Small Craft Advisory 979, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
430 PM AKST Thu Dec 17 2020

.ANALYSIS AND UPPER LEVELS...

At 300 mb, a strong upper level jet remains in place south of the
Aleutian Chain. An upper level trough over the Northern Gulf is
anchored by a surface low centered south of Kodiak Island. This
low gradually continues to track toward the northeast, spreading
in cloud cover across Southcentral. Meanwhile out west, an upper
level trough over the central Bering continues to lift
northeastward. At the surface, a low centered just southwest of
the Pribilofs is spreading clouds and precipitation across the
central and eastern Bering. Light snow showers are falling across
the northern Alaska Peninsula this afternoon.

&&

.MODEL DISCUSSION...

Model solutions are generally in good synoptic agreement for the
immediate short term through early Friday. A key difference that
arises in the upper levels Friday afternoon is the interaction
between two upper level troughs over the Gulf. Right now the GFS
continues to advertise these two troughs will phase, thus steering
a surface low farther west over Southcentral. Meanwhile the
NAM and Canadian models maintain two distinct upper level
troughs, thus sending the bulk of the energy farther east.
However, this was a recent development in the 18Z NAM, as the
previous 12Z run was advertising these troughs phasing. This
major difference will drive the track of the surface low into
Southcentral. With this in mind, and given the drastic changes
across run to run, forecast confidence remains low. By the
weekend, models do seem to be locking onto a broad complex low
over the western Bering very well with great synoptic agreement.

&&

.AVIATION...

PANC...North winds will continue through the evening hours. VFR
conditions are expected overnight until midday Friday when MVFR
ceilings are likely.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through
3/Thursday through Saturday)...

The area of low pressure we have been monitoring is currently east
of Kodiak Island, moving towards the northern Gulf coast. Warm sea
surface temperatures have kept Kodiak City as mostly rain,
therefore, a wintry mix of rain/snow is anticipated as the
backside of the low (and cold air) move over the area. As long as
this low stays the course, there is a chance for a quick shot of
snow for the Anchorage Bowl tomorrow afternoon/evening. Due to the
intricate details of the low track, however, there is high
uncertainty this will occur at this time.

The Winter Storm Watch for Thompson Pass has been upgraded to a
Blizzard Warning in effect from 9 p.m. this evening until noon
tomorrow. Total snow accumulations of 4 to 8 inches, 50 mph winds
and visibility to a quarter mile or less are expected. A foot of
transportable snow on the ground combined with the aforementioned
wind and new snowfall, will work together to keep visibilities at
or below the quarter mile threshold.

Following this low, models are all over the place with the
formation of two upper level shortwaves and their phasing, or not
phasing, together. The 12Z NAM and GFS phase these two together
and spin up another low, heading west towards Southcentral. The
Canadian, the EC and the 18Z NAM do not phase these two together,
with the track of any low spun up clipping the southern Gulf and
moving to the east. This accounts for uncertainty with the low
possibly moving as far north as Kodiak Island or as far south as
through the southern Gulf. All of these solutions have various
potential impacts including: chances for snow in Anchorage
Saturday, another round of wind/snow for Thompson Pass or no
impacts whatsoever. For now, the GFS solution was preferred, but
further refinement is necessary to really hone in on the details.

&&

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

A weakening low continues traveling eastward across the Southern
Bering towards Bristol Bay through Saturday, pushing onshore by
Sunday. Once it makes landfall the low fizzles out and becomes an
open wave. On Friday, the front off the Bering low will push
approach the Southwest coast, bringing a round of light snow
showers to the coastal areas. QPF and accumulations are
unimpressive so no significant amounts are expected. A second
strong low enters the Western Bering Sunday morning. The
associated leading front is expected to track across the Bering
through Sunday night and reach the Southwest coast by Monday. This
will be the next system of concern for the Southwest region and
will be monitored over the next couple days.

&&

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

A weakening low continues traveling eastward across the Southern
Bering towards Bristol Bay through Saturday, pushing onshore by
Sunday. Impacts will be minimal with this, mainly looking at gales
south of the low, weakening to small craft by tomorrow night.
Tonight and tomorrow the low lingers briefly over the Pribilof
Islands so consistent light rain/snow showers are expected during
that time. By Sunday morning, the leading front of an incoming
storm force low west of Shemya enters the Western Bering. This
front will push across the Bering Sunday into Monday with a band
of gales and precipitation, mostly rain and a rain snow mix. A
brief period of storms are expected south of the Central Aleutians
Sunday as well.

&&

.MARINE (Days 3 through 5/Sunday through Tuesday)...

Gulf of Alaska:

There is high confidence an area of low pressure will exist over
the Prince William Sound through Monday morning. Westerly winds
up to 35 kts are possible but most of the Gulf will remain within
small craft advisory criteria. On Monday afternoon there is high
confidence a gale force front will move into the western Gulf
where it will slowly move east at gale force through Tuesday. Long
fetch southerly winds will result in waves greater than 20 feet.

Bering Sea:

A strong, complex low with multiple centers is expected to move
into the western Bering on Sunday, moving slowly into the central
Bering by Tuesday. There is high confidence the front will move
across the Bering at gale force through Monday morning. Gales are
expected to persist around the low with isolated areas of storms
possible near the individual low centers. Forecast confidence is
low in regards to the location of the meso-lows and therefore the
area(s) of stronger winds.

&&

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

A deep trough is expected to remain over the Bering Sea while a
high amplitude ridge builds over western Alaska Monday and moves
east over Southeast Alaska through Thursday. This pattern will
result in active weather throughout the AOR. Over the Bering, a
broad low with multiple centers is expected to persist through
Thursday. The main feature for southern Alaska will be the large
warm front expected to reach Southwest Alaska on Monday. The
moisture feed extends down into the tropics, so moderate to heavy
rain is possible for the AKPen and portions of Southwest. A
transition to snow is likely over the interior but the exact
location of the transition remains uncertain at this time. The
front then reaches Southcentral on Tuesday, with rain likely over
lower coastal elevations. The exact rain/snow line is uncertain at
this time due to the large number of upper level and potential
surface lows developing along the front. By Wednesday forecast
confidence remains low; however, active weather should continue as
Southcentral will remain in a region of upper level divergence
and therefore favors storm development.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WSW 131.
MARINE...Gales 119,120,131,132,170,173,35,352,413,414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...KO
SOUTHCENTRAL ALASKA...CK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KM
MARINE/LONG TERM...CJ