Small Craft Advisory 887, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
506 AM AKST Thu Nov 19 2020

.ANALYSIS AND UPPER LEVELS...

The pattern continues to resemble the Greek letter Omega which is
a blocking pattern that is in place across Alaska. One upper level
low was present over the southeastern Gulf near Southeast Alaska.
The other much stronger upper level low was present just to the
south of east Russia. The center of the upper level ridge was
present over the Interior of Alaska just north of Fairbanks with
the axis extending south through Southcentral Alaska.

Underneath the upper level ridge, much of Southcentral Alaska is
observing a mostly clear sky with the exception of a few places
where some pockets of low stratus/fog are still present. These
areas of low stratus/fog were present near and over Gulkana this
morning. The other pockets were along the Knik Arm to the
northern Kenai Peninsula. The aforementioned pockets of fog were
bringing lower visibilities and low ceilings to the airport in
Anchorage, along the Glenn Highway, and north to Palmer and
Wasilla. Other than that, where the sky is clear, temperatures
have dipped to the teens and 20s below zero in the Copper River
Basin with -24 degrees so far in Gulkana. The Susitna Valley was
observing teens to single digits below zero. For the Matanuska
Valley, Anchorage Bowl, and the Kenai Peninsula, temperatures have
been in the single digits and teens. Campbell Creek in Anchorage
was one area reading below zero with -6 degrees being observed.
Kodiak was also clear with temperatures in the low 20s.

A mostly clear sky was being observed across the Interior of
Southwest Alaska as the area remains under the influence of the
upper level ridge. Temperatures in the Middle Kuskokwim Valley
have fallen into the single numbers and teens below zero with
northern Bristol Bay in the single digits and teens above zero.
For the lower Bristol Bay area including King Salmon and
Dillingham west to the YK-Delta are observing a stratus deck due
to a front moving onshore. Toksook Bay was observing light snow
and 1 1/2 mile visibility overnight. Temperatures along the coast
were generally in the 20s to low 30s with teens to low 20s further
inland.

The big weather story is the storm force low currently moving
over/near Adak. This was as seen on satellite and was moving
swiftly off to the northeast towards the Pribilofs. ASCAT
Scatterometer data passed over the storm earlier in the night and
was showing a wide area of gale force winds with a small area of
storms. These winds were also found to be mainly on the southern
and southeast side of the system. Adak was reporting light snow
and 1 1/4 mile visibility as the low was passing just to their
east this morning.

&&

.MODEL DISCUSSION...

Model guidance is generally in really good agreement through the
first 36 hours of the forecast (through Fri morning). It is beyond
this time that the details of the forecast greatly diverge. For
Southcentral, the Gulf waters and for Kodiak, there is generally
good agreement among the models on a strong storm force front
moving through the Gulf. There are, however, still differences on
the timing of this front, when/if a new low will develop south and
east of Kodiak Island and if so how strong will this low be.
Therefore, moderate to high confidence in the forecast for the
first 36 hours but then low confidence on what occurs beyond 36
hours and where things track/develop.

Bering Sea/Aleutians: Similar situation is unfolding across the
Bering Sea and Aleutian Chain in terms of the confidence in the
forecast. While confidence is high the first 30-36 hours of the
forecast for the storm force low currently moving into the Bering
Sea, where it goes once it reaches the Pribilofs is still
uncertain. This low may go anywhere from well west of Saint
Matthew Island by Fri night which the NAM and GFS solution show or
as far east as Nunivak Island which the EC shows. Therefore,
confidence is average to high for the first 30-36 hours with low
confidence beyond that. Continued model runs will occur throughout
the day and we will be continuing to fine tune the forecast as the
models try to grasp a solution.

&&

.AVIATION...

PANC...Once again fog remains in and around the terminal this
morning. This will continue to bring ceilings and visibility to
LIFR conditions when it is over the terminal. Once again though,
the fog will continue to come in and then dissipate or go back out
over the Knik Arm keeping the challenging forecast in place for
the terminal. When the fog is out over Knik Arm, expect mostly VFR
conditions. Light winds will persist the duration of the TAF
package. Winds have been out of the southeast all night but we
expect a light north wind later this morning through the remainder
of the package.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Saturday night)...
After several dry and frigid days, a pattern change is on the way.
An upper trough and surface cold front will swing into the western
Gulf today, then dissipate tonight as the bulk of energy in the
trough drives eastward toward British Columbia causing the front
to elongate. Kodiak Island will see snow early this morning
transitioning to rain as we head through the day. Showers, aided
by weak upslope flow, will spread across the Prince William Sound
region, falling mainly in the form of snow. Expect one more dry
and chilly day for the remainder of Southcentral, with some
lingering fog in the Anchorage area.

