Winter Weather Advisory 12, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
541 AM AKST Thu Feb 18 2021

.ANALYSIS AND UPPER LEVELS...

A complex pattern is coming together across the Gulf and into
Southcentral this morning. An inverted trough near the surface
extending from the Gulf into the Kenai Peninsula and Mat-Su
Valleys is sandwiched between a ridge extending along the AK
Panhandle and an upper shortwave trough axis extending from the
interior Southwest/AKPEN to the far western Gulf. This inverted
trough extends away from a powerful storm force low now moving
north into the eastern Gulf. A dense, cold cloud shield extends
from the low and along the low level inverted trough axis, with a
broad deformation band of snow and coastal rain/snow mix spreading
from southeast to northwest across the Prince William Sound and
Kenai Peninsula. The band of snow is now spreading into the Mat-
Su Valleys where a more narrow band of snowfall should continue
to set up this morning along an axis roughly from Willow to
Kenai, including much of the Anchorage area.

Weather remains comparatively quiet out west across much of the
Eastern Bering/Aleutians, with northwest flow and low marine
stratus dominating the pattern here. A strong upper ridge is
building into the western Bering ahead of a potent front
approaching the Western Aleutians, with southeast gales just
beginning to spread into the area out ahead of the boundary in
the far southwest Bering. The front is associated with a deep low
south of the Kamchatka Peninsula that continues to drift slowly
north this morning.

&&

.MODEL DISCUSSION...

The central theme in the immediate to near term continues to be
the subtle but impactful uncertainty with the deformation band of
snow developing along a northeast to southwest axis across the
Mat-Su Valleys and down along the western Kenai Peninsula. At this
point, observational data (including radar, webcams and ASOS
stations) is rapidly becoming more useful than model output for
diagnosing the progression of the band snow setting up over
Southcentral today. Nevertheless, hi-res models appear to finally
be getting a better handle on where this band is setting up, and
most models now show the band stalling along an axis approximately
from Willow to Kenai through midday today. The exact behavior of
this band will ultimately rely on mesoscale details on where low
to mid level convergence and lift are most strongly focused, which
is what models have had the most difficulty capturing up to this
point.

Looking into tonight and beyond, models are in good agreement that
a potent Arctic trough will arc southeast across the southern
mainland, and models show excellent agreement for an associated
push of much colder air to end the snowfall from west to east
over Southcentral. By Friday night and Saturday, models diverge
notably with the behavior of the pattern across the
Bering/Aleutians. Several shortwaves and associated lows will
cross the region during this period, but the individual tracks of
each wave vary significantly across all model solutions. Thus,
forecast confidence across the Aleutians and Bering is lower than
average for the end of the week into this weekend.

&&

.AVIATION...
PANC...Snow and low ceilings will continue to lead to MVFR to IFR
conditions through this morning. Conditions should improve to
mainly MVFR by around midday as snow gradually lets up, eventually
improving to IFR by this evening as colder and drier air work in
from the northwest and scour out any lingering low stratus and
falling snow.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Thu and
Fri)...Our snow event is upon us. Plenty of moisture over the
Northern Gulf of AK is pushing from east to west across
Southcentral this morning. An inverted trough at the surface is
helping to take advantage of the moisture and produce widespread
snow across most of the area. Over the past few days, we have been
closely watching to see exactly where the heaviest deformation band
of snow would set up. From radar trends and the hi-resolution
models overnight, it now appears that it will align over the
northern tip of the Kenai Peninsula stretching into the Southern
Susitna Valley. West Anchorage will be just on the periphery of
this feature, so expect continued steady snow over the Anchorage
Bowl through the day with a general 2 to 4 inches of accumulation.
The 12z Anchorage upper air sounding and satellite imagery do not
show any overly impressive instability, so models have backed off
on the intensity of the precipitation under the core of the band.
Regardless, any time there are mesoscale snow bands, there is
potential for some localized higher snow accumulations.

As cold air starts to move in tonight courtesy of a potent upper
level wave, this will serve to wring out any lingering moisture in
the atmosphere. Thus, expect snow to continue into the evening for
Anchorage up through the Matanuska Valley, holding on the longest
near the mountains. The orientation of the wave will allow for
very light, fluffy snow to continue over the Copper River Basin
into early Fri. As the cold air ushers in some gusty north to
northwest winds, this light snow could be easily blown around.

