Winter Weather Advisory 26, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
542 AM AKST Wed Mar 10 2021

.ANALYSIS AND UPPER LEVELS...

The upper level pattern continues to be very amplified in nature
this morning. A deep longwave trough spans across the Bering and
Aleutians, with a sharp longwave ridge axis to the east extending
from the Gulf into the AK Panhandle and Yukon. A weak (90 kt)
meridional jet axis extends across much of the mainland to the
north of a potent shortwave trough and associated frontal system
approaching the Kenai Peninsula from the northern Gulf. A
substantial cloud shield spans across much of Southcentral ahead
of the wave, with snowfall now spreading across the Cook Inlet and
up into much of the Susitna Valley. Precipitation is falling more
in the form or a mix or even all rain along much of the North
Gulf Coast, including areas near Homer, Seward and Cordova. To the
south, a low center is consolidating along the front over the
Gulf, or just to the east of Kodiak Island.

Out west, conditions are comparatively more inactive for the most
part. A tightly-wound low continues to drift eastward to the
south of the Western Aleutians, but the stronger gale force winds
remain confined well south the Aleutian Chain to the immediate
vicinity of the low center. A showery regime continues over the
Bering, with numerous snow showers drifting steadily east
underneath the longwave trough aloft. Conditions are likewise
fairly quiet across the Southwest, with the exception of some
locally dense fog and snow showers across parts of the Y-K Delta.

&&

.MODEL DISCUSSION...

Models continue to feature good agreement for the larger scale
pattern, and forecast confidence consequently remains moderate to
high. Some differences appear with the handling of the surface low
expected to move into the Prince William Sound by Wednesday
evening. While there remains solid agreement that the low will
stall over PWS through Thursday morning, models differ with the
track of the low as it begins to drift back south later on
Thursday into Friday. The NAM appears to be a northeast outlier,
as it attempts to drag the low briefly inland near Valdez on
Thursday morning before taking the center more southeast than
other solutions as it retreats back to the Gulf from Thursday
night to Friday. This results in some key differences with the
strong outflow winds anticipated to pick up near the Gulf Coast by
this time, with the NAM favoring much stronger outflow winds
through Passage Canal on Thursday with the low making it to the
north end of PWS. There is also the question as to whether
Thompson Pass could see significant blowing snow as their outflow
winds potentially become quite strong by Friday morning, and this
will be highly dependent on how much snow accumulates in the pass
before the strongest winds begin.

&&

.AVIATION...

PANC...Southeast winds and downslope flow are helping to dry out
the low levels early this morning, leading to generally improved
ceilings and visibility to VFR with snow tapering off to showers
for the next several hours. Occasional periods of MVFR snow cannot
be ruled out through midday with any showers passing over the
terminal. By the early afternoon, expect a brief but intense band
of snow to move back in from the west, bringing back IFR
conditions. North winds will quickly pick up after the onset of
snow, with gusts up to 25 knots possible during the late evening.
Snow should also let up by this evening before ending late
tonight. These gusty winds may lead to minor blowing snow at times
during the afternoon through tonight. Some uncertainty persists
for the snowfall end time, but generally expect return to VFR
conditions after midnight tonight at latest. Gusty north winds
will continue through Thursday morning.

&&

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

A strengthening area of low pressure will track from east of
Kodiak Island this morning into western Prince William Sound by
Thursday. As it does, a deep fetch of moisture from the Pacific is
moving north with the low and overspreading Southcentral Alaska.
Snow continues to fall in earnest across the Susitna Valley this
morning, with the heaviest snow beginning to shift west of the
Parks Highway as the southwesterly flow aloft yesterday turns
to the southeast. This southeasterly flow has shut off the
snowfall across the most of the western Kenai Peninsula and
Anchorage Bowl this morning as the downsloping and cross barrier
flow increase. Elsewhere, a mix of light rain and snow is falling
this morning along the immediate Southcentral coast. Light snow
has also pushed into the Copper River Basin.

The mixed precipitation will transition to all snow by early
afternoon as the low progresses toward Prince William Sound and
colder air begins to wrap around the system. A front associated
with the low will push over Cordova, bringing gusty easterly winds
and mixed rain/snow for a time this morning, before the front
lifts onshore and weakens. As it does, the mixed precipitation
should also switch over to all snow for Cordova. Southerly winds
will also increase in the Copper River Basin with the front
lifting north; however, temperatures will remain cold enough for
all snow.

Farther west, the snow will fill back in over the western Kenai
Peninsula, Anchorage Bowl, and Matanuska Valley by early afternoon
as the low moves east of the region and the cross-barrier flow is
replaced by a colder northwesterly flow. Snow will continue over
these parts through early Thursday with a general 2 to 5 inches
expected. Upwards of 6 to 10 inches is possible for higher
elevations such as the Anchorage Hillside and the Glenn Highway
north of Palmer before the snow tapers off.

