Storm Warning 136, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
505 PM AKDT Wed Nov 2 2022

.ANALYSIS AND UPPER LEVELS...

A large vertically stacked low is moving slowly east-southeast
across interior Southwest Alaska this afternoon. This feature
continues to bring areas of snow across the Kuskokwim Delta and
snow showers along Bristol Bay and the AKPen as it begins to fill
and weaken. Northerly winds along the coast remain gusty this
afternoon, but are also on a downward trend as the low moves
farther inland.

Downstream of the main low, a triple-point low is spinning up
south of the Kenai Peninsula. This feature is rapidly deepening
due to upper-level support from a potent shortwave trough
ejecting eastward from the main low, as well as a stout 160+ kt
jet streak situated from south of the AKPen to the northern Gulf.

This new surface low has spun up along an occluded front and is
beginning to move toward Prince William Sound. Strong
southeasterly flow and a deep fetch of moisture is producing a
widespread area of precipitation from the northern Gulf and
Southcentral coast to Cook Inlet and the Susitna Valley. A band of
heavy snow set up right along the western Kenai Peninsula coast
overnight, with locations from Ninilchik to Nikiski receiving
anywhere from 6 to as much as 20 inches of snow. This band is now
finally moving northeast with the back edge beginning to move
through Kenai as of noon Wednesday. Upwards of a foot of snow has
also been reported over the western Susitna Valley, where a
Winter Weather Advisory remains in effect. For locations along the
lee of the Kenai and Chugach Mountains, light snow overnight shut
off quickly due to the strong southeasterly (cross-
barrier/downslope) flow. However, with the low now moving east,
those winds are tapering off and snow is beginning to build back
in across the Anchorage Bowl and Matanuska Valley.

Farther west, another shortwave trough is dropping south across
the Bering, helping to maintain shower activity and further
enhance the surge of colder air moving southward.

&&

.MODEL DISCUSSION...

Models remain in good synoptic agreement overall through the
short term. The biggest challenge in the short term will be the
extent of light snow across the Anchorage Bowl and Matanuska
Valley through Thursday afternoon. Guidance is all showing that
main upper-level low moving through by 15z; however, the there is
some difference on strength and position of the wave. These
discrepancies will likely have some influence on where any
subsequent snow bands following the current system set up.

&&

.AVIATION...
PANC...Snow is expected to develop over the terminal by late
afternoon, with the heaviest snow falling through early evening.
Expect MVFR conditions to drop to IFR in steadier snow. There is a
possibility that visibility could briefly drop to LIFR in any
heavier snowfall later this afternoon. Variable winds will shift
to the south by late afternoon or early evening. Snow is expected
to taper off late tonight, but is then expected to return by early
Thursday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 to 3: tonight
through Saturday)...

A low pressure center has now begun to shift south of the Kenai
Peninsula on it's way to the Prince William Sound (PWS) area, with
snow continuing to spread into the Chugach range out ahead of the
low and it's attendant front, with some mixing with rain recently
observed along the coast. Out farther west, a deformation band has
now pivoted from the western Kenai Peninsula up into the Anchorage
and southern Mat-Su Valleys near a secondary upper trough axis
moving out into the Cook Inlet region from the west. This snow is
expected to continue into tonight as the entire upper low over
Southwest shifts overhead, and as a push of cold air begins to
advance from the southwest up the Cook Inlet into the Mat Valley.
This push of southwesterly flow should aid in upslope enhancement
of snowfall across the Anchorage hillside, as well as near higher
terrain between Palmer and Eagle River. A Winter Weather Advisory
has been issued for Anchorage through tomorrow morning, with the
expectation for storm total snowfall amounts up to 10 inches
across the Anchorage hillside through tomorrow morning. For the
Anchorage Bowl and much of the Mat Valley, 4 to 6 inches of new
snow accumulation is possible by the time all is said and done
later tomorrow. A Winter Weather Advisory also remains in effect
through this evening for the Susitna Valley, where the ongoing
snowfall event should be winding down during the next few hours.

Looking out towards Thursday evening and out into the start of the
weekend, the pattern taking shape looks more typical of January
than early November. An upper ridge will begin to amplify out in
the west as a series of strong upper troughs dig south across
Southcentral into the Gulf, each accompanied by a push of Arctic
air. Low pressure will remain anchored over the eastern Gulf
through at least Friday night as these shots of cold air and upper
level forcing move past, setting the stage for a significant gap
wind event beginning Thursday evening and lasting well into Friday
and Saturday. The strongest winds will be through the typically
favored areas with strong north to northwest flow coupled with
the early cold air outbreak, including the Valdez Arm, Passage
Canal, Resurrection Bay, and out of gaps along the northern Alaska
Peninsula into Kodiak Island. Peak gusts during this gap wind
event for the above areas could range from 50 to 70 mph, with the
highest gusts likely through Thompson Pass and around portions of
Kodiak Island at times from Thursday night to Saturday morning.

