Small Craft Advisory 937, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
505 AM AKST Thu Nov 24 2022

.ANALYSIS AND UPPER LEVELS...

A active, progressive pattern continues for Southern Alaska as a
series of lows will make their way to and through our region. A
low in the Northern Bering has driven a front across the Bering
and is now entering Southwest Alaska. Appended to the southwestern
tail of this Bering front is a North Pacific low that is
providing precipitation to the central Aleutians as it sweeps the
Chain to the east, reaching the Alaska Peninsula by tonight. In
Southcentral, a lull in precipitation has been filled with
scattered areas of fog in the Mat- Su valley. This will clear out
as the aforementioned North Pacific low approaching from the
Alaska Peninsula, will breach the Gulf by tonight. Additionally,
an inverted shortwave approaching from the east will begin the
precipitation in the PWS area, spreading westward, reinforced by
the low moving in which will keep precipitation going for
Southcentral through Friday evening.

After these systems move through, a ridge begins to build again
over Central Alaska which may create a similar blocking pattern
seen recently. The next active system to approach our region will
be another North Pacific low, moving into the Western/Central
Aleutians by Friday night. This system will advance westward,
however how it interacts with the building ridge in mainland
Alaska has yet to reach a consensus among the models. There is a
high likelihood the way things are looking now, that this low
will be blocked and kept out of Southwest and mainland Alaska.

&&

.MODEL DISCUSSION...

Regarding the relatively slower pattern currently, the models are
in excellent agreement. This consensus rapidly decays to moderate
uncertainty in Southcentral particularly, regarding the westward passage
of an inverted shortwave late this afternoon/evening. It is agreed
that the PWS area will receive a shot of snow beginning Thursday,
however how far west this wave will progress is still contested.
Additionally, the GFS and Euro 00Z runs bring a wave of
precipitation up Cook Inlet, originating from a low coming off of
the Alaska Peninsula. This wave is not represented as intensely
by the Canadian and NAM runs and is adding to the uncertainty
regarding how much snowfall for Anchorage to expect. How long
these systems linger in Southcentral is also contested, with the
GFS and Canadian runs keeping the precipitation in Southcentral
until Friday night, whereas NAM and Euro back the precipitation
out by Friday morning. Out west, agreement is higher with a calmer
pattern expected following the passage of the front and North
Pacific low making their way through now. Agreement is also high
currently regarding the entrance of the next North Pac low to
impact the Western/Central Aleutians starting early Saturday
morning.

&&

.AVIATION...

Light winds and VFR conditions will persist through Thursday
afternoon. Thursday night a drop in ceilings and visibility to IFR
criteria is expected along with snowfall beginning. There is some
model uncertainty regarding the longevity of the snowfall
however, most likely scenario currently appears to keep the snow
showers over the terminal through Friday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 2:
Today through Friday night...

Gap winds will decrease today as the upper level low moves east
and the pressure gradient relaxes. A surface low will track east
across the northern gulf and continue to produce light snow
across the Copper River Basin and Prince William Sound with most
of the accumulating snow in the higher terrain with the snow
levels at 500 to 1,000 feet. Our focus then shifts to a strong
shortwave moving towards Southcentral this evening this will shift
the upper level flow out of the southwest with snow developing
across the Mat-Su valleys and Anchorage area by late afternoon
into this evening. This shortwave will become negatively tilted
overnight with a surface low developing across the Gulf. However,
there is some uncertainty on the track of the surface low as most
models track it near the eastern Kenai Peninsula hugging the coast
towards Prince William Sound while the GFS continues to be the
outlier with a deeper surface low that is slower and farther
south. This track will affect how much snow falls from Anchorage
to the Mat-Su valleys. But with strong dynamics and southwesterly
flow continuing through Friday morning this should allow for 3 to
5 inches across Anchorage and the Mat- Su valleys. If the GFS
solution is correct we could see 4 to 8 inches across the
Anchorage and Mat valley. A secondary shortwave will move towards
the northern Gulf late Friday which will shift the flow to
northwesterly which will keep light snow going along the Anchorage
hillside and Eagle River areas through Friday evening with
additional accumulations up to 2 inches. As this system pushes
east Friday gap winds will re- develop with Gales expected across
parts of the Gulf and near the Barren Islands.

