Small Craft Advisory 885, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
458 AM AKST Wed Nov 18 2020

.ANALYSIS AND UPPER LEVELS...

An upper level ridge of high pressure remains dominant across much
of the state of Alaska yet again this morning. The axis of the
ridge is further east than yesterday now centered over the central
Interior. When looking at the water vapor imagery with the 500 mb
heights overlaid, the pattern continues to resemble the Greek
letter Omega as it takes on that shape. These usually are
stagnant patterns that remain in place for several days as much of
the Mainland has been observing the past couple days.

For Southcentral, a mostly clear sky is being observed with cold
temperatures at the surface. The Copper River Basin has seen
temperatures fall in the teens and low 20s below zero with -24
degrees so far observed in Gulkana. The Susitna Valley has had
temperatures also fall below zero in the single digits and low to
mid teens below zero. From the Matanuska Valley through the
Anchorage Bowl and to the Kenai Peninsula, temperatures have
fallen to the single digits and teens above zero. Some areas in
and around Anchorage have fallen below zero with Campbell Creek at
-4 degrees and here at the office falling to -2 degrees this
morning. Kodiak and Valdez were the warm spots with temperatures
in the mid to upper 30s. Valdez is warmer mainly due to the gusty
winds occurring there this morning. There are areas of low
stratus/fog being observed along the Knik Arm into the Matanuska
Valley including Wasilla to Palmer this morning. There is another
patch of low stratus/fog in east Turnagain Arm including Portage.
There was another area of low stratus/fog being observed on the
Kenai Peninsula including Kenai itself. There was one more area to
the east of Tyonek. The areas of fog continue to drift back and
forth. One other area of low stratus/fog was also observed near
McCarthy.

Southwest Alaska is observing the beginning of a pattern change with
a front pushing onshore but is falling apart as it does so
(frontolysis). The Middle Kuskokwim Valley and northern Bristol
Bay are observing a mostly clear sky this morning as the clouds
associated with the front are banging up against the ridge and not
having much progress moving eastward. As a result, temperatures
have been able to fall into the single digits and even below zero.
An area of low stratus/fog continues to be in place from King
Salmon over to Dillingham and into and including much of the YK-
Delta. Areas of flurries and mist or very light snow is being
observed across Bethel and even King Salmon.

A sub 960 mb storm force low continues moving rapidly to the
northeast towards east Russia this morning. ASCAT Scatterometer
data show mostly gales all around the low center and into the
Bering Sea. There was an area of storms on the northwest side of
the low as well. Shemya was observing sustained winds of 30-35
kts with gusts up around 40-45 kts this morning. The associated
front with this system was moving through the Pribilofs bringing
small crafts with gale force gusts (~30-40kts) this morning.

&&

.MODEL DISCUSSION...

Model guidance remains in good agreement for the next two days on
the synoptic scale features before diverging beyond 48 hours into
Day 3. Main issues going forward for the southern Alaska Mainland
will be the outflow gap winds as the same pattern remains.
Therefore, hi-res models and hand edits were used. Low stratus/fog
remains in place across areas of Southcentral this morning leading
to the continued hardship of figuring out where this fog will go
and when it will be in a particular area as models are nearly
useless regarding this. This will more be based off the current
obs and atmospheric profile data. Kodiak also continues to be an
issue as models seem to be showing more snow for them than rain.
NAM and GFS are in agreement on the snow amount and timing of the
precipitation but are having a tough handle on determining if it
will be snow ending as some rain or a rain/snow mix.

Bering Sea and Aleutians: All models continue the great agreement
on the sub 960 mb low currently west of the Bering continuing
northward towards east Russia. All models also agree on the front
being the only part of the storm moving across the Bering and into
Southwest Alaska for Wed night into Thursday. Attention then turns
to the next low that all models agree on the track, placement, and
intensity up until the point it reaches near the Pribilofs,
though, there are some minor differences on where the center will
exactly be. Beyond the 48 hours (Thu night), models continue to
diverge significantly with the NAM holding the low nearly
stationary south of the Pribilofs. GFS moves it a little to the
north of the Pribilofs with the EC and Canadian-NH being the
furthest north and west and fastest with the system. Models are
then showing another triple point developing southeast of Sand
Point which then moves northward into Bristol Bay. EC and GFS are
close in terms of the track and the strength but the Canadian-NH
which as of now appears to be the outlier is stronger and slowest
of them all.

&&

.AVIATION...
PANC...Yet again this morning there is low stratus/fog along the
Knik Arm. This will continue the forecast hardship for the
terminal as to whether this fog/low stratus makes it into the
terminal or not. When it does expect briefly reduced visibility
and/or a low ceiling to LIFR conditions. Otherwise, VFR
conditions and light winds are expected the duration of the TAF
package. There is the threat yet again also for vicinity fog again
this evening into overnight hours, though, the risks seems lower
for tonight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Friday night)...
Strong winds in the Valdez and Copper River Delta areas this
morning will diminish through the day as the low near Southeast
Alaska weakens and pressure gradients slacken. Patchy fog will
linger in northern Cook Inlet/Anchorage/Matanuska Valley
underneath a strong temperature inversion (as seen in the 12Z
Anchorage sounding). The low level flow will shift around tonight
and begin to strengthen Thursday morning, so expect we're nearing
the end of this foggy pattern. Other than the fog, clear and cold
conditions will hang in for another day.

