Small Craft Advisory 237, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
635 PM AKDT Thu Mar 17 2022

.ANALYSIS AND UPPER LEVELS...

The primary of feature of note across the Alaska region is a deep
cold closed upper level low along the Alaska Peninsula. Over the
past couple of days Arctic air has advected southward across all
of the Bering Sea and Aleutians and into the North Pacific. This
air mass has now wrapped all the way around the upper low center,
extending northward across the western Gulf and back into
Southcentral. A short-wave trough rounding the west side of the
low is enhancing snow showers along the southern Alaska Peninsula,
leading to severely restricted visibilities in snow and blowing
snow. Meanwhile, an Arctic short-wave trough is digging southward
from Siberia down across the heart of the Bering Sea, bringing a
re-enforcing shot of Arctic air. This is resulting in strengthening
winds and increasing snow shower activity across the Bering and
Pribilof Islands. Ridging over the western Bering Sea is leading
to very quiet conditions across the western Aleutian Islands.

Meanwhile, a short-wave trough digging south of the upper low
center is leading to intensification of a surface low over the
Pacific, with the low tracking northward toward the Gulf of
Alaska. Farther north, a series of weak short-waves are lifting
northward across Southcentral. This is producing areas of light
precipitation, mainly in the form of rain along the coast and
snow inland.

&&

.MODEL DISCUSSION...

Model agreement with the large scale pattern remains good. The
primary differences are with the track of the deep surface low
across the Gulf of Alaska Friday night through Sunday. The GFS
looks to be the biggest outlier in more aggressively tracking the
low to Kodiak Island early Sunday, while the rest of the guidance
is clustered over the western Gulf waters. Will go with the
majority of solutions and keep the low over the Gulf. Regardless,
the differences in low track will largely impact the marine
forecast and would expect some continued shifting of the track
in future forecasts.

&&

.AVIATION...

PANC...The short-wave which brought snow and IFR conditions to the
terminal today will exit northward this afternoon, with ridging
and light winds in the wake. While this may have caused concerns
for fog and/or low clouds to settle in, the amount of daylight
and increasing sun angle should be enough to prevent that from
occurring. Thus, expect ceilings to rapidly lift as we head
through the late afternoon and early evening hours.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Thursday night through Sunday)...

A passing shortwave coupled with onshore flow is keeping light
snow across the eastern Kenai Peninsula and Prince William Sound.
Elsewhere, light snow continues in parts of the Susitna Valley and
along the Alaska Range. Relatively little precipitation is
falling from Anchorage north to the Matanuska Valley. Meanwhile, a
North Pacific low is anticipated to move into the southern Gulf
by Friday. The leading front will move into the coast, bringing
moderate to heavy precipitation along the north Gulf coast, with
isolated snow moving further inland.

As the low lifts slowly north by Friday night, winds will shift
more easterly and focus the onshore precipitation for the favored
upslope mountains as well as into Whittier and Portage Valley.
Weak dynamic forcing and east-northeasterly flow will help push
some of the snow into the Copper River Basin, including upslope
areas of the Glenn Highway around Eureka. Much less precipitation
can be expected towards Seward and Valdez given the predominant
easterly winds. For Kodiak, cold, northerly winds and surface
temperatures around freezing will bring wet snow initially before
switching to rain late Friday afternoon. By late this weekend,
most of the precipitation will taper off from north to south as
cold and drier air filters across the mainland.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Thursday night through
Sunday)...

A relatively quiet weather pattern will continue across Southwest
Alaska over the next 3 days as an upper level trough extends
across the area. Intermittent snow showers are likely along
coastal areas, especially along the western Alaska Peninsula as
strong cold air advection will continue over the eastern Bering
Sea and allow for these convective snow showers to continue. These
snow showers will remain along the west coast. Farther inland
light winds will continue with a slight chance of snow showers.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Thursday night through
Sunday)...

Unsettled weather continues, especially across the eastern
Bering/Aleutians as strong cold air advection continues over the
water. This set up of cold air over the water results in ocean-
effect snow showers. Some of these convective snow bands may
produce heavy snow at times. Please the Winter Weather Advisory
for the Alaska Peninsula for blowing snow for further details.
This pattern will persist until Sunday morning. Ocean- effect
snow band activity will decrease on Sunday as the ridge- axis of
high pressure over the western Bering/Aleutians moves eastward and
allows for a weakening in the cold air advection.

&&

.Marine (Days 3 through 5: Sunday through Tuesday)...

Gulf of Alaska: A weakening low exits the Western Gulf as a
strong low enters the Southcentral Gulf late Sunday. Its front
extends across the Gulf through Tuesday. Forecast confidence is
good. Gusty winds look likely with areas of gales over the
Western Gulf, Barren Islands, Cook Inlet and Shelikof Strait,
set to diminish Sunday. Seas will be to 15 feet, subsiding by late
weekend. The Central Gulf low spreads high end gales over the
Gulf through Tuesday. Seas will to 16 feet. Freezing spray likely
south and west of Kodiak Island.

Bering Sea: A ridge moves across the Bering, pushing against a
large area of low pressure over interior Alaska through Tuesday.
Forecast confidence is moderate, but decreases with the western
system by Tuesday. Widespread high end northerly gales move over
the Eastern Bering and Aleutians/Alaska Peninsula, set to diminish
on Monday. Seas will be to 15 feet on the Bering side, with 20
feet on the Pacific side, subsiding by the end of the period. A
well developed low and front moves into the Western Bering through
Tuesday. Widespread high end southerly gales move over the
Western Bering, diminishing Tuesday. Seas will build to 20 feet.
Freezing spray expected over the Central and Eastern Bering open
waters.

&&

.Long Term Discussion (Days 4 through 7: Monday through Thursday)...

Agreement among global models is decent to start out on Monday,
but rapidly deteriorates by later in the week. A persistent upper
low currently near the Alaska Peninsula is projected to still be
drifting somewhere over the Gulf by Monday, with the center most
likely sitting south of Kodiak Island by this point. Another low
is expected to pivot north and west around the main upper level
center out of the North Pacific and into the Gulf through Tuesday,
wrapping winds up to gale force across open Gulf waters. This will
spread coastal rain and mountain snow into the north Gulf Coast
as the low retrogrades west towards the southern Kenai Peninsula.
Meanwhile, quiescent conditions are favored out west as a sharp
upper ridge moves across the Bering. This ridge axis should become
increasingly stretched and narrowed, eventually separating from
the prevailing flow aloft and becoming a cutoff high drifting up
past the Bering Strait and into the Chukchi Sea by Wednesday. The
upper level low should concurrently continue to retrograde back
into the Alaska Peninsula, possibly allowing a Rex block regime
to set up across the state by the middle of next week. Few details
are decipherable from the middle of next week and beyond, with
notably poor run-to-run consistency among operational models
appearing for even large scale elements of the pattern through
Thursday. There remains a general signal in model ensemble means
for an upper level trough to remain in place over the Alaska
Peninsula and eastern Bering through Thursday. This would keep the
best chances for precipitation across the domain concentrated
near Kodiak Island and up along mainly coastal areas of
Southcentral with the continued southeasterly/onshore flow. Most
of the Southwest and Bering should continue to enjoy a stretch of
relatively calm weather, although potential increases for a new
system to move into the Aleutian Chain near the end of next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 181.
MARINE...Gale Warnings 119 120 130-132 138 351 352 150 155 165
					   170-176 178 179 180 181 185 412-414.
Heavy Freezing Spray Warnings 160 165 170 179 180 185 412 414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB/AF
SOUTHCENTRAL ALASKA...MF
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ED
MARINE/LONG TERM...MK/AS