Small Craft Advisory 880, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
509 AM AKST Tue Nov 17 2020

.ANALYSIS AND UPPER LEVELS...

A highly amplified pattern continues this morning across the state
of Alaska and the Bering Sea. The pattern resembles the Greek
letter Omega when looking at the water vapor imagery with the 500
mb height fields overlaid. This kind of a pattern is usually
associated with the same weather prevailing across a region until
it begins to break down. Much of Southcentral is enduring a
mostly clear sky this morning with temperatures in the teens and
20s below zero. Temperatures across the Susitna Valley have fallen
to the single digits and teens below zero. From the Matanuska
Valley through the Anchorage Bowl to the Kenai Peninsula,
temperatures have fallen into the teens and single numbers. When
looking at the nighttime microphysics this morning, there were
some areas of low stratus and fog across Anchorage up through Knik
Arm and also along Turnagain Arm. This has been causing some low
ceilings and visibility issues occasionally. Areas where the low
stratus and fog set up have put a halt on temperatures falling
this morning and even helped to raise temperatures some. Areas
where it stays clear and have light to calm winds will continue to
have decent radiational cooling to drop temperatures near and
below zero.

Southwest Alaska remains under the axis of the upper level ridge
this morning leading to a mostly clear sky for a good portion of
the area. However, some low level moisture trapped beneath an
inversion is causing low stratus and fog for King Salmon and areas
nearby. The fog stretched out into Bristol Bay and across to Cape
Newenham and was moving up towards the coast this morning.
Dillingham remains on the edge this morning but the risk is there
that the fog moves into town. Temperatures across Southwest were
generally in the single digits and teens with 20s along the coast.

Lastly, across the Bering Sea and Aleutian Chain, one storm system
continues to move northward while weakening northwest of the
Pribilofs. Winds at the Pribilofs were sustained around 30kts for
much of the night with gusts 35-40kts. This is also supported with
the ASCAT Scatterometer pass that showed gale force winds across
the Pribilofs earlier in the night. These have since come down to
small crafts. Further west, the next storm force low and its
associated front is currently impacting Shemya with periods of
rain and winds sustained out of the southeast 25-30 kts and gusts
up to 35kts. This storm system is being supported by a 120-140 kt
jet stream aloft.

&&

.MODEL DISCUSSION (through Friday morning)...

Model guidance is in great agreement on the synoptic scale
features out through Wed night/early Thursday morning. For
Southcentral, besides for some low stratus and fog across the
Anchorage area and in parts of the Copper River Basin, weather
remains fairly benign. Temperatures and winds will remain the main
concern over the next couple of days as the upper level ridge
remains over the region. As a result, hi-res models and hand edits
were used for the winds through Day 3. A weak front is expected to
move its way towards Kodiak on Thursday with small crafts over the
waters but this front may bring a rain/snow mix to Kodiak
Thursday. The NAM shows a snow changing to rain situation whereas
the GFS shows the precipitation going back and forth. This will
continue to be fine tuned over the coming days. Attention will
then turn to the Gulf of Alaska to near Kodiak towards the end of
the period as a gale force front begins to move through the area.
This will continue to be fine tuned over the coming days as there
are some timing discrepancies as well as precipitation type issues
for Kodiak.

Bering Sea and Aleutians: A very active storm pattern is in place
across the region with a parade of lows expected over the course
of the next week. Models remain in good agreement on this next low
moving northward just west of the Bering towards east Russia. We
just expect its front to push through the Bering Sea and the Chain
with the center itself staying west. The front is expected to
slowly decay (frontolysis) as it approaches the Kuskokwim Delta
Coast, though this second front will have more energy with it than
the one approaching this afternoon. This is supported on all model
guidance.

Attention then turns to the next low that is expected to round the
base of the trough and move into the Bering near Adak. Models are
in fairly good agreement on the track of this low through 18Z
(9AM) Thursday morning but then quickly diverge from the low
going to the eastern Bering (NAM solution) or moving northward
towards the northwestern portion of the Bering Sea (Canadian-NH)
with the GFS and EC in the middle. Either way, this looks to be a
storm force low with gusty winds all along the central and
eastern parts of the Aleutian Chain.

&&

.AVIATION...
PANC...Areas of low stratus/fog have made its way down the Knik
Arm and into the terminal overnight and lowered visibility and
ceilings to LIFR conditions. The fog/low stratus is expected to go
back and forth with being over the terminal at times or just east
this morning. When it is just east, expected conditions to be
VFR. VFR conditions and light winds are otherwise expected through
the afternoon and to the end of the TAF package. There is the risk
for some vicinity fog once again this evening into the overnight
hours that may limit visibility and lower ceilings again.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Thursday night)...
The forecast for Southcentral Alaska remains largely on track for
the next couple days as cold and dry conditions dominate, with
gap winds along the coast. The one wrinkle introduced last
evening was the expansion of fog from Knik Arm down across the
Anchorage Bowl. Now that it is back in place, it's going to be
hard to get rid of, with a steep low level inversion in place (as
per the 12Z Anchorage sounding) and little change in the pattern.
Thus, expect some improvement by this afternoon, but fog will
likely expand again this evening.

