Heavy Freezing Spray Warning 108, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
524 AM AKST Tue Mar 9 2021

.ANALYSIS AND UPPER LEVELS...

A weak front is working its way across all of mainland AK,
stretching from the high arctic all the way south into the North
Pacific. Behind this front, showery precipitation exists in the
Bering, which is being enhanced by shortwave troughs embedded in
the flow. All of this is supported in the upper levels by an
exiting high amplitude ridge moving east into the Yukon and a
broad long wave trough that starts in Siberia and encompasses all
of the Bering Sea.

&&

.MODEL DISCUSSION...

Models continue to be in good synoptic agreement. Discrepancies do
continue to exist identifying more nuanced shortwaves in the
Bering and models continue to refine their forecast for
precipitation in Southcentral AK. The forecast was adjusted
accordingly where possible. Overall, we have moderate to high
confidence in models.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist to start the
TAF period. However, some light snow showers are expected this
morning at the airfield. These showers should not drop flight
category below VFR. Later today, snow will become more steady and
increase in intensity. This will drop the airfield to IFR conditions.

&&

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

A front draped from the Alaska Range south to Kodiak Island is
beginning to produce precipitation from Cook Inlet north to the
Susitna Valley this morning. Although there is some rain and
mixed precipitation from Kodiak City to Homer, the bulk of the
precipitation is falling in the form of snow. North of Kenai, the
precipitation has yet to reach the ground as it falls into a drier
airmass near the surface.

The aforementioned front will stall as it tracks to the western
Gulf. A digging shortwave from the Aleutians and a second lifting
north from the Pacific will then phase south of Kodiak Island,
resulting in the development and rapid strengthening of a surface
low along the front. These features will then lift into the
northern Gulf by Wednesday and stall over Prince William Sound
through Thursday.

A deep, southerly fetch along the front is tapping into moist,
sub-tropical air and lifting copious amounts of moisture
northward. This moisture feed will only be enhanced by the
developing low, resulting in a long duration snow event for much
of Southcentral. Along the coast, the southerly flow will
initially allow for a changeover to rain later today as
temperatures climb into the upper 30s. However, as the low tracks
toward Prince William Sound, colder air will wrap back in, with
any lingering liquid precipitation quickly changing back to snow.
Farther inland, the precipitation should remain all snow, with
upwards of a foot of snow accumulation across parts of the
Susitna Valley and 12 to 18 inches possible for Hatcher Pass
through early morning Thursday.

For Anchorage and other locations along the lee of the coastal
mountains, periods of light snow are expected today, before the
surface low tracks to the eastern Kenai Peninsula. Once there, a
cross-barrier flow will set up, shutting off the bulk of the
precipitation for a time on Wednesday. By Wednesday afternoon,
however, the snow should once again pick up as the low moves to
Prince William Sound and the southeasterly flow aloft turns
northerly. The snow will then linger over the Matanuska Valley,
Kenai Peninsula, and Anchorage Bowl through Thursday as colder air
advects over the region. 4 to 6 inches of snow is possible from
Palmer to Kenai with up to 10 inches possible along the Anchorage
Hillside due to upslope from the colder westerly winds on the
back side of the low. A foot of snow is also possible from
Whittier to Valdez and Cordova, with over a foot for Turnagain
Pass. Blowing snow may become a concern for coastal locations late
Wednesday through Thursday as the colder outflow winds ramp up
and temperatures drop into the 20s. The gusty outflow winds
continue through the end of the week as colder temperatures
return.

&&
.SHORT TERM FORECAST SOUTHWEST ALASKA (Day 1 through Day 3: Today
through Thursday night)...

A weak front reinforced by a North Pacific low fueled by tropical
moisture could bring light to moderate snow over the Alaska Range
and lower Kuskokwim Valley tonight. For the rest of Southwest
Alaska, a relatively quiet forecast is expected except when low
pressures and troughs from the Bering Sea enters the Y-K Delta and
Bristol Bay between Wednesday night and Thursday morning. These
weather patterns could increase winds and snowfall. Meanwhile,
cold air advection and temperatures between -16C to -24C at 850 mb
will remain present over the region. Essentially, anticipate
overnight temperatures to continue declining to below zero in the
next couple days.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Day 1 through Day 3:
Today through Thursday night)...

Light and heavy freezing spray will persist across the Bering and
north of the Eastern Aleutians in the next couple days.