Attention will then shift to a digging trough over the North
Pacific and an approaching frontal system extending from a storm
force low in the Bering Sea. The amplification of the upper flow
combined with arctic air with the trough is favorable for
cyclogenesis at the triple point of the frontal system. It's not
clear exactly where this low will form and how deep it will get.
However, we can say with some confidence that the low will form
south of Kodiak and track northwestward along the front, all while
the front approaches the western Gulf. Therefore, expect storm
force winds to develop ahead of the front and low Friday morning
and spread northward across the western Gulf waters Friday through
Friday night. With high pressure sitting over mainland Alaska,
this will tighten up the north-south pressure gradient leading to
strengthening northeasterly winds over the Matanuska Valley as
well as in Cook Inlet.

There is then potential for yet another triple point low to form
and deepen over the Gulf Friday night through Saturday. The extent
of low development will affect how far north the initial front
tracks and how far onshore precipitation makes it. At the moment,
it looks like the vast majority of precipitation with this storm
system will remain along the coast, with interior Southcentral
remaining dry. With low level easterly flow ahead of the front,
would expect the heaviest precipitation along western Prince
William Sound. Warmer low level air will accompany the front,
so expect a transition from snow to rain for areas at or near
sea level Friday night through Saturday. The upper trough looks
like it will lift northward Saturday night, which would provide
increasing chances of snow inland. Because of all the uncertainty
in the details of the low positions and depth, forecast confidence
in the details is low, so stay tuned as we track this very dynamic
weather pattern.

&&

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

High pressure over the interior is breaking down this morning as
an occluded front makes its way slowly across Southwest. Snow is
expected over the mainland along the front with rain likely this
morning over the Alaska Peninsula. A much stronger front is
rapidly approaching the southern tip of the AKPEN this morning and
is expected to reach Cold Bay around midday. Strong winds and
rain are expected along the front as it moves east through Friday
night. Behind the front, cold air is expected to make its way in,
which could provide a chance for snow or mixed precipitation to
the AKPEN by early Saturday morning. Strong winds are also
expected along the Southwest coastline but colder temperatures
should keep precipitation as snow.

&&

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

A storm force low is expected to make its way from Adak to the
Pribilofs over the next 24 hours. Although the storm has already
occluded, cold air to the south of the storm center should keep
winds at storm force through Friday morning. Gale force winds and
rain are expected along the occluded front, which should be near
the Southwest coastline Thursday night. By Friday afternoon storm
force winds are expected between Sand Point and Kodiak Island as
the occluded front moves through.

Over the western Bering, cold air advection will provide a chance
for snow showers through Saturday morning with Shemya reporting
snow this morning. Otherwise, calmer conditions are expected over
the Bering by Friday night as the low moves north towards Saint
Matthew Island.

&&

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

Gulf of Alaska:

There is increasing confidence in a strong low moving into the
Gulf Saturday with the potential for storm force winds, however
there continues to be extremely low confidence in the track of
this low. By Saturday night, conditions look to improve with sub
small craft wind levels expected. By Sunday, there is some
potential for another low to arrive over the Gulf from the south,
however a wide variety in model solutions has kept confidence very
low regarding track and strength of this low over the Gulf
through Monday.

Bering Sea:

There is potential for a strong low to impact the eastern Bering
Saturday, however there is extremely low confidence as model
solutions are vastly different. One solution places a storm force
low over the eastern Bering while another depicts the low centered
south of the Alaska Peninsula. The only insight at this time for
the weekend is that it will be an active pattern over the Bering
through Monday with the potential for storm force winds. By
Monday, another low enters the western Bering introducing
southerly gale force winds over the western Aleutians.

&&

.LONG TERM FORECAST (Days 4 through 7...Sunday through
Wednesday)...

The long term continues to show plenty of uncertainty, however
model solutions have still keyed in on several features. A broad
upper level trough looks to maintain its hold over the eastern
Bering/western mainland on Sunday while upstream, a ridge builds
over the Kamchatka Peninsula/western Bering. An upper level jet in
place just south of the Aleutian Chain will shift eastward late
Sunday, allowing for a more amplified pattern to develop Monday. A
strong Kamchatka low approaches the Bering Monday, forcing the
ridge to amplify and the broad upper level low to shift eastward
over the mainland, resulting in several upper level shortwaves
that will likely keep an active pattern across Southcentral. As
this pattern becomes more amplified, the increased pressure
gradients will likely bring a gusty northwesterly outflow wind to
the Alaska Peninsula Monday, especially given the signatures of
ample cold air advection at this time. By Tuesday, the ridge
continues eastward while the Kamchatka low deepens and its
associated front pushes eastward across the Bering through
midweek.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 352 413 414 130 131 132 150
155 165 170 172 173 174 175
Gale Warning 351 412 119 120 136 137
138 139 160 171 176 179 180 181 185.
FIRE WEATHER...NONE.

&&

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