&&

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

The strong surface cyclone moving inland over Southwest Alaska
continues to track eastward this evening. Snowfall with this
system is expected to gradually wind down for the Kuskokwim Delta
and Lower Kuskokwim Valley as the low pushes across the Alaska
Range and colder, dry air wraps in behind on northwesterly flow,
bringing clearing skies tonight. The Bristol Bay region, which is
already virtually dry north of King Salmon, can expect to see
continued clearing skies spreading southward over the northern
Alaska Peninsula, where it will put an end to the rain and snow
showers streaming in off of Bristol Bay. The combination of cold
northwesterly flow and mostly clear skies tonight will make for
cold low temperatures in the single digits across interior
Southwest Alaska, with low 10s near the coast. This surge of
cold, dry air is expected to cross the Alaska Peninsula on
Thursday, leading to gusty northerly gale-force winds out of gaps
on the south side of the peninsula. High temperatures rebound into
the 20s region-wide on Thursday afternoon under clear skies.

The next round of active weather begins on Friday, with the
arrival of a shortwave trough crossing the northern Bering Sea
into the Kuskokwim Delta early Friday morning. There is increased
confidence in the track and timing of this feature compared to
previous forecast packages. As the low dives southeast across the
region, flow will shift southwesterly, bringing a surge of warmer
air which will warm temperatures into the low 30s along the coast.
While some rain could mix in along the immediate coast, snow is
expected to be the sole precipitation type observed inland, with one
to two inches of snow accumulation possible around the Kuskokwim
Delta for Friday, with accumulations less than an inch expected
elsewhere. Winds are also expected to be 25 to 35 mph in the
Delta, which may result in brief periods of low visibility from
blowing snow, though the intensity of snowfall is expected to be
light, and possibly showery. This wave will also bring a second
surge of cold air behind it, overspreading Southwest Alaska and
reaching the Alaska Peninsula by Friday night. This will
contribute to another round of strong, gusty gap winds, likely
stronger than the last. With 850 mb temperatures decreasing to
around -16C and upper level jet support in place, winds are
expected gust to storm force out of gaps along the south side of
the Alaska Peninsula Friday night, continuing into Saturday.

&&

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

Persistent northwesterly flow sets up across the domain for the
next several days as a deep low pressure system tracks east into
Southcentral Alaska. Rain and snow showers associated with a weak
shortwave dropping south across the central and eastern Bering
Sea/Aleutians will gradually diminish through Thursday as the
disturbance crosses into the North Pacific. High pressure pushes
in from the west on Thursday, yielding generally calmer and drier
conditions through Friday. However, the Eastern Bering Sea and
Aleutians are expected to experience gusty northwesterly winds as
a shortwave dives south across Southwest Alaska and brings another
round of cold air advection Friday night. A low tracking near
Kamchatka on will begin to push a gale force front into the
western Aleutians Friday night. Winds in the western Bering
Sea/Aleutians gradually weaken on Saturday as the low tracks north
into Chukotka.

&&

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

Ensemble guidance is in excellent agreement in the longwave
pattern that a high amplitude ridge and omega block will exist
over the Alaska Mainland on Sunday. The ridge axis is expected to
be over central to western Alaska, which will place the Bering Sea
and Southwest in an active, warmer, and wetter pattern while
Southcentral will stay clearer and colder, though a slight warming
trend is expected through Wednesday. Models often struggle on the
breakdown of these ridges and tend to break them down faster than
what occurs. This will lead to some uncertainty in the long term,
particularly for how far east precipitation is able to make it
into Southcentral. Most deterministic and ensemble solutions
indicate a North Pacific low will cross over the Aleutians on
Sunday and travel north into the Chukchi Sea by Wednesday. This
storm track will send a slug of warmer air into Southwest Alaska,
which will likely result in rain along the coast and freezing rain
and snow for interior areas late Sunday into Monday.
Deterministics try to form a low along the system's front near
Kodiak Island, but how far north and east this system is able to
travel remains uncertain due to ridging. It's most likely that
this will bring precipitation to coastal areas before the system
is deflected south by the strong area of high pressure. In
summary, the Bering Sea and Southwest Alaska is looking to be
warmer and wetter, while Southcentral will remain colder and
drier.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...
 Winter Weather Advisory: 101 145.
MARINE...
 Storm Warning:130 131
 Gale Warning:119 120 125 132 136 138 150 160 165 180 351 352 414
FIRE WEATHER...NONE.

&&

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