Small Craft Advisory 868, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
556 PM AKDT Tue Nov 1 2022

.ANALYSIS AND UPPER LEVELS...
A complex pattern is taking shape aloft with a number of
features coming together. A large mature upper level low pressure
is hanging out in the northwestern Bering Sea. Strong high
pressure sits over the central Pacific Ocean. Between the two
features, a very energetic jet streak is interacting with a
shortwave trough rounding the base of the Bering trough. A strong
surface low is developing in response, moving onshore near the
Kuskokwim Delta. Snowfall is overspreading the southwest mainland
as winds are also increasing due to the approaching system. Winter
Weather Advisories are posted for snow/blowing snow in the Bristol
Bay area, and blowing snow in the Kuskokwim Delta. The remnants of
our previous system are still spinning in the northern Gulf of
Alaska, continuing snow showers for the Valdez and Cordova area.

&&

.MODEL DISCUSSION...
Even at this late hour, differences in the models are still
apparent with both the main developing parent low and the triple-
point low forming to the east. The good news is strength and track
of the main surface low is relatively well agreed upon, in the
upper 960s coming ashore near the Kuskokwim Delta and the impacts
will be similar with snowfall/blowing snow no matter which model
you pick.

The developing surface low at the triple point is a different
story. In general, guidance develops the low through the Shelikof
Strait/Barren Islands area and moves it into Prince William Sound.
A couple of the higher resolution models (NAM/RDPS) try to bring
at least part of the surface circulation into Cook Inlet which has
repercussions in winds/precipitation type in the Anchorage area.
Currently we are favoring the GFS solution even though it is
slower than the rest, moreso for keeping the surface low east of
the Kenai Mountains. However, if the more western track pans out
there is potential for high winds along the hillside of Anchorage
and Turnagain Arm.

&&

.AVIATION...
PANC...Lot of uncertainty in the forecast for the upcoming system.
Right now there are two scenarios regarding low track and impacts.
The first is a more eastward track of the surface low into Prince
William Sound which would make for generally light northerly winds
at the terminal with IFR snowfall Wednesday morning. The other
scenario is a low track up Cook Inlet which has the potential to
bring low level wind shear. Currently the forecast favors the
first scenario, stay tuned this evening for any possible changes.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 2: Today
through Wednesday night)...

A complex setup is taking shape for the next 24 hours or so
across the outlook area, with stubborn uncertainty for small scale
details continuing to undermine overall forecast confidence for
the incoming storm system. The outer fringes of the approaching
frontal system are already making headway into the southwestern
Gulf, with radar returns (still mostly virga) already picking up
on the Kenai radar at the southern end of the Cook Inlet.
Meanwhile, ongoing snow showers should continue well into this
evening and tonight near Valdez and Cordova as a mesoscale low
east of Middleton Island lifts into the coast and shears apart
ahead of the approaching front out to the west.

The exact details for the remainder of the forecast are hinging on
the fate of a triple point low expected to form near Kodiak Island
and work northeast towards the Kenai Peninsula though tomorrow
morning. Model guidance has diverged with the behavior of this
surface low, with namely higher resolution models attempting to
essentially split the feature around the Kenai Mountains as it
lifts northeast, forming a secondary center near the western Kenai
Peninsula as the main low approaches the Prince William Sound
(PWS). Most global model solutions, namely the GFS and ECMWF,
have persisted in keeping a single low track skirting south of
the Kenai Peninsula, with all of the low's energy driving into the
PWS. Given the mainly westerly orientation and location of upper
level flow and the better run-to-run consistency for the latter
scenario, the current forecast leans in favor of this outcome.
This uncertainty mainly impacts the forecast for winds across
Anchorage and along the Turnagain Arm, with much higher winds than
currently forecast possible on Wednesday morning if the secondary
low formation moving along Cook Inlet comes to fruition.
Regardless of this central forecast challenge, widespread snowfall
is still expected across most of the outlook area tonight through
tomorrow afternoon, with the heaviest amounts concentrated around
the PWS vicinity and eastern Kenai Peninsula, as well as across
the western half of the Susitna Valley, where upslope enhancement
will boost totals with the strong southeasterly low level flow
along and ahead of the front. Winter Weather Advisories are now in
effect for both the Susitna Valley for moderate snowfall, and for
Thompson Pass due to a combination of falling and blowing snow
expected.

