Small Craft Advisory 983, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
427 PM AKST Fri Dec 18 2020

.ANALYSIS AND UPPER LEVELS...

An upper level low present over the northeastern Gulf has opened
up into an upper level wave. This wave was bringing snow to Valdez
and Cordova for much of the morning hours. The rest of
Southcentral remains under southerly flow aloft which is keeping
the region with a good amount of cloud cover. Strong outflow winds
remain at the surface in areas such as the Valdez Narrows and
Thompson Pass. While winds were much higher earlier in Thompson
Pass creating blizzard conditions, these winds have since
subsided enough to lessen any blizzard conditions. Much of
Southwest Alaska also remains under southerly to southeasterly
flow aloft that is keeping a good amount of cloud cover across the
region. Temperatures are very cold also in the teens below zero
across the Middle Kuskokwim Valley.

A broad upper level trough remains in place across the Bering Sea
with an associated surface low just west of the Pribilofs. Some
gale force winds were occurring mainly on the southern side of the
system. Besides the winds, on and off rain showers were occurring
for the Pribilofs.

Lastly, an upper level wave was present on satellite imagery this
afternoon with an associated surface low over the North Pacific.
This low is the one currently being watched for snowfall across
Southcentral on Saturday.

&&

.MODEL DISCUSSION...

The biggest challenge going forward even though we are inside 24
hours now is the snow potential across Southcentral for Saturday.
While most guidance agrees on Anchorage seeing snow, the question
remains just how much. The trend on model guidance this afternoon
has been to the west but they can just as well go back to the east
again. Even though they came west, model guidance still has
different tracks on exactly where the center of this low moves.

Further west, model guidance remains in great agreement on the low
in the Bering. Attention then turns to the next low that is
expected to impact the Bering and the Aleutian Chain Saturday
evening into Sunday. Model guidance continues to spin up various
different lows with the GFS being the strongest one with a low
developing along the front. Thinking the parent low will be the
low hanging back to the southwest of Attu Island. While model
guidance is spinning up these different lows along the front, the
intensity, and placement of each of these is in disagreement
across the board. This could have an impact on just how strong
this front is as it moves through the Bering/Aleutian Chain. Where
the parent low goes beyond Sunday afternoon remains in question as
well as the intensity of this low as it moves into the Bering.
Either way, guidance is indicating this being a strong low in the
Bering but how strong.

&&

.AVIATION...

PANC...MVFR cigs will be intermittent through the afternoon hours,
before becoming VFR later this afternoon. Snow is expected to
develop late tomorrow morning and persist into the evening.
Confidence is gaining that the airport should see an inch or more
through the event. However, confidence is low where the strongest
banding will develop. There is the potential for conditions to
drop to IFR for short times if banding is near or over the field.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

A weakening low in the north central Gulf this afternoon will
continue to lift northward through the night, allowing for the
gusty winds to diminish across the Copper River Delta. A second
low moves into the North Pacific overnight into Saturday morning,
which will bring gale force winds to the northern Gulf through the
afternoon hours. The Anchorage Bowl will also see snowfall
associated with this system starting in the morning and continuing
through most of the day, with accumulations of 2 to 4 inches
expected. However, there is still a bit of uncertainty in the
forecast as to when the snowfall will start and how much will
fall. A shift in the track of the low, either east or west, could
change the amount of snowfall received throughout the day on
Saturday. Areas along the eastern Kenai Peninsula could also see
about 3 to 7 inches of new snowfall with this low. As the
aforementioned low lifts northward into the North Gulf Coast by
Sunday morning, chances for snowfall starts to diminish across
Southcentral through early Monday morning.

