Small Craft Advisory 930, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
535 AM AKST Tue Nov 22 2022

.ANALYSIS AND UPPER LEVELS...

There is a broad longwave trough from the Eastern Bering. At 500
mb there is a closed low near Nunivak and a weak trough over the
Gulf Of Alaska both features with multiple embedded shortwaves.
At the surface there is a weak low over the northern Gulf and
another surface low with a magnitude of 991 mb. On satellite
imagery (GOES Nighttime Microphysics RGB), there is a front moving
into western Alaska and the remnants of an occlusion over the Gulf
and Kenai Peninsula. There are some showers over Cook Inlet and
Turnagain Arm on the Kenai radar this morning. In the Central
Bering there is a jet core with an estimated magnitude of 110
knots. There is a dome of high pressure south of Amchitka this
morning.

&&

.MODEL DISCUSSION...

For initialization purposes, the radar was compared to the models.
The GFS and NAM missed the showers over Cook Inlet early this
morning and the ECMWF and Canadian models did ping into them.
Synoptically, the models are in sync with the major synoptic
features especially through 12z Wednesday. However, there are some
precipitation issues straight out of the gate. Southcentral is
going to be challenging the next 2 days. More significant model
divergence emerges Wednesday night and Thursday morning with the
next low track to enter the Gulf.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist through
tonight. There are some showers over Cook Inlet on the Kenai radar
this morning. Precipitation could move back over the terminal.
Amendments will be made as needed.

&&

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

The forecast challenges today are very similar to the ones from
yesterday. There is an inversion with above freezing temperatures
below about 3000 ft in the Anchorage area, Matanuska Valley and
Kenai Peninsula. In addition there is weak and unorganized lift
over the region which is causing some areas of precipitation.
A few areas of very light freezing rain are possible this morning
from the northern Kenai Peninsula through the Anchorage area. What
is a little different to day is that temperatures aloft are
expected to be cooling through the day so the dominant
precipitation types in those areas will go from freezing rain/
rain to a rain-snow mix during the day and then predominately snow
by this evening and beyond.

Rain is once again occurring around the Prince William Sound
region. A wave lifting through the area tonight will bring more
numerous rain and snow showers. Temperatures aloft will be cooling
from the west to east so more mountainous areas will see more
snow as the precipitation type and lower elevations around
Cordova will remain all rain.

The wave moving into the region this evening and overnight is in
conjunction with an upper level low that is moving from Bristol
Bay, over southern Kodiak and south of the Gulf of Alaska. This
track will make a big difference in where the extra lift with the
wave will occur. The slightly farther south track of the GFS would
cause less lifting especially over Anchorage, but the NAM track
would put the increased lift over Anchorage and the northern Kenai
Peninsula. The forecast leans more toward the NAM solution due to
other high resolution models indicating better chances for
precipitation tonight where the NAM is favoring them.

As the upper level low moves south of the Gulf on Wednesday, a
strong surface low will head toward the Gulf from the south. This
low will bring Gale force winds, and possibly Storm force winds to
the Gulf of Alaska late Wednesday through Thursday, but the low
will not proceed farther north as it runs into a ridge building
into Southcentral. Depending on the interaction of this low and
the ridge, there could be some periods of heavy snowfall over the
eastern half of the Copper River Basin Thursday into Thursday
night. This is still very much up in the air at this time.

&&

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

An upper level low is now beginning to make its way over the Y-K
Delta as an attendant surface wave progresses into the same
vicinity from the northwest. Meanwhile, a markedly cooler air mass
has advanced into all but far eastern parts of Bristol Bay behind
a cold front now reaching the Alaska Peninsula. A band of snow has
developed along an axis from King Salmon to Aniak, and this band
is anticipated to lift northeast into the Lower Kuskokwim Valley
through this afternoon and evening. Another batch of snow showers
should also push across the Y-K Delta today as the surface wave
and upper trough move past. The best chance for brief periods of
heavier snowfall through this evening will remain focused along
the Kuskokwim Delta Coast, where convective snow showers/bands
developing out over open water will likely move onshore at times.
Gusty northwest winds around 20 to 30 mph through tonight could
result in localized blowing snow as snow showers move across,
especially near and north of Kipnuk.

