Small Craft Advisory 923, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
505 AM AKST Sun Nov 20 2022

.ANALYSIS AND UPPER LEVELS...

There is a broad longwave trough from the Central Aleutians to the
Gulf Of Alaska. At the surface there is a low over the northern
Gulf and another surface low south of Unalaska. On satellite
imagery (GOES Airmass RGB), there are back to back occluded
fronts. South of the chain there is a jet core with an estimated
magnitude of 170 knots. The radars at Kenai and Middleton Island
have been active this early morning.

&&

.MODEL DISCUSSION...

Models are in decent agreement with the major synoptic features
especially through 12z Tuesday. However, there are some minor
differences with the shortwaves aloft. And NAM and GFS some
discrete differences with the rain/snow line over the southern
half of mainland Alaska as well as pockets of mixed p-type. The
big forecast challenge through Tuesday is precipitation types with
freezing rain potential for portions of the western Kenai
Peninsula and Anchorage bowl.

&&

.AVIATION...

PANC...VFR conditions and light winds will prevail through the
TAF package. There is potential for freezing rain at PANC and it
is actively being monitored. Currently, -FZRA is in the TAF from
22Z to 06Z, with the best chances during the afternoon. Any
accumulations of freezing rain are expected to be less than a
tenth of an inch.

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

The Gulf of Alaska low has pushed to the north Gulf coast early
this morning with the front bringing rain to the Prince William
Sound (PWS) area and radar reflectivity to the western Kenai
Peninsula. As of 5 AM, there has been no reposts of rain in the
western Kenai Peninsula as dewpoint depressions remain between 5
an 10 degrees so it appears the precipitation is evaporating
before it reaches the ground. With the freezing level around 4500
ft in Anchorage this morning (and this should be fairly
representative of the Cook Inlet and PWS areas) there should not
be much snow except at elevation. That leaves rain and or freezing
rain for the populated areas and roadways from Talkeetna
southward. For the Copper River Basin, the atmosphere aloft is
considerably colder so snow would be the precipitation type from
Chickaloon/Tahneta Pass eastward through the Copper River Basin.

So the big issue is the potential for freezing rain from the
western Kenai Peninsula northward into the MatSu Valley to about
Talkeetna, including Anchorage and Wasilla. Temperatures will be
above freezing in much of that area especially by this afternoon.
However, there will be plenty of areas that remain near or just
under freezing which will have some light freezing rain.
Therefore, the winter weather advisories for freezing rain remain
in effect for the areas from the western Kenai northward into the
Susitna Valley. This does not include the eastern Kenai Peninsula
where temperatures have all sufficiently warmed to keep
precipitation rain for both inland and coastal regions.

After today, the atmosphere will cool down aloft so for additional
precipitation chances after SUnday it should start to resemble a
more typical situation of snow below freezing and rain above
freezing without the freezing rain threat. Temperatures start to
more significantly cool aloft on Tuesday as a cold upper level
low approaches the area from the west.

&&

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

The remnants of an upper-level ridge over Central Alaska has left
a col over the Alaska Peninsula. Unsettled weather will commence
today across Southern Southwest Alaska as the lack of a strong
pressure gradient will do little to thwart the advancing
shortwaves originating from the approaching North Pacific low.
Colder temperatures now in place over much of Southwest will
provide ample opportunity for multiple precipitation types as the
weak precipitation has begun. Later this morning, a front
approaching the Greater Bristol Bay area will provide consistent
light, showery, precipitation to persist through the weekend.
Temperature profiles suggest an environment favorable for a mix of
freezing rain and snow during this time from Dillingham to
Koliganek. Any ice accumulations associated with this wave is
expected to minimal however with a transition in precipitation to
snow and plain rain by the evening. Precipitation that makes it
north of the Goodnews Bay is expected to all fall as snow. There
is a chance for light precipitation across Southwest Alaska
through tonight and the beginning of the workweek. Several lobes
of upper level energy will transit through the region bringing
areas temporarily higher likelihoods of precipitation. One such
wave for the Kuskokwim Delta region will move in Monday morning
and will likely provide light snowfall into Tuesday morning. In
general, precipitation amounts for the next few days are expected
to be relatively minimal.

As the precipitation plays out over Southwest, weak off-shore
flow will be replaced by colder northerly flow by Tuesday morning,
drawn down by the North Pacific lows movement south of the
Eastern Aleutians. Activity driven by this low is expected to
diminish by mid-week with colder air then present across Southwest
Alaska.

