Small Craft Advisory 1001, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
539 AM AKST Tue Dec 14 2021

.ANALYSIS AND UPPER LEVELS...
A low pressure trough remains over the Gulf this morning but is
in the process of exiting to the south and east as multiple
smaller disturbances continue to help dig the trough southward.
Overnight snow has been active across Cordova in conjunction with
a smaller inverted trough along the coast. Snow showers are noted
on radar from Southeast Prince William Sound up into the Copper
River Basin with various towns from Cordova to Glennallen
experiencing reduced visibility and light accumulations. A new
blizzard warning is in effect for Thompson Pass where winds
gusting to 40 knots are contributing to blowing snow and reduced
visibility to a quarter mile or less at times. The good news for
Southcentral is that temperatures are warmer this morning and
cloud cover is clearing from west to east across the Susitna
Valley. This clearing trend should gradually continue through the
rest of Southcentral into Tuesday night. The Anchorage Bowl
experienced a period of light snow during the overnight hours but
this too is quickly coming to an end as upper-level forcing
associated with the trough over the Gulf shifts southeast and out
of the forecast area.

Previously elevated conditions across Southwest, including
blizzard warnings for the Pribilofs, the Kuskokwim Delta and
Bristol Bay have largely come to an end. The pressure gradient
over the eastern Bering has relaxed somewhat as low pressure to
the east makes further separation and warmer temperatures move
into the southern Bering. A ridge of high pressure is
overspreading the Eastern Aleutians and Alaska Peninsula with
clearing and diminished winds. However, morning satellite shows
the next longwave trough over the western Bering, which will
eventually pivot northward across the Aleutians through Wednesday.
Its warm front is already bringing showers to the the Western
Aleutians and has moved atop Adak this morning.

&&

.MODEL DISCUSSION...

Models in good agreement through Thursday. Regarding an area of
low pressure approaching the Aleutians on Thursday, the GFS/NAM
are a touch faster and further north with the low whereas the EC
and Canadian lag behind. Even with these differences overall
impacts to the Western Aleutians should be similar with warm air
advection bringing increasing showers and southerly winds to the
Aleutian Chain.

&&

.AVIATION...
PANC...Ceilings are quickly rising this morning with the
departure of light snow from the region. VFR conditions to
continue through Wednesday as a high amplitude ridge rolls over
Southcentral. This should keep PANC relatively clear with calm
winds through Wednesday night.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 to 3: today
through Thursday)...

A surface low located downstream of an upper-level shortwave
trough continues to race southeastward across the Gulf. The low
tracked south of Kodiak Island last night and produced several
inches of snow and reduced visibility for most of the island. Any
remaining snow potential for this area is ending this morning.
Anchorage and most of Southcentral saw trace amounts of snow and
reduced visibility for a few hours courtesy of this system.

An inverted trough continues to linger around Prince William
Sound and produce snow primarily for the northeast part of the
sound. Cordova has been experiencing moderate to heavy snowfall
with several inches of snow accumulation and very low
visibilities. Moist, southerly flow from the previously mentioned
low over the southern Gulf is aiding in snow production for this
area. In addition to the snow, cold air in the Copper River Basin
continues to drive gusty northeasterly outflow winds through
Thompson Pass. The combination of these two features is creating
blizzard conditions in Thompson Pass. We have upgraded the Winter
Weather Advisory to a Blizzard Warning to account for heavier snow
rates. Snow will begin to taper off later tonight as the surface
low exits the Gulf and northwesterly flow pushes cold, dry air
into the area. Gusty outflow winds will persist through Wednesday
night until an upper- level ridge of high pressure pushes into the
Gulf and relaxes pressure and temperature gradients. A new system
looks to track across Kodiak and through Cook Inlet early Thu.
This system will bring another quick shot of snow to much of the
Gulf coast, but there might be enough warm air to allow some rain
to mix in for Kodiak.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Today
through Thursday morning)...