The ridge that has been dominating the weather over Southcentral
for the past several days will then weaken and give way to systems
moving in from the Bering Sea. A weakening trough and front will
track to Kodiak Island Thursday, then completely dissipate as it
lifts northward across the Gulf. The airmass will initially be
cold enough for snow on Kodiak Island Thursday morning, though
expect a changeover to mostly rain by afternoon, with just some
very light snow accumulation prior to the change. Snow showers
will affect the Gulf coast and Prince William Sound areas out
ahead of the front, though precipitation generally looks quite
light.

A much stronger frontal system associated with a storm force low
in the Bering Sea will arrive in the southwestern Gulf on Friday.
Conditions are favorable for a triple point low to develop south
of Kodiak Island, though confidence in the track and depth of this
new storm is low. With a strong upper level jet and arctic air
feeding in from the west, will likely see another triple point
low quickly form as the front lifts northward across the Gulf.
Despite all of the uncertainty in the details of these features,
confidence is fairly high that the Gulf waters will see high end
gale to storm force winds Friday through Friday night. Expect the
bulk of precipitation to affect Kodiak Island and the Gulf coast,
while inland areas remain dry. Increased cloud cover across the
region will lead to a gradual warming of temperatures as we head
through the remainder of the week.

&&

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

Remnants of a weak frontal system that stalled over the Southwest
coastline yesterday are bringing low stratus and fog this morning
from King Salmon to Hooper Bay. Over the interior, high pressure
will continue the pattern of clear skies and cold temperatures
for one more day. The next frontal system is expected to reach the
AKPEN Wednesday afternoon, pushing east to the Mainland early
Thursday morning. Mostly rain is expected for the AKPEN while snow
is likely over the Mainland. The front should reach as far east
as Sleetmute by Thursday afternoon before losing its moisture and
upper level support. At this time yet another occluded front will
reach the AKPEN. Like the front on Thursday, rain is expected for
the AKPEN while snow is more likely along the coastal areas of the
Southwest Mainland. The main difference with this front will be
much stronger winds.

&&

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

An active pattern will continue for the Bering through the
forecast period. On Wednesday, an occluded front will continue to
move slowly east, reaching the Alaska Mainland early Thursday
morning. Behind the front, moderate cold air advection is
expected. This cold advection will set up a strong temperature
gradient over the North Pacific, which will lead to rapid
cyclogenesis near Adak Wednesday night. This storm is expected to
quickly reach storm force early Thursday morning. The expected
track is to the north-northeast, centering itself over the
Pribilof Islands Friday morning. Although the storm itself rapidly
occludes, the tight circulation and cold air advection behind the
low should lead to storm force winds for the central and eastern
Bering/Aleutian Chain through Friday morning. To the south of the
low center, cold air could lead to hurricane force gusts to the
north of the Island Chain. By midday Friday the stronger winds
should subside, but widespread gales, rain and snow are expected
over the central and eastern Bering.

&&

.MARINE (Days 3 through 5: Friday through Sunday)...

Gulf of Alaska:

A storm force low forming south of the Alaska Peninsula will bring
gale and storm force winds to the Gulf waters through Saturday.
Winds will weaken to small craft levels Saturday night while weak
low pressure remains over the Gulf. There is increasing
confidence a gale force low enters the western Gulf Sunday night.

Bering Sea:

A storm force low over the Bering Friday lifts into the northern
Bering Friday night. There is potential for a second storm force
low to form south of the Alaska Peninsula Friday morning and lift
northward into the Bristol Bay region for Saturday, however there
is still uncertainty with the track and strength of this low.
Broad low pressure will remain over the Bering through the weekend
with gale force winds likely across the Bering waters through
Sunday.

&&

.LONG TERM FORECAST (Days 4 through 7...Saturday through
Tuesday)...

Two key features in the long term include a broad upper level
trough over the Bering extending along an axis from Kamchatka to
the Alaska Peninsula and an upper level ridge over the northern
AK-Yukon border. A series of lows will develop as shortwaves
embedded within the upper level trough eject northeastward toward
Southcentral. This will make for an active weather pattern for
much of the southern half of the state for the start of the
weekend. In the midlevels, a sharp baroclinic zone is positioned
south of the Aleutian Chain as a cold airmass with northerly flow
persists over the Bering waters through Saturday night. While the
initial pattern looks to be zonal, over the course of the
weekend, a more amplified pattern is likely to set up by early
next week as a ridge moves in from the west. This ridge will shift
the broad upper level trough eastward over the Mainland. With a
ridge over the Bering and a trough over the Mainland, the
Southwestern part of the state can expect strong north to
northwesterly outflow winds, especially over the Alaska Peninsula.
By late Monday into Tuesday, models are keying in on a strong
Kamchatka low arriving over the western Bering. This low will
ultimately force the ridge eastward as its front traverses the
Bering early next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 414 155 165 170 172 173 174 176 179
Gale Warning 411 413 171 175 178 127 128.
FIRE WEATHER...NONE.

&&

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