Looking out into Wednesday, areas of snow should clear out from
north to south as the aforementioned upper low moves south across
Bristol Bay and soon after into the western Gulf. Cool northerly
flow will continue behind this system as an shortwave ridge
approaches from the Bering, with partial clearing and diminishing
winds likely allowing temperatures to drop down into the single
digits in some of the most interior valleys late Wednesday night.
A weakening front is also expected to reach the Y-K Delta by this
point, sending in another round of snowfall beginning from west to
east. Light snow will spread into western parts of Bristol Bay on
Thursday as the front advances, eventually reaching the Lower
Kuskokwim Valley in the afternoon as snow tapers off across the
Y-K Delta. Forecast uncertainty increases markedly by Thursday
evening, with significant model spread in the track of a low
approaching the Bering and eastern Aleutians evident by this
point. Moderate mixed precipitation could move across much of
Bristol Bay starting late Thursday night, but this will be highly
dependent on how far north the low moves as it approaches from
the west.

&&

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

Cold northerly flow will dominate most of the northern and eastern
portions of the Bering for the next day or so, with snow showers
likely to continue across the eastern half of the Chain and
southern Alaska Peninsula through Wednesday. Gusty northwest gap
winds reaching gale force in places will continue along this same
stretch, with gusts into gale force range likely through the
typically favored gaps and passes until cold northerly flow
diminishes late in the day on Wednesday. Meanwhile, much warmer
air will stream north and west ahead of a strong front entering
the western Bering, resulting in strong southwest winds and rain
spreading across the western Aleutians on Wednesday and into the
Pribilofs by Wednesday evening.

Looking into Thursday, forecast uncertainty picks up notably. As
the frontal axis stalls out north of the Aleutians, a North
Pacific low is expected to approach the tail end of the front near
the central Aleutians. Models differ considerably on the track of
the low from this point, with some solutions taking the low
northeast into the eastern Bering as a potent, strengthening low
that would bring at least gale force winds across the eastern
Aleutians and southern Alaska Peninsula. However, it's possible
this low will take a much more southerly track, perhaps staying
entirely on the Pacific side of the Aleutians. Additional
forecast adjustments will likely be required for the Thursday and
Thursday night timeframe as confidence for this next system
improves.

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

A very progressive pattern looks likely through the long term
forecast period as a trough sets up in the Gulf of Alaska and
shifts eastward as longwave ridging builds back in for the
weekend. However, temperatures look to remain below average for
Southcentral Alaska with a shortwave tracking from the Bering Sea
into the Gulf late in the period. On Friday, gusty gale force gap
winds develop south of the Alaska Peninsula supported by weak cold
advection across the Aleutian Range as a digging shortwave
begins to move into the Gulf of Alaska. Snow showers will also be
possible with the shortwave on Friday, especially over Southwest
Alaska behind a decaying front pushing inland. As a North Pacific
low pressure system enters the eastern Gulf and draws a
stronger surge of cold air across the Aleutian Range, winds will
intensify Friday night into Saturday out of bays and passes along
the western Gulf coast. As the low in the Gulf exits east in
tandem with the deepening trough aloft on Saturday, cold, offshore
flow will promote drier conditions across southern mainland
Alaska.

Out west, uncertainties abound as run-to-run consistency is
limited with a North Pacific low tracking into the Bering Sea
toward the end of the week. Compared to yesterday, the front
associated with the Bering low is more meridionally (south-to-
north) oriented now, indicating the potential for a much stronger
push of warmer air and rain into the region. Its gale force front
pushes across the western/central Aleutians on Friday, reaching
the northern Bering and eastern Aleutians by Saturday. Early on
Sunday, the front reaches Southwest Alaska. With a strong
southerly push of warm air aloft, mixed precipitation, including
freezing rain, will be a distinct possibility with this system.
The low and its associated shortwave trough aloft then ride over
top of a strong ridge into the Gulf for Sunday night, according to
the current spread of tracks indicated by model guidance. This
suggests that Southcentral should stay cold enough that any
precipitation on Sunday would primarily remain snow. However, this
evolution of the pattern is certainly not set in stone, and a
further northerly shift to the track/stronger ridge would increase
the likelihood of warmer air reaching Southcentral, so stay tuned
for further refinements to the forecast.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Heavy Freezing Spray 185. Gale 411 413 414 150 155 165
170 172 173 174 175 176 177 178 180 181 185 130 138 351 352.

&&

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