Small Craft Advisory 817, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
459 AM AKDT Tue Oct 27 2020

.ANALYSIS AND UPPER LEVELS...

A positively tilted trough extends from northwestern Canada
through Southwest Alaska, with a potent vort max moving over
the Alaska Peninsula. Water vapor imagery shows drying across
much of Southcentral, though a few lingering showers are seen near
Prince William sound under a weakening surface low. Predominately
zonal flow will develop to the west.

&&

.MODEL DISCUSSION...

Model agreement remains good in the short term with no impactful
weather over Southcentral. There remains some disagreement with
regards to a low approaching the western and central Aleutians
Wednesday into Thursday. The GFS is the strongest, while other
models/ensembles are slightly weaker (up to 10 mb). The strength
of the cold advection on its north side will determine how deep it
will ultimately become. Confidence remains moderate for this
system.

&&

.AVIATION...

PANC...Scattered low clouds and brief showers remain possible in
the early morning, leading to MVFR conditions. Skies are expected
to clear thereafter with VFR ceilings and light winds persisting
through Wednesday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Thursday night)...
A short-wave trough over Southcentral Alaska early this morning
will stall along the north Gulf coast this afternoon and weaken
as ridging from the north takes over. Expect no more than a few
showers with this trough, primarily from the southern Copper River
Basin southward into the Gulf. Clouds will decrease behind the
trough and colder air will continue to filter into the region.

Temperatures will drop more significantly tonight underneath the
mostly clear skies and these colder temperatures will become the
norm for the foreseeable future. A series of short-waves crossing
the northern half of the state will bring colder and colder air
southward into Southcentral. The advection will be weak, so this
will be a gradual process with each day become a little bit
colder. By Thursday, temperatures for all but the immediate
coastline are expected to top out in the 20s. A surface low will
linger over the northeast Gulf, with clouds and some light snow
possible over the eastern Copper River Basin. The presence of the
low combined with the weak cold air advection will maintain gap
winds along the Gulf coast and Prince William Sound. However, none
of these winds look particularly strong.

Thursday through Thursday night looks a little more interesting.
A higher amplitude short-wave trough will dive out of eastern
Russia, southward across western Alaska and toward Southcentral
Alaska. Meanwhile, a deep low tracking along the Aleutians will
approach the Gulf. The timing of each of these feature will be
critical to determining how much of the arctic air accompanying
the trough makes it into Southcentral and where the leading
frontal system with the Aleutian storm tracks. At the moment,
model guidance is in general agreement on the frontal system
leading the way into the Gulf and the short-wave/arctic air then
causing development of a triple point low which eventually tracks
to the northeast Gulf (on Friday). This has potential to be a
decent snow producer with ample arctic air in place over
Southcentral. The current track would favor eastern Prince William
Sound and the Copper River Basin. However, timing of each feature
could shift the snow to the west or to the east.

&&

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

A quiet weather pattern is setting up across Southwest Alaska for
the next few days. Lingering snow/rain showers this morning will
taper off throughout the day as the low system over the eastern
part of the state exits the region. However, an upper level wave
will keep precipitation confined to the AKPEN through Wednesday.
The cooling trend throughout the week continues as and upper level
trough and cold air advection moves into the Southwest.

&&

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

A low situated near the western Aleutians and Bering allow a weak
front to moves eastward through Wednesday. Small craft advisory
level winds are expected with this system as it move into the
eastern Bering.

A much stronger North Pacific system approaches the Aleutians
late Wednesday. Models are still in good agreement with the track
and strength of this system, bringing high end gales force winds.
Storm force winds cannot be ruled out, especially over the eastern
Bering Thursday afternoon through Friday as cold air advection
should help keep higher winds in the forecast.

&&

.MARINE (Days 3 through 5)...

A surface low will linger over the Northern Gulf through late
Friday. Models are in good agreement for the low tracking just
south of the Aleutians later this week. The latest model runs
have increased the overall strength of this system, thus, gales
will accompany this low, with storms becoming increasingly likely.
An associated trough pushes into the Southern Gulf by late
Friday, with good model agreement on the formation of a surface
low. Models disagree on where the low forms and the strength, with
the GFS being the strongest. Depending on which solution pans
out, gales may be possible.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Multiple polar waves will track south across the state, bringing
colder air to the Southwest/Southcentral regions. In conjunction
with cold advection, possible clearing will aid in dropping
temperatures significantly, especially overnight, for the
Southcentral and inland Southwest areas by the end of the week.

Models agree quite well for the North Pacific low traveling just
south of the Aleutians. Both the ECMWF and Canadian solutions have
increased the strength of this system to better match the GFS,
which has been consistently stronger. Gales are very likely along
the Aleutian Coasts, with storms becoming increasingly possible.
By the end of the weekend and into early next week, a second
system approaches the Aleutians out of the west. Models are in
fair agreement on the path but vary greatly on center location and
strength.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 130.
FIRE WEATHER...NONE.

&&

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