Heavy Freezing Spray Warning 120, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
504 PM AKDT Mon Mar 15 2021

.ANALYSIS AND UPPER LEVELS...

In the upper levels, a complex pattern has set up with a broad,
longwave trough with embedded shortwaves moving over the
Bering/Aleutians, AKPEN, and Southern Mainland. Coupled with a
cold, northwesterly flow aloft and interacting with the
relatively warm waters, these shortwaves are bringing widespread,
convective snow showers over parts of the Aleutian Chain, Pribilof
Islands, and Coastal Southwest AK. Further west, a high amplitude
ridge extends along its axis from the North Pacific, western
Bearing Sea, and into eastern Siberia. The Nighttime Microphysics
this morning showed the Southwest and AKPEN completely blanketed
with low fog/stratus with many stations reporting a reduction in
ceiling and visibility.

At the surface, a rapidly, intensifying low with a well defined
circulation and wrap-around moisture is currently moving
southeastward of Sand Point. Additionally, a Gulf low that has
been responsible for spreading snow showers over much Southcentral
since early this morning is slowly weakening as it moves onshore
in the vicinity of the Copper River Delta. The 12Z ANC sounding
has an impressive steep, moist profile above the warm nose at 1,500
feet, with variable winds near the boundary layer and increasing,
southeasterly winds aloft.

&&

.MODEL DISCUSSION...

Models are overall in good agreement with the synoptic scale
features and still on track for continued snow showers today
associated with the aforementioned Gulf low with conditions
improving tomorrow over the Southern Mainland. Moreover, model
solutions are in agreement with a low and associated front moving
toward the western Aleutians on Tuesday, and a transient shortwave
and associated surface low moving southeastward toward coastal
Southwest AK on Wednesday.

&&

.AVIATION...

PANC...Light snow is expected to continue through Tuesday morning
with generally IFR ceilings and visibility. A few breaks in the
cloud cover and precipitation are possible this afternoon through
this evening which could allow for some improvement to MVFR
conditions. However, steadier snow is expected to move back over
the terminal by later this evening, continuing through Tuesday
morning before tapering to flurries.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Tonight through Thursday night)...

Snow will persist into early this evening, with the weather
conditions improving later tonight into Tuesday morning over much
of Southcentral. A weakening low over Prince William Sound (PWS)
combined with a shortwave aloft will continue to produce snow
showers, particularly for eastern Prince William Sound. As this
low dissipates Tuesday afternoon, it transitions into a surface
trough mainly over the eastern end of PWS with very minimal to no
effects over Cordova and Valdez.

A developing stacked low will move over the northern Gulf between
Wednesday night and Thursday morning. This combined feature may
stay farther south of PWS but it could also induce a bit of
weather to move near Cordova by Thursday morning. Upper level
wind direction at 500 mb seems to disagree for the first 24 hour-
period in response to the upper low, but by Wednesday morning,
model solutions all come into agreement on direction and speed.
The cold air advection will remain present over the area, but not
significantly strong for most areas. Strong northwest winds at
850 mb extends from Susitna Valley through Kenai Peninsula into
Wednesday morning. The -20 degrees Celsius (at 850 mb) will spread
east of Talkeetna Tuesday night, then spreading southeast toward
Thompson Pass through Wednesday morning. Colder temperatures will
settle in across nearly all of Southcentral. Diurnal ranges will
be fairly large due to the rapid increase in amount of daylight,
but temperatures will remain below normal for this time of year.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Monday
afternoon through Thursday)...

A broad upper level trough in place over much of the mainland is
keeping active weather across Southwest. A weakening low centered
south of the Alaska Peninsula is keeping snow showers in place
across the southern AKPEN. Snow will taper off this evening as
the low gradually moves southward. Meanwhile across the Bristol
Bay coast, persistent fog this morning has lifted to stratus, and
will likely stick around during the overnight hours. Patchy fog
can be expected along coastal locations again for Tuesday morning.