&&

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

A weakening Bering front has arrived on schedule across western
portions of Southwest this morning, with snow and reduced
visibility observed across the Kuskokwim Delta overnight into
early this morning. Radar returns are beginning to show up on the
King Salmon radar as the outer fringes of frontal precipitation
begin to advance into Bristol Bay, and light snow is expected to
spread east into the Lower Kuskokwim Valley and Bristol Bay as
the day progresses. A pocket of warm air aloft will also attempt
to work into southern portions of Bristol Bay near the front
before surface temperatures rise briefly above the freezing mark
later this afternoon. This warm nose moving atop colder air at the
surface may provide a narrow window for a mix of light freezing
rain mixing with the snow for an hour or two between Dillingham
and King Salmon, changing to a plain rain and snow mix this
afternoon as temperatures warm. In any case, any freezing rain
should remain minimal in duration and intensity, and no ice
accumulation is expected. The bulk of remaining precipitation
(mostly snow) should come to an end across all of Southwest by
this evening as the front progresses into the western Gulf.

A relatively benign early winter pattern will set up shop for the
rest of the outlook period from Friday to Saturday. Cool
northwesterly flow will once again develop as a high pressure
ridge builds over the Bering. The return of a colder air mass
should help notch temperatures back down into the 20s for highs
and 0s to 10s for lows for both Friday and Saturday. Any new
snowfall through the end of the week should stay generally
confined to areas favored for upslope flow along the Kuskokwim
Mountains and Aleutian/Alaska Ranges. One exception will pertain
to a small, fast-moving shortwave that will move southeast on
Friday between the Kuskokwim Delta and Bristol Bay. A quick flurry
of snow showers will likely follow this system as it tracks
through, with the best chance for light snow over southern
portions of Bristol Bay and the Kuskokwim Delta on Friday
afternoon. Otherwise, expect cool and quiet conditions to persist
across the region through Saturday night.

&&

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

A front currently draped across the southern Bering will continue
to produce gusty westerly winds and rain from the southern Alaska
Peninsula to the eastern Aleutians for most of today. This front
should cross over to the Pacific side by this evening, allowing
rain to shut off as gusty small craft to gale force winds to
quickly become lighter and northwesterly overnight. A lull in the
pattern is expected for most of Friday into Friday night as a
broad upper ridge expands across the Bering, with perhaps the most
impactful conditions limited to westerly gale force winds near St
Matthew Island and occasional snow showers moving into the
Pribilofs. On Saturday, the ridge axis will begin to shift east as
a North Pacific low approaches the western Aleutians. Winds will
pick up from the east and rain will begin as the low's front
approaches on Saturday morning across western parts of the Chain
on Saturday morning. Rain will spread east out to Atka and Dutch
harbor by Saturday evening as the low crosses into the western
Bering. Similarly, easterly small craft to gale force winds should
shift into the central Aleutians through Saturday evening.

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

As a low exits east through the Gulf of Alaska over the weekend,
confidence is high that a ridge will build near the eastern Bering
Sea and western Alaska. While a ridge will keep the weather drier
over Southcentral, the strength and tilt of the ridge axis is a
little less certain and could lead to implications in the synoptic
flow and temperature forecast along the western mainland.
Upstream of the ridge is a developing shortwave beginning over the
western Aleutians Sunday. This shortwave is expected to ride
along the ridge, reaching mainland sometime Monday and dip into
the northern Gulf Monday night. Precipitation can be expected with
the shortwave but model differences do not point to any specifics
on quantity. The position of the low as it transits the Gulf will
also have implications to the Southcentral forecast. This appears
to be a quick and brief disturbance and the upper level ridge
generally prevails over the eastern Bering and western Alaska.
Midlevel temperatures suggest cold air and perhaps surface high
pressure over the mainland that could help to enhance any gap
winds along the Gulf coast, especially as the low transits the
Gulf and increases not only the temperature gradient, but also the
pressure gradient. Model consensus breaks down substantially
around midweek.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE... Gales Warning 125 130-132 150 155 172 174 181 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CL
SOUTHCENTRAL ALASKA...RC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AS
LONG TERM...VR