Isolated large scale features will bring elevated seas and winds
briefly, and increased snowfall and patchy fog over the Bering
and parts of the Aleutians through the next couple days. First, an
upper-level trough from the Western Bering responds to a surface
low over the North Pacific, and this could bring the potential of
increased snow over central Aleutians on Tuesday night through
Wednesday. As soon the upper level trough exit the Bering
Wednesday night, an amplified upper level ridge from the North
Pacific treks eastward across the Bering and the Aleutians through
Thursday night. Meanwhile, a deck of low stratus and patchy fog
spreads underneath a surface ridge over the western and central
Aleutians. Across the Bering, a few low pressures will find their
way to either move northeast past Nunivak Island through Norton
Sound by this evening, while other lows and troughs of low
pressure embedded within the westerly flow make their way closer
to Bristol Bay or inland between Wednesday night and Thursday
morning. The other interesting weather phenomenon brewing over the
North Pacific will boost winds to near gales, and ushers
rain/snow mix to the southeast of AKPEN starting this evening.
With the presence of cold air advection over the area combined
with mid to upper level troughs, snowfall is favored through the
forecast period except when warm temperatures ascends near western
Aleutians by Thursday afternoon in response to a front moving in
from the western Bering. This next system could bring gales as
early as Thursday morning. For now, will monitor this situation
closely and provide updates. Stay tuned!

&&

.MARINE (Days 3 through 5)...

Bering: Models are in good agreement with a strong front moves
over the western Aleutians and Bering on Thursday. This system
looks to bring high end gale force winds, however, there is the
potential to see sustained storm force winds throughout the day on
Thursday. The system will weaken as it rack eastward through
Friday. By Friday afternoon, a North Pacific low lifts northward
towards the western Aleutians. Models are in decent agreement with
the track of the low, but differ when it comes to strength. This
system will likely be gale force strength, but could be strong
enough to see storm force winds as it lifts into the northern
Bering through Saturday afternoon.

Gulf: A low in the northern Gulf will keep the active pattern
throughout the day on Saturday. Models agree that gale force winds
will accompany this system, but the position varies slightly
between models as it exits into the southern Gulf on Friday. By
Friday afternoon, models begin to diverge with another potential
North Pacific low entering the eastern Gulf. The GFS has a much
deeper low with gale force winds spread across the entire Gulf,
while other models are keeping a weaker solution. In any case,
conditions become quieter for Saturday afternoon.

&&

.LONG TERM FORECAST (Days 4 through 7: Friday through Monday)...

The upper level pattern this weekend is highly amplified. Model
guidance discrepancies are lower than expected for the long term
in both the upper and lower levels. Solutions do continue to
struggle with the smaller scale features and shortwaves behind the
longwave troughs. The upper levels are highly amplified over the
weekend. A longwave trough extends over Northern and Southcentral
Alaska, with a vertically stacked low over the Gulf, on Friday,
and a ridge over Southwest pushing east through Sunday. Behind the
ridge is an another upper level longwave trough entering the
western Bering late Friday. There is some surface reflection via
a surface low passing over the Western Aleutians. This surface low
increases in strength as it enters the Bering, becoming
vertically stacked by Saturday. The front pushes the Southwest
coast as the parent low tracks north. This is a good setup for
strong winds and moderate snowfall across the Kuskokwim Delta over
the weekend, especially along the coast and Nunivak Island. There
will be a warm air push, however, it will likely remain over the
AKPEN and Aleutians, therefore, milder temperature will be
confined to these areas.

For Southcentral, offshore flow will dominate towards the end of
the week, thus, expect winds for Seward/Resurrection Bay,
Whittier/Passage Canal, and Valdez/Valdez Narrows to become
increasingly gusty. The leading low in the Gulf will keep the
active weather along the coast on Saturday. Cold air is likely to
flow into Southcentral ahead of the ridge Friday and Friday night,
supporting colder temperatures during this time. As the ridge
continues pushing east this cold air will retreat over western
Canada. Both Southwest and Southcentral look to enter a more
active patter with the front, as well as the shortwaves both ahead
and behind it, with snow showers and rain/snow showers expected
across the Mainland and the Coasts respectively.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BJB
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
MARINE/LONG TERM...MTL

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
1104 AM AKST TUE MAR 9 2021

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

1103 AM     SNOW             3 NW NIKISKI            60.72N 151.37W
03/09/2021  E3.0 INCH        KENAI PENINSULA    AK   PUBLIC

            3 INCHES OVER 8 HRS.

&&

KUTZ

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
222 PM AKST Tue Mar 9 2021

AKZ181-101430-
Alaska Peninsula-
Including the cities of Cold Bay and Sand Point
222 PM AKST Tue Mar 9 2021

...TRACE ASHFALL POSSIBLE IN THE VICINITY OF VENIAMINOF...

The Alaska Volcano Observatory has detected continued activity
and low level ash emission in webcam and satellite data from the
Veniaminof Volcano located near Chignik. Any ashfall near Chignik
and Castle Cape is expected to be of trace amounts.

Low-level winds are expected to be out of the north this evening,
turning out of the west overnight.  Any potential ashfall would
be south and east volcano.