A new front enters the western Gulf and moves over Kodiak Island
Monday morning, which will continue over the Kenai Peninsula
through the afternoon. This system will bring a significant amount
of moisture with it, but there is still a bit of uncertainty as
to whether it will fall as snow or rain as warmer temperatures
will move into the region. Be sure to check back for the forecast
as models come into better agreement with this shift in the
weather pattern for the beginning of the work week.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

A weakening low continues moving slowly across the Southern
Bering, reaching the Bristol Bay coast Sunday morning. Light
snow showers will be confined to the Southwest Coast through
tomorrow before extending into the southern portions of Bristol
Bay, the Southern AKPEN and the higher elevations. No significant
accumulations are expected with this. There is deep amplification
in the upper levels by Sunday as a deep low approaches the Western
Bering. The associated leading front is expected to reach the
Southwest Coast by Monday morning. Winds will become gusty for
the Kuskokwim Delta, through passes in Bristol Bay and the AKPEN,
and along the Coast on Monday. Precipitation will be heaviest
along the Eastern Aleutians/Southern AKPEN as the front
approaches, as well as along the eastern Kuskokwim Mountains.
Along the Chain/AKPEN, this will start as a rain/snow mix
transitioning to all rain and then back to a mix. Light snow
showers are expected across the rest of the Southwest Mainland on
Monday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

Low impact system tracks across the Southern Bering through
tomorrow night. A few gales and small craft winds expected along
the Aleutian Coasts as well as light showers south of the low. By
Sunday morning a leading front enters the Western Bering. Model
agreement by this point is not ideal, with different solutions
spinning up embedded lows in different areas within the system.
However, confidence is high enough to expect a band of gales along
the front to travel across the Bering towards the Southwest Coast
through Sunday night, with the chance for periods of storms
likely. As the front approaches the Southwest coast, periods of
moderate to heavy precipitation, will work its way across the
Eastern Aleutians/Southern AKPEN Sunday night into Monday. This
will mostly be a rain/snow mix transitioning to all rain then back
to a mix, with snow along the higher elevations.

&&

.MARINE (Days 3 through 5/Monday through Wednesday)...

Gulf of Alaska:

There is good confidence on a quiet start to the period with high
pressure extending across much of the Gulf. By Monday night
southerly winds will be increasing across much of the western Gulf
of Alaska as a front approaches. The current forecast calls for
widespread Gales, but some areas of storm force winds appear
possible. On Tuesday we should see these southerlies subside.
Wednesday there is significant model disagreement on the
development of an area of low pressure in the northeastern Pacific
that would bring stronger easterlies back into the Western Gulf,
for now the forecast leans towards this development remaining too
far south and too weak to have any significant influence on
winds/waves in the western Gulf.

Bering Sea:

A broad complex low with multiple centers will be centered over
the Bering Monday into Tuesday. The strongest winds will be found
along the northern fringes of this broad low across the Northern
Bering, with good confidence on generally low end Gales. By
Wednesday the broad low looks to break up into multiple weak lows
across the Bering Sea. Confidence on the placement of these
individual lows is very low, but none of them look to produce more
than some very small areas of Gales.

&&

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

On Tuesday a deep trough will extend across much of the Bering,
with a ridge across the Gulf of Alaska through eastern Mainland
Alaska. A pronounced shortwave will be rounding the base of this
Bering trough with a low deepening across the western Gulf. This
low will have a great connection to some moist air from the
tropics and thus looks to set up widespread precipitation,
potentially heavy from the AKPEN through the Northern Gulf Coast.
With the precipitation will come a significantly warmer airmass
so a quick change to rain is possible for portions of the AKPEN
and Kenai Peninsula. Some very heavy snows look possible for all
higher elevation areas surrounding Prince William Sound. On
Wednesday this system should begin to weaken with only some
lingering precipitation for the Northern Gulf coast. For
Thursday/Friday another storm system looks to lift out of the
North Pacific into the eastern Gulf of Alaska with most of its
impacts remaining east of our AOR. Across the Bering a complex of
weak lows look to keep the pattern active across the central and
western Aleutians, but with none of these lows looking
particularly impactful/strong.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gales 119 120 165 177 178.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MV
SOUTHCENTRAL ALASKA...AH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KM
MARINE/LONG TERM...BG

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