Small Craft Advisory 802, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
505 PM AKDT Thu Oct 22 2020

.ANALYSIS AND UPPER LEVELS...
The large low in the Central and Western Aleutians has a double
center that can be seen particularly well on visible satellite.
These two centers should merge sometime tonight or tomorrow
morning. The front associated with the low in the Bering Sea and
stretching over the Alaska Peninsula is meeting up with the jet
stream just south of the Alaska Peninsula today. This set-up often
causes a triple-point low to develop, but that does not look like
it will happen possibly due to the surface low not quite aligning
with the upper-left exit region of the jet stream. The ridge over
mainland Alaska has held firm overall, but it is being eroded
away slowly by this approaching front allowing rain to finally
reach Kodiak.

&&

.MODEL DISCUSSION...
Models remain in good synoptic agreement into early next week.
They are also in better agreement today with slowing the speed
the front is progressing into Southwest and Southcentral Alaska
which should have an impact on precipitation type in these areas.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

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

Rain and easterly gales will continue for the western Bering
through late Friday as a front lifts slowly from the western
Bering to the Barren Islands. As it lifts north and east, the
front will stall and start to weaken, due in large part to an
expansive 1042 mb high parked over the Yukon. Rain and snow will
push north to the southern Kenai Peninsula by Friday morning and
to Prince William Sound and portions of interior Southcentral
through Saturday. A strengthening southeasterly cross-barrier
flow along the Chugach and Kenai Mountains will limit
precipitation for the western Kenai Peninsula and Anchorage Bowl.
However, some overrunning of moisture out ahead of the front may
produce flurries or a brief period for very light snow from Kenai
to Palmer late Friday night into Saturday morning. Any
precipitation that does make it to these locations will also have
to deal with a dome of cold and dry air near the surface. Again,
this will act to limit snowfall, as any precipitation falling into
a drier layer of air will evaporate before reaching the ground.
So, it will take some time to saturate the drier column of air
and get any light snow to reach the ground.

Gap winds will also increase Friday as the front draws closer to
the Southcentral coast. These winds will bend down toward Cook
Inlet given the pressure gradient, but the northerly winds from
Talkeetna to the Matanuska Valley and Anchorage will help keep
the warmer air attempting to move in from the Gulf at bay.

Rain or a mix of rain and snow will linger across the
Southcentral coast with snow lingering over the northern and
western portions of the Susitna Valley through Saturday. The next
system then tracks into the western Gulf late Sunday into Monday
with a front pushing to the coast. This low looks to track into
Prince William Sound and push its front onshore. This would bring
an initial round of snow to much of the region late Sunday that
should quickly change to rain for most locations with the
exception of the mountains and the far interior Southcentral
Mainland.

&&

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

Strong winds associated with the frontal boundary will continue to
spread across the Kuskokwim Delta and Bristol Bay tonight into
Friday night. A series of continuous rain will impact mostly the
coastal area and portions of Southwest Alaska the next couple
days, with snow falling over much of the region. The wet pattern
will continue as the second front brings more rain through the end
of the week.

&&

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

A broad occluded front wrapped into a complex low south of the
western Aleutians will continue to bring storm force winds over
Bristol Bay tonight. Gales will persist over much of the Bering
Sea and most of the Aleutians tonight through Friday night.
Hazardous seas and widespread rain showers will continue
especially along the path of the storm force low and its front. A
good portion of snow will fall in the area around Nunivak Island
between tonight and Friday, or near the most northeastern cold
sector of the front. With the cold air advection moving across the
Bering, only rainfall is expected over much of the region through
the weekend.

&&

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

Gulf of Alaska:

There is medium confidence that a slow moving front will lead to a
barrier jet across the northeastern Gulf coast on Sunday. Differences
in models exist over the strength of the winds associated with
this feature and its extent. For now, a NAM/EC solution is
preferred with gale force winds from Prince William Sound east.
After this, forecast confidence decreases with an area of low
pressure from the North Pacific late Sunday afternoon. At this
time, the majority of model guidance indicates broad southwesterly
flow from the southern Gulf into southeast Alaska on Monday.
However, small craft to gale force winds are a possibility,
including continuation of the barrier jet from the day before.
These intricacies should be better determined with future model
runs. This low looks to move inland across southern Alaska and
dissipate Monday into Tuesday.

Bering Sea:

There is high confidence that the low currently over the eastern
Bering will continue to be stationary and weaken through Sunday.
As it weakens from a low pressure center to a trough, it will be
absorbed by an area of low pressure south of the eastern Aleutians
Sunday night into Monday morning. Winds should remain at small
craft criteria or below during this time. Additionally, a complex
low pressure system off the Kamchatka Peninsula will bring a
series of fronts over the western Aleutian waters, bringing small
craft winds to the area. Expect the first front Sunday,
dissipating Monday, and the second front Monday moving into the
central Aleutians through Wednesday.

&&

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

Models have started coming into better agreement in regards to the
ridging and longwave patterns on Sunday/Monday. A high amplitude
trough and associated weakening low slowly push east towards the
Southwest shore on Sunday, while a ridge builds over the North
Pacific. Warmer air flows in behind the ridge, increasing
temperatures slightly for Southern Alaska. The GFS was the
previously the biggest outlier with how far inland this warmer air
goes, however, the ECMWF has started showing more agreement with
the GFS, bringing warmer air into the region. The trough pushes
east, flattening the ridge, before being forced south by an
incoming strong wave out of the north. Model solutions are
agreeing well that this arctic wave pushes southeast across the
state Tuesday into Wednesday, which will bring colder air towards
Southcentral along with it. Any precipitation associated with this
wave will likely be snow.

A ridge sets up over Southwest/eastern Bering behind the longwave
trough. This pattern may be short-lived as a low pushes out of
the south towards the Central Aleutians late Wednesday night. The
path is relatively consistent across most models, but strength
and center location vary greatly.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...STORM WARNING 160.
GALE WARNING 130-132,136-138,351,352,150,165,170,172,174,177,178,
180,181,185,411,412,414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
MARINE/LONG TERM...CK/KM