Looking into Wednesday night and beyond, most of the snow should
taper off as the main surface low and associated upper trough
shift towards the northeastern Gulf. The big story for Thursday
and Friday will be the return of Arctic air and associated gap
winds blasting in from the northwest. Multiple upper level waves
will drive south from the interior towards the Gulf in concert
with strong cold advection beginning Thursday, yielding impressive
support for virtually all of the typical gap winds along the
northern Gulf Coast. The most intense winds are expected out of
gaps and passes near Shelikof Strait, Kamishak Bay and across
Kodiak Island, where northwest gusts up to Hurricane force will be
possible between Thursday afternoon and Friday morning.

&&

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

A strong, and still strengthening, low pressure system situated
in the eastern Bering will track inland over the Ahklun Mountains
tonight as it pushes its front across Southwest Alaska. With
strong dynamic forcing and an ample moisture tap, the passage of
the front will produce a brief round of heavy snowfall for areas
in Bristol Bay and the Kuskokwim Delta this evening through early
Wednesday morning. Near the coast, these conditions have already
begun, with snow combining with strong, gusty east to
northeasterly winds in many areas to reducing visibilities to as
low as one half mile. With the current Winter Weather Advisories
in effect, these conditions are expected to continue and spread
inland tonight. The front will push through quickly, with the
back end moving ashore late tonight for Bristol Bay. A Coastal Flood
Advisory is also in effect beginning this evening from Jacksmith
Bay to Togiak along the Bristol Bay coast. Minor coastal erosion
and flooding are possible as a result of strong westerly to
southwesterly winds moving in on the back side of the low.

Behind the front, winds will shift out of the south, pushing a
warmer airmass inland and leading to the potential for rain
showers for some time along the coast overnight. Locations further
inland are anticipated to stay all snow even after the completed
frontal passage turns precipitation showery. Colder air will
remain entrenched over the Kuskokwim Delta, keeping precipitation,
snow, even along the coast. This cold air will wrap around
the low as it moves over the Ahklun Mountains later on Wednesday,
allowing snow showers to return for Bristol Bay. Precipitation
will gradually clear through the day on Wednesday, though some
areas of stronger winds and heavier snow showers will continue to
contribute to low visibilities, particularly in the Kuskokwim
Delta and near the Western Capes.

As the low exits into Southcentral Alaska, cold, dry
northwesterly flow takes over across the region. Temperatures drop
back into the 20s for Thursday in this regime and most of
Southwest Alaska can expect to see drier conditions through the
end of the week. By Friday, a weak surface low from the northern
Bering may skirt the region and lead to increased snow chances
near the coast, but there is presently low confidence with the
timing and placement of this feature.

&&

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

A gale force low in the eastern Bering pushes into Southwest
Alaska tonight, setting the stage for persistent northwesterly
flow across the Bering Sea through the end of the week. A
decaying low pressure system in the northern Bering Sea will also
rotate southward, bringing rain and snow showers across the
central Bering and Pribilofs tonight into Wednesday. A surge of
colder air begins to move southward across the Bering Sea tonight
as well, bringing gusty northwesterly winds and snow showers as
far south as the eastern Aleutians for Thursday. Winds increase to
gale force out of gaps on the south side of the Alaska Peninsula
and persist into Friday. Conditions will be calmer further west
as a large surface high pressure moves into the central Aleutians
by Thursday and lingers through Friday.

&&

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

At the onset of the forecast period, the longwave pattern features
a positive tilt trough axis over the Gulf Of Alaska through to
the Northwest Territory, a high amplitude ridge axis over the
Central/Eastern Aleutians, and another trough axis over eastern
Russia. It is a highly amplified pattern. At the surface on
Saturday, cyclonic flow continues over the Gulf Of Alaska, the
ridge axis has shifted to the eastern Bering and a frontal
boundary is now east of Kamchatka. By Sunday there is a tight
thermal gradient along the eastern Gulf of Alaska and wide area
of high pressure at the surface which spans from Nunivak to the
interior of Alaska.Also Sunday the models bring a surface low
towards Adak but the tracks are divergent. On Monday the global
models begins to struggle with the North Pacific and the southern
Bering. Cold temperatures will dominate the forecast period with
the most brutal temps expected in the Copper River Basin, which
could see temperatures into the negative 20s to negative 30s,
hinging on the presence of fog or low stratus.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory: 131 (Thompson Pass), 145, 155,
                                  161.
         Coastal Flood Advisory: 161.
MARINE...Gale Warning: 119, 120, 125, 130, 131, 132, 136, 137,
                       138, 141, 150, 160, 170, 179, 180, 181,
                       185, 351, 352, 412, 413, 414.
FIRE WEATHER...NONE.

&&

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