Lingering snow showers across the Kuskokwim Delta will eventually
diminish by the evening hours tonight. By Tuesday night, a
shortwave trough moves in from the north, bringing a quick shot of
snow and gusty northerly winds. A brief period of blowing snow
with limited visibility are possible Tuesday night through early
Wednesday morning. As this shortwave quickly moves southeastward,
a quick burst of snowfall is expected with a couple inches
possible across higher elevations, especially over the Kilbuck
Mountains. By Thursday morning, quieter conditions are expected
across the region, ahead of an incoming front arriving by the end
of the work-week.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3:
Monday afternoon through Thursday)...

Northwesterly flow and a showery pattern will continue across much
of the Bering as an upper level ridge in place over the western
Aleutians gradually builds in. Snow showers will persist for much
of the central and eastern Aleutians and Pribilofs tonight before
diminishing early Tuesday morning. Fair conditions are expected
across the Bering waters with the ridge in place Tuesday. By
Tuesday night, another front approaches the western Aleutians.
Warmer southerly flow and rain will spread in from west to east
across the Chain as the front reaches the central Bering Wednesday
afternoon and weakens considerably. By Wednesday night, model
solutions suggest a reinforcing upper level low arrives over the
western Bering. This stronger low will likely phase with the
weaker one, and will send a stronger front eastward by the end of
the week.

&&

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

Gulf: A low moves across the Northern Gulf and dissipates Sat.
Forecast confidence is good. Widespread small craft winds and
waves south of the low move across the Gulf, dissipating Fri. Gale
force gap winds exit into the Gulf from the Barren Islands and
Sitkinak areas, and dissipate Fri as the low moves east. A new
front approaches the Southwest Gulf late Fri into Sat.

Bering: A Western Bering low moves to the Central Bering by Sat.
Its front extends through the Eastern Bering and Alaska Peninsula
through Sat. Forecast confidence is good. Widespread high end
small craft winds and waves with areas of gale force winds
wrapping the low and front across the Bering through Sat. Winds
dissipate south of the low and front, but remain across the
Northern Bering through Sat. Seas to 22 feet over the Western
Bering, 18 feet on the Pacific side, subsiding through Sat.

&&

.LONG TERM FORECAST (Days 4 through 7)...

The long term forecast begins late Thursday with a persistent
broad trough over much of Mainland Alaska. Meanwhile, a ridge
situated over the eastern Bering and Southwest AK coast begins to
breakdown as a low moves east across the Aleutian Chain. By
Sunday, models diverge greatly as to how far eastward this system
progresses and how it interacts with the aforementioned ridge. The
GFS pushes this system into the southern Gulf by Sunday while the
EC and GEM keep the low farther west, with some semblance of a
ridge remaining in place. The GEM goes so far as to establish an
omega block, with an amplified ridge extending to the Chukchi
Sea. The latter solutions would allow a second shortwave to move
underneath the aforementioned Aleutian low and steer it north into
the Gulf by Monday. The GFS collapses the ridge by dropping a
shortwave southeast from the northern Bering. The interesting
thing of note, is that while these solutions are different in the
evolution of the shortwaves through the weekend, they are
convergent on the solution of the upper-level trough re-establishing
itself over the state with an amplified ridge over the Bering by
the start of next week. In terms of sensible weather, the EC and
GEM would keep conditions cooler and drier over the Southern
Mainland through the weekend, while the GFS would allow for some
precipitation to develop from the AKPEN east across Bristol Bay
and perhaps the immediate Southcentral coast beginning Saturday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 130 138 177 178.
         Heavy Freezing Spray Warning 121 130 139 180 181 185.
FIRE WEATHER...NONE.

&&

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

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
1047 PM AKDT MON MAR 15 2021

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

1046 PM     SNOW             10 SSE ANCHORAGE        61.08N 149.78W
03/15/2021  E8.0 INCH        ANCHORAGE          AK   PUBLIC

            8 INCHES STORM TOTAL.

&&

CJ