A pattern shift will take place Wednesday night as a trough
crossing the Bering Sea pushes the remnants of the upper level
ridge into northern Alaska. This will allow a weakening occluded
front to reach the western Gulf and Kodiak Island late Wednesday
night through Thursday. The airmass in place ahead of the front
will be cold enough to support snow and there won't be much
warming aloft with arrival of the front. However, the limiting
factor with all of these early winter storms is warm sea surface
temperatures which keep the air near the surface above freezing.
Expect Kodiak will see some light snow accumulation before mixing
with or changing to rain during the day Thursday. The front will
dissipate as it reaches the southern Kenai Peninsula Thursday
night. Thus, precipitation will be light and mostly confined to
the southern to eastern Kenai Peninsula. Some clouds will spread
into the rest of Southcentral leading to a moderating trend in
temperatures.

&&

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

Two frontal systems are expected to bring increasingly active
weather to Southwest through Thursday. The high pressure system
that brought clear, cold weather over the past weekend will begin
to move off to the northeast; however, northeasterly surface flow
will keep the interior portions of Southwest cold and dry through
Wednesday. The first front will reach the Y-K Delta coastline
Tuesday afternoon bringing a chance for snow. As the front moves
inland, warm air aloft and onshore flow should transition snow to
rain along the coast with a chance for light freezing rain inland
as far east as Bethel. The front is very weak and has little
moisture, so any precipitation will be light with little
accumulation. Precipitation should also be confined to within 100
miles of the coastline before the moisture mixes out with the
residual dry air over the interior.

On Wednesday, a front associated with a strong system over the
western Bering will reach the AKPEN. Rain and southerly winds are
expected to sweep east over the AKPEN through Thursday morning. As
the front reaches the Southwest Mainland on Thursday,
precipitation should switch over to all snow due to residual cold
air over the mainland as well as cold air advancing behind the
front. Higher moisture content and stronger upper level forcing
should allow this front to move as far inland as Sleetmute, thus
bringing snow to the majority of Southwest on Thursday.

&&

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

An active pattern is expected across the region with two storm
force lows impacting the Bering. The comma shaped head of the
first strong low can be see on satellite this morning intensifying
off the eastern coast of the Kamchatka Peninsula with a front
extending south over Shemya and into the North Pacific. This
system is expected to occlude rapidly with the low center
remaining west of the AOR; however, cold air wrapping around to
the south of the low center is expected to bring storm force winds
and high seas to the western Bering. Along the front, strong
winds and rain are expected to sweep east across the Bering before
reaching the Alaska Mainland on Thursday. Behind the front, cold
air, wind and snow are expected.

As the strong cold advection moves across the Bering, a tight
thermal gradient will set up over the North Pacific. A slight
perturbation in the upper level flow combined with the left exit
region of a 300 mb jet streak will allow for rapid
intensification along the baroclinic zone. This is expected to
lead to the second storm force low of the week, which should form
south of Adak early Thursday morning. There are still a few minor
model differences in how fast the low center will track northeast
towards the Pribilof Islands on Thursday. The strongest winds are
expected to be south of the low center, likely impacting the
central and eastern Aleutian Chain.

&&

.MARINE (Days 3 through 5: Thursday through Saturday)...

Gulf of Alaska:

A low in the southeastern Gulf will produce gap winds along the
northern Gulf coast through early Thursday morning. Another low
arrives over the western Gulf Thursday afternoon bringing small
craft level winds with gale force winds likely through narrow,
channeled terrain including over Kamishak Gap and the Barren
Islands. By Friday morning, a storm force low approaches the
western Gulf, spreading in gales and storm force winds over Gulf
waters through Saturday.

Bering Sea:

There is increasing confidence that a storm force low in the North
Pacific will lift northward into the Bering Thursday. This storm
force low is expected to track northward into the northern Bering
Friday evening. A second gale force low forms on the heels of the
first and tracks northward over the Bristol Bay area. There is
lower confidence in the strength and track of this second system,
though storm force winds are possible. A third low arrives late
Saturday introducing potential for gale force winds, though
confidence is extremely low at this time given extensive model
discrepancies.

&&

.LONG TERM FORECAST (Days 4 through 7...Friday through Monday)...

The long term begins with a highly amplified pattern as a deep
upper level trough over the Bering remains in place. Meanwhile, an
upper level ridge builds over the mainland. As the upper level
trough ejects shortwaves eastward, this pattern will support a
series of surface lows developing near the Alaska Peninsula area
and tracking eastward into the Gulf. By the weekend, the ridge
retreats to the north allowing for the formation of a broad upper
level trough that will persist over the Mainland through Sunday
night. By Monday, the active pattern continues over the western
Bering as there is growing model consensus that a strengthening
low enters the western Bering Monday afternoon.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 411 178
Gale Warning 412 413 127 128 173 174 175 176 177 185.
FIRE WEATHER...NONE.

&&

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