&&

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

A North Pacific low is spinning south of the Central Aleutians
and will track to the western tip of the Alaska Peninsula by
Monday morning. The low then continues east and will exit into the
western Gulf by Tuesday morning. This low will be spreading vast
swaths of light precipitation across the Aleutians during its
track eastward, as well as bringing cold arctic air south from
Russia and the Northern Bering. This cold northerly flow will
create cooler windy across the Aleutians, spreading eastward on
the backside of the low track. Cool northerly winds will be
reaching Adak tonight, the Pribilofs by Monday afternoon, and
Unalaska by Monday night. Precipitation across the Aleutians
associated with this low today and early tonight will be largely
plain rain, however, a transition to mixed precipitation and snow
will occur with the colder air moving in. Precipitation will
likely switch to wintry mix for the Pribilofs by early Monday
morning and Unalaska by late Monday afternoon. Not more than
dusting of snowfall is expected for places like Cold Bay and Dutch
Harbor.

A transient upper ridge will build in the wake of the
trough and precede the next system of interest. The warm front and
cold front extending from a Kamchatka low is advertised to enter
the western Bering on Tuesday morning and then Wednesday morning,
respectively. General southerly flow with this system will return
warmer temperatures to the Bering and Aleutians by the end of
Tuesday.

&&

.LONG TERM FORECAST (Days 4 through 7: Wednesday through Saturday)

There is good agreement on a pattern change in the long term
forecast period, with a shift to colder weather likely the latter
half of this week as broad longwave ridging pushes east of the
state. On Wednesday, a decaying low in the northern Gulf of Alaska
in the vicinity of Prince William Sound will support upslope
showers along the coastal mountains through the evening hours.
Meanwhile, a strong shortwave trough stretching across western
Alaska helps to advect a cold air mass over Southwest Alaska
eastward, leading to colder temperatures and supporting the
eastward progression of a cold front into Southcentral Alaska. As
a result, Southcentral Alaska can expect snow showers inland from
the coast on Wednesday as the front pushes east and weakens. The
cold air mass also streams into the North Pacific across the
Aleutian Range. In combination with a strong low in the eastern
Gulf of Alaska, this promotes gusty winds to gale force out of
bays and passes on the south side of the Alaska Peninsula and
within the western Gulf of Alaska. The development of offshore
flow in response to low pressure in the Gulf also shuts off
precipitation along the north Gulf coastal mountains Wednesday
night.

The next impactful weather system for southern Alaska also exits
far eastern Russia into the northern Bering Sea on Wednesday. The
large low pushes eastward with a potentially storm-force front
moving through the Aleutian Islands. Wednesday night, the front,
weaker further north, reaches the Southwest coast. While there is
some uncertainty, this looks to bring mostly snow to the region
with a sufficiently cold air mass already in place. Beyond this
point, model guidance diverges significantly. GFS and GEM agree
that the bulk of the upper-level energy associated with the low
dive south across the eastern Bering Sea with the front decaying
quickly as it moves inland. The EC presents an alternate
solution, whereby the low remains consolidated much further north
in the Bering Strait and the front pushes eastward, surviving to
bring another round of snow to Southcentral by Friday. The
question will be how far west along the Aleutian Chain the cold,
northerly flow on the back side of the low will begin to surge
across. The further west solutions of the GFS/GEM bring gale force
northwesterly winds across the central/eastern Aleutians on
Thursday, while the more northerly track of the EC spares the
islands and initiates winds out of gaps in the Alaska Peninsula on
Friday. This will also impact where widespread showers will result
if cold air advects over the relatively warm waters of the
central/eastern Bering Sea, which would also affect parts of the
Aleutian Chain and the Pribilof Islands.

By Friday, there is good agreement that low pressure will be
present in the Gulf of Alaska, which will help to draw in and
reinforce colder air over southern mainland Alaska while enhancing
precipitation chances along the north Gulf coast. Gap winds also
shift south of the Alaska Peninsula and into the western Gulf as a
result. At the same time, the next system begins to track into
the far western Bering and pushes its front northward, bringing
warmer conditions and rain to the western/central Bering Sea and
Aleutians through Saturday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 101 111 121 145.
MARINE...Storm 413. Gale 170 173 174 175 176 177 178 179 185 411
412 414.

&&

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CL
LONG TERM...CQ

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
151 AM AKST Sun Nov 20 2022

AKZ161-210100-
Bristol Bay-
Including the cities of King Salmon, Dillingham, Naknek,
and Pilot Point
151 AM AKST Sun Nov 20 2022

...FREEZING RAIN POSSIBLE TODAY THROUGH TONIGHT...

Dillingham and inland areas up to Koliganek and New Stuyahok may
see freezing rain/drizzle today into tonight. Any ice
accumulations that occur are expected to be minimal with a
transition to periods of plain rain and wintry mix.