Small Craft Advisory 912, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
454 AM AKST Wed Nov 16 2022

.ANALYSIS AND UPPER LEVELS...

The stout North American Ridge extends from southern California
along the West Coast all the way to Southcentral Alaska.
Overhead, a portion of this ridge is closed off, allowing for
clear skies over much of the Southern Mainland, with the
exception of some fog and low stratus in the Copper River Basin.
Very dry air is evident on low level water vapor imagery, with the
Chugach and Wrangell Mountains being visible. Over the North
Pacific (NPAC), a massive, well occluded low centered at 43N 158W
is sending a series of weak shortwaves and surface features north
into the Alaska Peninsula and Kodiak Island. This will be the main
area of active weather today with the addition of Bristol Bay as
a few shortwaves move north. Over the Bering, a north-south
oriented stationary front will continue to weaken in place.

&&

.MODEL DISCUSSION...

Models are in generally good agreement but minor differences
remain, particularly with the speed and north/south deviation of
shortwaves moving north from the large NPAC low. These
differences will result in timing and magnitude differences for
precipitation along coastal Southwest. As we move into Friday,
considerable difference begin to develop in the evolution of
stronger shortwaves revolving around the NPAC low, which try to
break down the strong high near the ALCAN border and Yukon
Territory. Deterministic models often break down a ridge too
quickly, so it's unlikely that the more progressive solutions
00/06Z NAM and 00Z GFS will verify in driving a strong shortwave
through the eastern periphery of the ridge. The 00Z ECMWF and 06Z
GFS solutions of the low moving into the southern Gulf and
becoming more stationary over the weekend seems more likely.

&&

.AVIATION...
PANC...VFR conditions and light northerly winds will persist
through the TAF package.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Friday night)...

A strong upper level ridge sitting overhead of Southcentral will
slowly shift eastward through Thursday. As it does so, warm air
aloft will move up and over top of cooler air sitting at/near
the surface, causing existing temperature inversions to further
steepen. This is typically a great set-up for fog. However,
pressure gradients between high pressure over mainland Alaska and
a deep low in the North Pacific will produce just enough wind to
limit fog formation. The one exception is the Copper River Basin,
where fog and stratus are firmly entrenched and will likely remain
so until the pattern changes and we can break down or weaken the
inversion - which may happen on Friday.

As warm air advection ensure along the coast today, there could be
some very light precipitation across the eastern Kenai Peninsula.
Anything that falls will be in the form of rain. With sub-freezing
temperatures across inland valleys there is a chance of some light
freezing rain or freezing drizzle. Meanwhile, a series of short-
waves rounding the aforementioned low in the Pacific will cross
the southern Gulf, bringing periods of rain to Kodiak Island
through the remainder of the week. Even in breaks between short-
waves, persistent easterly upslope flow will maintain low clouds
and visibility along the east side of Kodiak Island.

By Friday the ridge will shift far enough east to allow the
trough over the Pacific to lift northward toward Southcentral.
There is quite a bit of uncertainty in the timing and track of
this trough and associated short-waves and surface features.
What can be said with some confidence is that precipitation is
likely along the Gulf coast by late Friday - and it could be
heavy precipitation along coastal mountains/Prince William Sound.
Stay tuned, as we figure out the details of this impending shift
in pattern.

&&

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

A blocking ridge continues its reign over Mainland Alaska,
keeping clearer skies this morning and warding off much of the
precipitation for Southwest Alaska today. However, some light
precipitation is expected today from the Alaska Peninsula up into
the Y-K Delta area. Light to moderate rain along the Alaska
Peninsula will begin early this morning. Precipitation from this
system will then push into the Eastern Bering/Bristol Bay area.
Precipitation in Dillingham to begin in the late morning early
afternoon in the form of freezing rain/drizzle quickly
transitioning to a light rain for the remainder of the day due to
an influx of warmer air. A chance for light showers will linger
across coastal Southwest through the night tonight and into
Thursday, with another wave of moisture maintaining the light
precipitation from Kipnuk to Cape Newenham. This wave will consist
mostly of rain along the Y-K Delta coast with a chance of some
freezing rain/drizzle in Bethel Thursday night. Precipitation will
ease across Southwest Friday with a return of clearer skies for
much of Southwest by Friday night.

&&

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

A stationary front currently in the Central Aleutians has been
effectively blocked by the ridge over Mainland Alaska. Winds and
rain along this front will weaken today dissipating by late
tonight. A shortwave south of the Eastern Aleutians/Alaska
Peninsula will push a shot of rain through the Eastern Aleutians
this morning, with rain reaching the Pribilofs by the early
afternoon today. This shortwave will then collide into the
stationary front in the Central Bering and both systems will exit
to the northern Bering. A break in precipitation is expected
across the Aleutians for Thursday. Thursday night, a North Pacific
low will approach the Western and Central Aleutians. The leading
front along this system will bring warmer temperatures coupled
with light precipitation in the form of rain across the Western
and Central Aleutians. Currently, this system is expected to
linger here for the remainder of Friday into Friday night.

&&

.LONG TERM FORECAST (Days 4 through 7: Saturday through
Tuesday)...

A persistent ridge of high pressure will stretch across much of
the Mainland through much of the long term. Deep southerly flow
with the ridge will result in above average daytime high and
overnight lows are advertised for both Southcentral and Southwest
Alaska through the weekend. Upper-level shortwaves within the
upstream ridge flow will track across Southwest Alaska and
maintain periods of precipitation through Saturday morning.
Meanwhile, an upper-level low and its associated surface low are
expected to move into the Gulf of Alaska. Heavy precipitation and
strong winds accompanying the aforementioned low and front look
will impact the coastal areas through Tuesday with some areas of
precipitation being hinted for Cook Inlet. Strong down inlet flow
is also anticipated with an increase in winds for the Copper River
Basin and Valdez regions as gradients tighten between the low and
high pressure situated over land. However, their is considerable
models spread on the timing and placement of the aforementioned
low.

Out west, a shortwave ridge will form over the eastern Bering for
Saturday. A North Pacific low is expected to reach the western
Aleutians Friday morning and track into the Bering through the
weekend. Model spread grows in association with the exact track
and strength of the low and its front, but a consensus of
widespread rain and gale force winds are currently advertised to
accompany the low along with a consensus of the low tracking
along the Chain. Models spread begins to grow when the low reaches
the Alaska Peninsula. Current GFS model runs are advertising the
aforementioned low tracking northward over the Alaska Peninsula
and into the eastern Gulf with widespread gale force winds located
in the across the Bering. Meanwhile, recent Canadian and EC runs
are showing roughly the same track, but more south with the low
and advertise a swath of small craft winds roughly located over
the eastern Bering. The Canadian even develops a second low over
the central Bering, but all of the other models are not
advertising this. Overall, models have yet to iron out the
intensity and track of the aforementioned low and its impacts.
High pressure will build into the western Bering at the end of
the long term, which will promote calmer conditions with the
possibility of marine stratus forming under the ridge.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 130-132 138 150 155 180-181.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CJ
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CL
LONG TERM...NSC