Winter Weather Advisory 27, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
546 AM AKST Thu Mar 11 2021

.ANALYSIS AND UPPER LEVELS...

The low that facilitated a deformation band that dropped abundant
snowfall from parts of the Kenai Peninsula to much of the Mat-Su
Valleys has now moved into the Prince William Sound this morning.
Snow showers continue to spiral around the low near the sound,
with snow also spreading north into the Copper River Basin. The
band responsible for the bulk of the heavier snowfall across the
Mat-Su Valleys is gradually breaking up and becoming increasingly
pushed against the Chugach front range as northerly flow and cold
air advection increase to the west of the low center. The
remaining portion of the band will continue to gradually weaken
through the rest of today as the cold, dry air continues to filter
in from the interior.

In the west, a very fast-moving shortwave over the Northern Bering is
speeding towards the Y-K Delta. Up to gale force westerlies are
quickly following the southern portion of the wave as it moves
east. Very cold air remains in place ahead of this system across
much of the Southwest, with temperatures dipping down as low as
-10 to -20 over the Middle Kuskokwim Valley. A ridge extends
across most of the Western Bering/Aleutians with open-celled
cumulus rotating anticyclonically around the ridge axis.

&&

.MODEL DISCUSSION...

Models remain generally in agreement at the synoptic level. Some
differences persist with the track of the low over the Prince
William Sound as it tracks back into the Gulf, but consistency has
improved overall. One other issue will be whether the stronger
winds following the shortwave/low over the northern Bering will
make it onshore or stay mostly over water. There is good agreement
that stronger winds will move into parts of the northern AKPEN and
Bristol Bay as the low moves into the area Thursday evening.
However, the signal is more mixed for the Y-K Delta, and some
guidance show the stronger westerly winds wills stay confined to
the immediate coastline as the low tracks across from west to
east on Thursday. This could lead to issues determining which
areas experience blowing snow on Thursday, mainly depending on
how far the winds make it inland. There remains decent agreement
on the storm-force front that will sweep across the western Bering
through Friday.

&&

.AVIATION...
PANC...Uncertainty continues with the end time for snow this
morning. PANC has reported breaks in the snowfall at times, but
much of the rest of Anchorage has continued to report IFR snow.
This has been inconsistent with radar trends that have indicated
moderate snowfall staying parked over all of west Anchorage, and
periods of IFR snow could move back over the airport for parts of
the early morning. It appears drier air is working in from the
north and west at the low levels, causing much of the snow to
sublimate before reaching the ground in the far western parts of
town. North winds gusting over 20 kts at times may continue to
cause minor blowing snow this morning. Any lingering snow should
clear out by the early afternoon, with rising ceilings and return
to VFR conditions expected. North winds will persist but gradually
become lighter through Thursday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 to 3)...

The long-duration snow event for Southcentral continues today as
a negatively titled upper level trough and associated 995 mb
surface low slowly drifts northward into eastern Prince William
Sound (PWS). A deformation band has set up west of the surface low
along the 700 mb trough axis. This is allowing the snow to
continue across the Anchorage Bowl and Matanuska Valley. A stalled
out N-S frontal boundary bisecting the state is also serving to
enhance precipitation as moisture wrapping around the
aforementioned low is fanning out along this boundary. Snowfall
has been impressive over these locations, especially where the
strongest snowbands developed overnight. Reports include almost 8
inches of snow in Talkeetna, 12 inches in Trapper Creek, 7.5
inches in Kenai, and 12-15 inches along the Anchorage Hillside. As
for the airport, the northerly winds have advected drier air at
the surface, resulting in much less snow over west Anchorage (2.6
inches)...even with some of the strongest radar returns overhead
last night.

The snow is expected to diminish in intensity and coverage by
around noon today as the surface low and upper trough start to
drift south and east. Thereafter, model guidance brings an upper
level shortwave with a cold, arctic airmass eastward in the wake
of the low. Outflow winds will be a main concern for the Barren
Islands, Thompson Pass, Valdez, and the Copper River Delta that
will continue through Saturday. A upper-level ridge of high
pressure will move over the Southern Mainland Saturday through
Sunday with outflow winds subsiding. Additionally, expect
temperatures to precipitously drop to the single digits Friday
morning through the weekend under clear sky conditions.

&&

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

The compact and fast moving low responsible for the issuance of
Winter Weather Advisories in zones 155 and 161 is currently
coming ashore. Light snow is falling in Southwest AK around
Bethel, with the strongest of the storm still offshore. Please see
the advisory statements for details. We still expect this to be a
very fast moving system, exiting later today.

After the exit of the system today, Southwest AK will stay quiet
and cold until Saturday. A front that has been tracking across the
Bering will arrive along the coast. The big weather change this
system will bring are warming temperatures for all of Southwest
AK, with temperatures warming to near freezing. Expect windy
conditions along the coast and snow as well with this system.

&&

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

A strong low located just of the east coast of Kamchatka is
sending a storm force front into the Western Bering. This front is
expected to track east over the next couple of days, weakening as
it goes. It will lose its storm force strength tomorrow as it
reaches the Central Aleutians. After that, it will track east as
a gale force system, reaching the Southwest Coast on Saturday.
Ahead of this front the Bering is quiet, save for a compact low
now moving into Southwest Alaska.

&&

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

Bering: A well developed front moves into western Alaska for Sun.
Its Northern Bering low continues to move across the Bering
through Mon. Forecast confidence is good. Widespread gale force
winds and waves with the front diminish Sun. Seas to 18 feet over
the Eastern Bering, subsiding. Upper level features bring
widespread small craft winds with areas of gale force winds across
the Bering through Mon. Seas to 26 feet on the Bering side, 24
feet on the Pacific side, subsiding.

Gulf: A low exits the Gulf on Sat. A new low and front moves to
the Central Gulf for Mon. Models in good agreement. Gusty outflow
winds from the North Gulf Coast extend from the Barren Islands and
Hitchinbrook Entrance. Gale force winds and waves diminish Sat.
Small craft winds elsewhere, diminishing Sun. A new low and front
moving to the Central Gulf brings small craft winds with areas of
gale force winds around the low Sun. Seas building to 15 feet with
the front.

&&

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

The long term pattern consists of two key features. A broad upper
level trough will be in place over the Bering while an upper level
ridge extends along an axis in place from Southcentral over the
Gulf of Alaska. By Monday, the trough will move eastward,
displacing the ridge, and several upper level waves embedded
within the base of the trough will move east over Southcentral.
With the upper level in trough situated over the mainland, active
weather is expected for Southcentral, with the potential for
several lows to form along a frontal boundary that extends from
the Alaska Peninsula southward over the western Gulf. Farther
west, more of a showery pattern is expected, with the possibility
of several upper level waves generating weak surface lows across
the Aleutian Chain Monday into Tuesday. By the middle of the
week, model solutions are hinting at the development of a strong
Arctic ridge building over the Bering Sea.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory: 101,111,155,161.
Winter Storm Watch: 131.
MARINE...Storm Warning: 130,131,175,177,178,185,411-413.
Gale Warning: 119-121,125,127-129,132,136-138,351,352,150,155,160,
165,170,173,174,176,179,180,414.
Heavy Freezing Spray: 130,138,139,150,160,180,185.

FIRE WEATHER...NONE.

&&

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

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