Winter Weather Advisory 22, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
520 PM AKST Wed Mar 3 2021

.ANALYSIS AND UPPER LEVELS...

A broad ridge building over the western Bering is forcing a more
amplified pattern to set up as an elongated trough shifts eastward
toward the Alaska-Yukon border. Farther west, there is an upper
level low over Kamchatka gradually tracking eastward. The
associated front is in place across the western Bering,
transporting warmer, southerly flow northward over much of the
Western Bering. Meanwhile over Southcentral, fog and low level
stratus in place this morning has dissipated, and northerly
outflow winds continue to aid the clearing process. A weakening
low in place over the central Gulf continues to bring a few
lingering showers to areas along the northern Gulf coast.

&&

.MODEL DISCUSSION...

Overall, model solutions remain in good synoptic agreement. The
finer details, however, reveal several discrepancies. As the
Kamchatka low moves east, its front will push inland over
Southwest Thursday. Model solutions have been struggling with the
location of the triple point that forms in the vicinity of Kodiak
Island Friday. At this time, the NAM seems to be the outlier,
keeping the developing low much weaker and broader, without a
defined circulation. In contrast, the GFS/EC/Canadian solutions
all advertise a well defined low spinning up and moving into the
western Gulf by Friday morning. The GFS solution even suggests
storm force winds on the backside of this low Friday morning,
however it is the lone model to suggest this. The track this low
takes will largely affect the weather over the next few days,
first for Kodiak Island, and then for Southcentral. How far north
or south this low tracks as it travels eastward across the Gulf
will determine snowfall amounts and wind fields. At this time,
confidence is high that temperatures will be cold enough to
produce an all-snow event for areas across the northern Gulf, but
confidence remains low with regard to the the track and position
of this low.

&&

.AVIATION...

PANC...VFR conditions are expected through the afternoon with
light northerly winds. Freezing fog over Cook Inlet may develop
again during the overnight hours, and drift into the terminal
during the early morning on Thursday, though expect fog to
dissipate by late Thursday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 3: Today
through Saturday afternoon)...

A cold air mass remains anchored in place over Southcentral with
quiet and clear conditions predominantly expected through this
evening along with continued cool temperatures. Gap and outflow
winds along the North Gulf Coast supported by offshore flow and
cold air in place over the interior will reach their peak this
evening. The strongest winds will blow through the Copper River
Delta and Thompson Pass, where northerly gusts as high as 50 to 60
mph are possible through tonight. These winds should shut off
rapidly during the day on Thursday as low level flow reverses and
becomes onshore by Thursday evening. Areas of fog are possible
once again over parts of the Mat-Su valleys after nightfall as
the boundary layer decouples and temperatures near the surface
cool into the single digits.

Attention from there shifts to the next system approaching from
the Southwest. A triple point low is expected to form along a
weakening front moving in from the Southwest, and this compact low
will track somewhere just south of Kodiak Island on Thursday Night
into Friday. A quick-hitting but possibly intense round of wet
snow is likely for Kodiak Island north of the low center, with
3 to 6 inches of snowfall currently expected for Kodiak City late
Thursday into Friday morning. However, some uncertainty persists
with the track of the low, and any shift north or south could
result in higher or lower totals than currently expected. As the
low pushes farther east Friday, snow will spread into the eastern
Kenai Peninsula along an inverted trough extending from the Gulf
low. Several inches of snowfall are possible from Friday into
Saturday with this band along parts of the Kenai Peninsula
including Seward, Homer and Whittier, but uncertainty here remains
for where the axis of heaviest snowfall will set up. Minor
snowfall accumulations are also possible on Saturday morning as
the band moves west over the Matanuska Valley prior to the
passage of an upper level shortwave, along with another shot of
much colder arctic air moving in from the northwest.

&&

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

An active weather pattern is in store for Southwest Alaska over
the next 3 days. An occluded front (warm occlusion) will approach
Southwest Alaska this evening and tonight. Winds will increase
overnight to gale force along the coastal Kuskokwim Delta and
Nunivak Island by Thursday morning. The strongest winds will occur
in and around Etolin Strait due to this area being prone to the
localized funneling of south to southeasterly winds. Snow will
begin falling early Wednesday morning across Nunivak Island and
reach the Kuskokwim Delta by mid-morning Wednesday. The snow will
then begin in Bristol Bay during the late morning hours on
Wednesday and the Kuskokwim Valley by mid-afternoon. Accumulations
range from 3-5" across the Kuskokwim Delta, 4-8" across Nunivak
Island, 3-6" across Bristol Bay and 3-5" across the Kuskokwim
Valley with locally higher amounts in spots. A Blizzard Warning
has been issued from 6am AKST Wednesday morning through 6pm AKST
Wednesday evening for the coastal Kuskokwim Delta and Nunivak
Island as gale force winds in addition to this snow will restrict
visibility to a quarter of a mile or less at times. Farther inland
in the Kuskokwim Delta and Kuskokwim Valley, winds will not be as
strong, though visibility can be restricted to a half of a mile
at times inside of the heaviest snowbands. A Winter Weather
Advisory has been issued for Bristol Bay for snow and blowing snow
from 11am AKST through 11pm AKST as visibilities will be reduced
to a half of a mile at times with areas in the heaviest snowbands
expected to see 6" of snow within a 12 hour period.

