thu, then diminish to small craft winds on Fri.
Waves build to 20 feet Thu, then subside Fri.

&&

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

The trend in the long term continues to be an active, zonal
pattern midweek transitioning to a more meridional pattern going
into the weekend. The pattern slows down (as one would expect)
when this transition occurs. However, the pattern looks like it
continues to send troughs and ridges through our area, just at a
higher amplitude and slower pace. So, active weather will continue
through the week. Outside of general agreement on the synoptic
pattern, models have a difficult time. This especially goes for
low pressure centers. So, the long term still looks quite messy.
This means moderate to high confidence in the synoptic pattern,
but low confidence for what comes of it with the weather.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning: 130.
Gale Warning: 120 131-138 165 170 172-174 176 178-185 351 352 411-414.
Heavy Freezing Spray Warning: 130 138 139 141 150 160 179 180 185
                              412 414.

&&

SYNOPSIS AND MODEL DISCUSSION...BJB
SOUTHCENTRAL ALASKA...KM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MV
MARINE/LONG TERM...MK/BJB