With the low moving into Prince William Sound and colder air
advecting from the northwest, gusty outflow winds will develop.
Gusts to 30 mph are possible over Anchorage and Palmer tonight with
gusts near 50 mph possible for Whittier and Seward through
Thursday. Localized blowing snow is likely for these locations,
especially with any lingering snow showers.

By late Thursday, the outflow winds will ramp up for Valdez,
Thompson Pass, and the Copper River Delta and continue through
Saturday. Winds could be especially strong for these locations as
both the surface and upper-level flow turns northeast. An upper-
level shortwave is also expected to moves over the region for
Friday. These factors could allow wind gusts to exceed 50 mph,
especially for Thompson Pass.

With the outflow pattern back in place over Southcentral,
nighttime temperatures will tumble into back into the single
digits, and even below zero over many inland locations, as winter
retains its grip across the region.

&&

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

A weak trough moving east through Southwest AK is dropping light
snow across the Y-K Delta as it goes. Meanwhile, wrap-around
moisture from a low pressure system moving through the Western
Gulf of Alaska is producing snow along the Alaska Range. These two
systems area expected to exit late this morning, allowing weather
to quiet down by this afternoon and tonight.

Thursday, a quick moving and compact low will dive southeast
through the Eastern Bering a scrape the coastline of Southwest
Alaska. This will provide the opportunity for another round of
snow for coastal areas. Models have shifted the track of this low
a bit to bring it more inland tonight and seem to be in agreement
on the new track. So, adjustments were made to the forecast
tonight to reflect this. Stay tuned for any further adjustments.
Despite changes in the low track, we can be certain this will
remain a coastal storm, so the interior will stay quiet. After
this storm exits Thursday evening, quiet weather is expected for
all of Southwest AK. Overnight temperatures are also on the
cooling trend, so expect quiet and cold conditions by the end of
the week.

&&

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

General westerly flow and quiet weather is expected for the Bering
through tomorrow. The only two items of note are the intermittent
activity Veniaminof continues to show and a compact low that drops
southeast through the Eastern Bering tomorrow. This is a quickly
moving system that will transverse the Eastern Bering in a day. It
does have a decent pressure gradient on the backside of the low,
which looks like it will induce gale force winds. This is
reflected in the forecast this morning. As far as the volcano
goes, winds are shifting the northwest today, so any ash that it
emits will go generally southeast over the North Pacific. Please
see other products issued by this office and the AAWU for further
details. Lastly, a storm force low enters the Western Bering
tomorrow into tomorrow night and tracks a front east through the
rest of the week, though it weakens as it goes and loses its storm
force strength by the time it reaches the Central Aleutians.

&&

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

Bering: A well developed front moves across the Bering through
Sun. Models in good agreement. Widespread gale force winds and
waves with areas of high end gale force winds with the front as it
moves into the Eastern Bering Sun. Upper level support sustains
gale force winds across the Bering after frontal passage through
Sun. Seas to 25 feet across the Bering, 30 feet on the Pacific
side through Sun.

Gulf: A low exits the Southeast Gulf on Sat. Models in good
agreement. Outflow winds from the Alaska Peninsula extend into the
Gulf through the Barren Islands and Sitkinak areas. Gale force
winds and waves diminish Sat. seas building to 18 feet over the
Gulf, subsiding Sat. A new front approaching the Western Gulf
brings small craft winds Sun. Seas building to 12 feet.

&&

.LONG TERM FORECAST (Days 4 through 7: Saturday through Tuesday)...

Upper level pattern becomes 'blocky' into the weekend, meaning a
high amplitude upper level ridge is sandwiched between two
vertically stacked lows. One low is over the western Bering and
the second over the eastern Gulf, while the ridge is positions
over much of the State. Skies over Southcentral Saturday through
late Sunday morning should be mostly clear while under the ridge.
The deepening trough over the Western Aleutians circles
cyclonically around the low center, forcing the ridge
southeastward by Sunday afternoon. Cloud cover, as well as some
precipitation, returns to Southcentral by Sunday afternoon as a
front pushes into the Gulf. Further west, the aforementioned low
moves across the Bering over the weekend. The associated front
approaches the Southwest Coast Saturday afternoon. Gusty southerly
winds across the Kuskokwim Delta and along the Southwest Coast
are typical with this pattern. Snow and blowing along the
Kuskokwim Delta and Bristol Bay coast are also likely as the warm
air mass looks to stay over the Southern AKPEN. Spotty snow
showers may linger across the Bering, Aleutians and Southern AKPEN
as a series of shortwaves track behind the front.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory: 101,111,145.
MARINE...Storm Warning: 130,131.
Gale Warning: 119-121,129,132,137-139,351,352,150,160,177,178,180.
Heavy Freezing Spray: 130,138,139,150,160,165,180,185.
FIRE WEATHER...NONE.

&&

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

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