A brief break in the weather is expected today as a shortwave ridge
moves over the region, keeping skies mostly clear and
temperatures cold. Offshore winds this morning are dropping wind
chill values over the interior of the Kuskokwim Delta near -40 F. The
main weather story in the short term is an occluding low in the
Gulf of Anadyr. Due to ridging over the eastern Bering, the storms
initial front will have a difficult time reaching the Kuskokwim
Delta Coast; however, with a surface high over the interior, the
pressure gradient and ageostrophic component to the wind will
likely produce some brisk easterly winds over the Kusko Delta
later tonight, which may cause some drifting to occur. Overnight
Tuesday and into Wednesday morning a shortwave will round around
the nearly stationary upper low, providing the energy to break
down the ridge and bring precipitation to the Kuskokwim Delta
Coast just before sunrise. With some stronger offshore winds and
falling snow, blowing snow is likely along the coast which may
reduce visibility below one half mile at times. Snow totals will
range from 2 to 7 inches across the region, with the higher
accumulations expected along the coast near Toksook Bay. Snow will
spread into the interior by the afternoon, though accumulations
will remain light.

By Wednesday afternoon the front will begin to move onshore in
Bristol Bay. Expect a gradual increase in temperatures as the
southerly winds bring in warmer air. Temperatures aloft are
expected to remain cold enough that precipitation will remain all
snow despite the onshore flow. Expect snow totals by Thursday
morning to range from 4 to 7 inches from Togiak to New Stuyahok,
with much lower amounts in the lee of the Aleutian Range due to
downsloping.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Today
through Thursday morning)...

An occluding low in the Gulf of Anadyr will send a front slowly
across the Aleutian Chain today through Thursday. Gales are
expected along the front over the Western and Central Aleutians
but will taper off to small craft before reaching Unalaska. Mostly
rain is expected along the front with snow showers and gale force
winds expected in the cold sector of the storm. As the first low
continues to weaken a stronger low is expected to form south of
the Kamchatka Peninsula and strengthen on its way northeast to
the Western Aleutians. Storm to hurricane force winds appear
likely with this system, though a considerable model spread
remains.

&&

.MARINE (Days 3 through 5)...

Gulf of Alaska: Expect winds less than 35 kts and seas less than
20 ft for the Day 3 through 5 period.

Bering Sea: A series of lows track across the Bering Thursday
through Saturday. Widespread gales and seas approaching 20 ft are
likely with each system. Areas of northerly storm force winds or
wind gusts are possible on Thursday night behind the first low,
with 20-25 ft. The following Friday night is the front off a
Kamchatka low pushing into the western Bering. Another round of
storm force winds with seas 20-30 ft possible along the front.

&&

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

A pattern change is expected during the extended period as strong
upper level ridging builds across the area from the eastern Bering
Sea through Southcentral Alaska. There is a medium to high degree
of ensemble consistency the ridge-axis will remain over Southwest
Alaska during this time. The surface pattern is also expected to
change from an offshore to onshore flow pattern as the ridge over
the Alaska Interior will weaken while the ridging off shore
associated with the aforementioned upper level ridge will
strengthen. This will allow for warmer temperatures to enter the
region. This pattern change will also allow for a greater chance
of moisture entering the region due to the change from off shore
to onshore flow. There is currently a moderate degree of agreement
that a weakening low will move inland on Saturday and bring snow
to the Kuskokwim Delta and Bristol Bay. A warm front is expected
to move across Southwest Alaska on Sunday and Monday and bring a
mix of snow and rain throughout the region. Confidence is low with
the specifics of this front at this time due to differences
between guidance.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 131.
         Winter Storm Watch 155.
         Wind Chill Advisory 155.
MARINE...Storm Warning 185.
         Gale Warning 119 127 131 132 173-178 181 411-413.
         Heavy Freezing Spray Warning 119 121 127 130 139 181 414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...AF
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CJ
MARINE/LONG TERM...ED/KM