Heavy Freezing Spray Warning 94, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
510 AM AKDT Mon Mar 22 2021

.ANALYSIS AND UPPER LEVELS...

The pattern across the region remains relatively calm, yet
is somewhat more amplified compared to 24 hours ago. A weak ridge
axis has now moved over Southcentral with clear, still and cold
conditions settling in once again across the area at the surface.
To the east, an upper level low is exiting the AOR into the Yukon,
with subsidence and very dry air trailing behind it over the
eastern Chugach Mountains and Copper River Basin. Another
deepening, closed low center resides over the North Pacific,
currently well to the south of the Eastern Aleutians.
Northeasterly winds up to gale force extend away from the low to
the north, with the strongest winds remaining mainly along and
south of the Aleutian Chain per recent ASCAT passes. A shortwave
trough is moving onshore farther north in western Alaska, with
expansive low stratus and patchy fog still blanketing the
Southwest out ahead of the shortwave axis.

&&

.MODEL DISCUSSION...

Models continue to be in excellent agreement overall for the next
several days, leading to generally quite good forecast confidence
through midweek. Models are also displaying much better agreement
for the track and progression of a low moving across the Gulf
from tonight into Tuesday. There is now better confidence for the
track to favor a period of snow for Kodiak Island. Some concern
exists as to whether the snow will mix with or changeover to rain
with the initially warmer easterly flow off the water along the
eastern shoreline of the island. However, models indicate cold air
advection and winds backing to become more northerly by early
Tuesday morning, likely keeping this window for mixing/rain
tonight fairly short-lived. Models also show good agreement for a
strong front to move quickly onshore by Tuesday night over the Y-K
Delta where there is good confidence for snow and blowing snow
with the frontal passage.

&&

.AVIATION...

PANC...Expect VFR conditions and light winds to continue through
tonight. There remains a slight possibility for patchy fog early
this morning with moisture coming off of the Knik Arm, which could
briefly hamper conditions at the terminal before the sun clears
out any fog patches by late this morning. A disturbance moving in
from the west may bring a period of very light snow on Tuesday
morning, likely leading to mainly MVFR conditions towards the end
of the period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Wednesday night)...
A trough crossing the Bering Sea today will interact with an
existing closed low and trough south of the Alaska Peninsula,
pulling moisture northward across the Gulf and toward
Southcentral. At the surface, a weak low and associated frontal
system will track into the southern Gulf, passing to the south and
east of Kodiak Island. The interaction of the troughs and track
of the surface low is all occurring farther west than previous
forecasts indicated. As a result, expect more precipitation for
Kodiak Island this afternoon through tonight, mostly in the form
of snow. Also expect the northern edge of the precipitation shield
to reach western Prince William Sound, leading to light snow
tonight through Tuesday morning.

Mid to high level moisture streaming northward across the rest of
Southcentral will begin to moisten what is currently a very dry
air mass in place. As the Bering trough reaches Southcentral early
Tuesday it now appears a bit more favorable for light snow from
the western Kenai to Anchorage and the Mat-Su during the morning
hours on Tuesday. However, don't expect much accumulation, with
generally a dusting to one half inch for most areas. The trough
will weaken as it continues eastward Tuesday afternoon, with just
a chance of light snow or flurries from eastern Prince William
Sound to the Copper River Basin. Meanwhile, the low over the Gulf
will deepen slightly as it tracks toward Southeast Alaska. Cold
air advection combined with tight pressure gradients behind the
low will lead to strengthening gap winds along the coast,
beginning along the Alaska Peninsula Tuesday morning then along
the north Gulf coast and Prince William Sound Tuesday afternoon
and evening.

Progressive flow aloft will bring another trough and front across
the Bering and Southwest Alaska, reaching Southcentral by
Wednesday afternoon. This looks like a much better set-up for
accumulating snow across Southcentral, especially along the west
side of the Chugach and Talkeetna Mountains thanks to robust low
level southwest winds and the resultant upslope flow. Snow will
likely overspread Southcentral Wednesday night and persist into
Thursday. Temperatures over the next few days will remain below
normal and largely below freezing for most of the region.

&&

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

Cooler temperatures will build into the region into this evening
under a deep upper level trough. Light snow is possible over
the Middle Kuskokwim Valley and interior Bristol Bay late tonight
as the trough departs to the east. Fog may potentially develop
Tuesday morning ahead of the trailing upper ridge and in
combination with weak surface flow over the Kuskokwim Delta and
Bristol Bay coast. A strong front will move onshore over the
Kuskokwim Delta coast Tuesday afternoon. Blizzard conditions are
likely to develop between Hooper Bay and Kipnuk through the
evening with visibilities reduced to one quarter mile at times.
Likewise, interior regions including Bethel may see visibilities
reduced to one half mile in blowing snow. Conditions will rapidly
improve Wednesday morning as the front weakens and winds diminish.
The long southwest fetch of moisture will continue through
Wednesday potentially leading to several more inches of
accumulation across western Bristol Bay and the southern Kuskokwim
Delta.

&&

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

Scattered rain/snow showers will persist over the central/eastern
Aleutians this morning along the fringes of a North Pacific low.
Strong northerly flow will develop behind the low, leading to
strong gusts out of bays and passes on the Pacific side of the
Chain. Generally dry conditions are expected for this area through
Wednesday under a positively tilted upper level ridge. A front
will extend from the Pribilofs through the western Aleutians
Tuesday afternoon into the western Aleutians. Light snow and
blowing snow will briefly occur over the Pribilofs late Tuesday
before changing to a rain/snow mix into Wednesday morning. Winds
will not be as strong over the western Aleutians, though
precipitation will linger through Wednesday morning before the
front tracks eastward.

&&

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

Gulf of Alaska: Northwesterly flow across the GOA Wednesday.
Expect areas of gale force winds from the Alaska Peninsula west of
Sitkinak. There should be areas of gale force gap winds through
the Barren Islands and Sitkinak Wed and Thu. Seas building to 14
feet.

Bering: Gale force winds out of bays and passes along the Pacific
side of the Alaska Peninsula and eastern Aleutian coasts are
expected for Wed. A gale (and potentially storm) force front
moves across the Bering from west to east Wed through Thu. Seas
over 20 ft are likely with this front.

&&

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

There is a noticeable amount of model divergence from the Bering
to the Gulf Of Alaska. This section of the forecast package
reflects a blend of several models to hedge. There are a few items
to glean past 96 hrs. At the onset of the forecast period, expect
a highly amplified pattern with a trough over the Central Bering
and ridge over Southcentral and the Gulf Of Alaska. Late Thursday
and early Friday the jet core will be directly over Anchorage.
There will be a weak frontal boundary at the surface on Thursday
that will bring a batch of moisture to the Eastern Bering and the
Y-K Delta. The Canadian, GFS and ECMWF have a weak disturbance
near Kamchatka that will bring showers to the Western Bering. The
precipitation will move eastward across the Bering reaching
Nunivak Island by Saturday. The GFS and ECMWF are spinning up a
storm force low in the NW Pacific Saturday which merits watching,
but the anchoring lows are more than 400 miles apart in these
diverging solutions by this point in time.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning: 155.
MARINE...Gale Warning: 130,155,170,172-179,181,185,411-414
		 Heavy Freezing Spray: 160,180,185.
FIRE WEATHER...NONE.

&&

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