With respect to precipitation type, there is high confidence that
the precipitation will remain as snow across the Kuskokwim Delta,
Kuskokwim Valley, Bristol Bay and the northern AKPEN. BUFKIT
soundings consistently show temperatures remaining below freezing
from the surface through the upper levels. Likewise, guidance is
consistent in the warm front occluding before it reaches Southwest
Alaska. This means the above freezing layer aloft will erode and
thus make for below freezing temperatures from the cloud to the
surface. The NAM was the warmest outlier with bringing this warmer
layer farther north compared to the rest of guidance and this
solution also showed the above freezing aloft become completely
eroded before the onset of precipitation. Farther to the south
(i.e. southern AKPEN), forecast soundings show this above
freezing layer aloft not eroding completely which will allow for
the snow to mix with rain at times. There is also no cold air
damming present and this above layer aloft will be short-lived
which inhibits the potential for freezing rain or sleet.

The snow will then gradually taper off across Nunivak Island late
Thursday afternoon, Thursday evening for the Kuskokwim Delta,
Thursday night for coastal Bristol Bay and Friday morning for the
Kuskokwim Valley.

We are currently monitoring the potential for elevated water
levels along coastal Bristol Bay Friday afternoon into Saturday.
Confidence remains low at this time, however with the current
pattern of widespread gale force westerly winds combined with long
period 20-30ft swell behind this occluded front it cannot be
ruled out.

&&

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

An active weather pattern is in store for the Bering/Aleutians over
the next 3 days. A quick moving transient ridge of high pressure
will pass over the eastern Aleutians and AKPEN tonight and thus
weaken the strong northwesterly gap winds throughout the coastal
marine zones south of the AKPEN and eastern Aleutian Chain. A
front that is currently approaching the central Aleutians is
producing widespread storm force winds in the western Bering Sea
with an even larger area of gale force winds. This front will
continue to move eastward and completely move over land by
Thursday afternoon. Areas along the Aleutian chain that are prone
to the localized funneling of winds from the south will experience
enhanced winds. Gusts associated with this front will reach low-
end hurricane force in localized areas. Behind this front,
widespread west-northwesterly gale force winds with localized
areas of storm force winds will produce large westerly swell that
will build to 30-35ft in spots across the central Bering Sea. The
combination of widespread gale force winds with a very long fetch
over the water allow for this large swell to form. This swell
will remain long in period. This 30-35ft westerly swell will begin
to gradually abate Thursday afternoon and will remain 20-25ft on
Friday.

&&

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

Bering Sea: A Northern Bering low tracks into Northwest Alaska,
and its front moves across the Aleutians and Bering into Western
Alaska Sat night. Widespread gale force winds and waves with some
areas of storm force gusts moves into the Eastern Bering and
diminishing Mon. Seas to 24 feet and heavy freezing spray subsides
by Mon. Forecasts become unclear towards Mon. A new low and front
moves into the Western Aleutians and Bering for Mon. Seas
building to 20 feet around the Western Aleutians Mon.

Gulf of Alaska: A low exits the Gulf Sun. Strong northwesterly
outflow winds from the Alaska Peninsula weaken. Model confidence
is good. Widespread gale force winds and waves with an area of
storm force winds near the Barren Islands and heavy freezing spray
along the Alaska Peninsula diminish to small craft winds for Sun.
Wave heights 22 feet over the Gulf subsiding late Sun. A new low
and front enter the Western Gulf Sun night. Widespread
southeasterly small craft winds spread across the Gulf. Waves
building to 12 feet for Mon

&&

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

Looking at the averaged big picture, models are in pretty good
synoptic agreement Sunday and Monday. An upper level low will
remain across the eastern portion of Siberia and is expected to
have a trough dig across the Bering and Aleutian Chain late this
weekend. Ahead of that trough, ridging sets up across
southcentral.

Overall this means temperatures will continue to steadily climb
throughout the daytime hours Sunday and Monday across much of the
region. Inland areas across southcentral could see temperatures
rise above freezing for the first time since the middle of
January. This pattern could potentially turn into a snowier
regime for Southcentral and in particular across the Anchorage
Bowl. However, many factors have to come together for Anchorage to
get the snow, though, the storm track looks favorable with model
guidance mostly bringing lows near and into Prince William Sound
with details of each uncertain.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 155.
		 Winter Weather Advisory 161.
MARINE...Storm Warning 177-179 181 185 411-414.
		 Gale Warning 131 150 155 165 170-176 180.
		 Heavy Freezing Spray Warning 121 130 138 160 165 180 185
									  414.
FIRE WEATHER...NONE.

&&

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