Attention then quickly shifts to the very cold airmass blasting
across the area. This will lead to rapidly clearing conditions for
most locations. However, it will also bring some gusty winds
through the typical gaps (Thompson Pass, Barren Islands,
Whittier). For some the marine areas (namely Shelikof and the
Western Barren Islands), heavy freezing spray will be a serious
threat by Fri morning. For inland areas, we will be keeping an eye
on wind chills as the gusty winds work in tandem with 850 mb air
of -20C. Cold and dry conditions look to hold as we head into the
weekend, with the exception of Kodiak seeing another round of snow
and rain.

&&

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

An upper level trough situated over Southwest gradually moves
eastward today. As a result, interior locations will see snowfall
waning through the day as residual showers clear out with the
trough. An upper level ridge over the central Bering will move
inland, trapping low level moisture and likely keeping stratus and
fog in place around the Bristol Bay region Friday morning. Farther
north, a much colder airmass will drop southward in the wake of
the trough, bringing temperatures down to sub zero values across
much of Southwest. Low temperatures tonight will drop to near 15
below (F) across the Kuskokwim Delta, while the Bristol Bay area
will see values right around 0. Attention quickly shifts to an
incoming front approaching the Southwest coast Friday afternoon.
Offshore winds will increase in response to a low forming along
the frontal boundary. Temperatures should be cold enough for
precipitation to fall as snow beginning along the southern Alaska
Peninsula Friday night. As the front pushes eastward Saturday
morning, it will keep a horizontal orientation, keeping
precipitation limited to the Bristol Bay region and Alaska
Peninsula. There is still some uncertainty with regard to snowfall
totals, however thinking around 2-4 inches at this time is likely,
with the greatest amounts across the Alaska Peninsula. By Saturday
night, the associated low tracks eastward across the Alaska
Peninsula. This could keep snowfall lingering longer along the
foothills of the Alaska Range through early Sunday.

&&

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

A low over Kamchatka continues to push a front eastward across
the Bering today. Warmer southerly flow has brought a rain/snow
mix to the western Aleutians this morning. As the front reaches
the central Bering Friday morning, a low spins up along this
frontal boundary just north of the central Aleutians. With enough
cold air in place over the northern Bering, the Pribilofs may see
a brief period of snow and blowing snow Saturday morning, however
this is largely dependent on the low track. A second low spins up
over the western Bering Saturday morning and quickly moves
southward toward the Aleutian Chain by Saturday afternoon. This
will keep cloud cover and snow showers in place across the
Aleutians through late Saturday. Cold air and northerly flow
return across much of the Bering by early Sunday.

&&

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

Bering Sea: An area of easterly gales over Bristol Bay will
diminish early Saturday morning. Heavy freezing spray will remain
possible through the day near the ice edge. Sub-gales and below 20
ft seas will continue through the weekend. Generally southerly
gales and greater than 20 ft seas will approach the western Bering
Monday, though the timing, strength and exact position remain
uncertain.

Gulf of Alaska: Brief easterly gales will possibly develop over
the Barren Islands and southern Cook Inlet Saturday evening,
though seas are not expected to exceed 20 ft. Model solutions
diverge thereafter, however westerly gales and greater than 20 ft
seas are possible in the southern Gulf on Sunday. Confidence is
actually greater on Monday as a low develops south of Prince
William Sound leading to potential high-end northwesterly gales
and building seas between the Kenai Peninsula and Kodiak.

&&

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

A series of lows will move over the Gulf through Tuesday leading
to offshore flow and cooler temperatures across much of the
mainland and precipitation along the coast. Snow is possible
across the Copper River Basin as the GFS shows a relatively
strong upper low tracking over Southwest Alaska Sunday into
Monday. All models show an upper ridge moving over Southcentral on
Tuesday which would bring warmer temperatures, before a trailing
trough and its associated front lead to more precipitation chances
for Wednesday. A series of upper troughs and surface fronts are
likewise expected to persist across the Bering/Aleutians through
mid-week. Overall, model agreement is quite good. However, any
time there is a pattern shift, it takes some time for the models
to hone in on specific details and thus confidence remains
average.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning: 130.
 		 Gale Warning: 120,125,131,132,137,138,351,352,165,
		 173-179,185.
		 Heavy Freezing Spray: 130,138,139,160,180,181,185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AS
SOUTHCENTRAL ALASKA...MSO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...AP

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
308 PM AKST THU FEB 18 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0308 PM     SNOW             UPPER POTTER VALLEY     61.06N 149.76W
02/18/2021  M2.2 INCH        ANCHORAGE          AK   TRAINED SPOTTER

            SPOTTER REPORTED 2.2 INCHES OF SNOW. STILL
            SNOWING LIGHTLY.

&&

MV

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