Storm Warning 123, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
522 AM AKDT Mon Oct 24 2022

.ANALYSIS AND UPPER LEVELS...

The jet stream extends from eastern Russia to the south of the
Aleutian Chain and into the southern Gulf of Alaska. Along the
jet, a shortwave trough and associated surface low in the eastern
Bering Sea is tracking inland into Southwest Alaska, bringing a
wintry mix of precipitation. A 130+ jet streak coupled with upper
level forcing is aiding gusty northwesterly gap winds along the
coastline, and for the northern side of the Alaska Peninsula.
Meanwhile, a transient ridge of high pressure is keeping the rest
of Southcentral cold and dry. Local effect winds out of the bays
and passes are also slowly decreasing as the ridge builds
overhead. Further west, a rapidly intensifying low pressure system
is advancing into the western Aleutians, with a leading front of
rain and southwesterly winds pushing along the Aleutian Chain and
across the Bering Sea.

&&

.MODEL DISCUSSION...

The latest operational model runs are in decent agreement with
the placement and intensity of synoptic scale features at both the
surface and in the upper levels. The aforementioned deep low is
on track to cross the Bering Sea between today and Tuesday. The
main challenges continue to be possible coastal flooding or high
surf and precipitation types for Southwest. As the low tracks
southeastward into the Gulf of Alaska, precipitation types and
snow amounts will be the next forecast challenge for Southcentral
between Tuesday and Wednesday.

&&

.AVIATION...

PANC...VFR conditions with northerly winds at 5 to 10 kts will
persist through the TAF package.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Wednesday night)...

Expect one more day of dry conditions before a winter storm bears
down on Southcentral on Tuesday. A short-wave crossing from
Bristol Bay into the western Gulf of Alaska this morning will
bring some rain showers to parts of Kodiak Island and some mid to
high clouds to to Southcentral, but these will all exit across the
Gulf this afternoon. High pressure over the Interior and lower
pressure over the Gulf will maintain localized gap winds along the
coast today. These winds will diminish tonight through Tuesday
morning as flow begins to shift in response to a storm in the
eastern Bering Sea.

The upper level flow is very progressive, with a strong west to
east jet stream. This keeps storms and weather features moving
quickly across the region. This will be a key to producing
widespread snow across Southcentral Tuesday/Wednesday. A strong
short-wave trough will amplify as it crosses the Bering Sea
tonight, then move across southern Alaska Tuesday. Light
precipitation will reach Kodiak Island tonight along a leading
warm front. The more significant weather maker will come on
Tuesday as the upper trough approaches Southcentral, with
precipitation overspreading the region from southwest to northeast
through the day. A deep surface low will bottom out and become
occluded as it nears the Southwest Alaska coast on Tuesday. As a
result, the warmest air with this system will never reach
Southcentral, instead becoming modified as cooler air wraps around
the low center. Furthermore, a triple point low will form over
the northwest Gulf Tuesday afternoon and serve to block warmer air
from continuing northward into Southcentral. Therefore, expect
precipitation to fall as all snow for much of Southcentral. The
exception will be the southern Kenai Peninsula and Gulf coast,
which will warm enough to see a transition to rain Tuesday
afternoon and evening.

One typical concern with formation of a triple point low near the
southern Kenai Peninsula is strong easterly downslope flow for the
western Kenai Peninsula, Anchorage, and the Mat-Su. However, the
progressive nature of the flow will keep the low moving eastward.
This low also looks particularly compact. As a result, expect some
downslope drying as the low passes south of Seward late Tuesday
afternoon-early Tuesday evening, but this should be short-lived.
Strong dynamic forcing and favorable south to southwest flow aloft
will bring periods of snow to the usual downslope areas. With the
initial short-wave driving right up Cook Inlet Tuesday, expect
development of a heavier band of snow over the Inlet and the
Sterling Highway corridor, extending up into the southern to
western Susitna Valley. Have issued a Winter Weather Advisory for
the western Kenai Peninsula, with heaviest snow amounts likely
from Soldotna northward to Nikiski. The heaviest precipitation
will shift eastward to Prince William Sound Tuesday night as the
upper trough and triple point low continue to progress eastward.

Areas of light precipitation will linger across the region
Wednesday while the best forcing and heavier precipitation moves
off to Southeast Alaska. Cold air advection will then ensure late
Wednesday through Wednesday night behind the exiting storm system.
This will help dry things out, but lead to redevelopment of
offshore gap winds. Additional short-waves tracking from the
Bering Sea to the Gulf will lead to particularly strong winds
across the western Gulf and Kodiak Island waters.

&&

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

A strengthening low in the western Bering pushes eastward towards
Southwest Alaska over the next couple of day. The driving force
is a strong jet streak stretching across the southern Bering,
driving that low onshore by Tuesday evening. The jet amplifies
through Tuesday night, which in turn will for the low to dig
southward, scooting along the Southwest Coast towards Kodiak
Island into Wednesday. Further west, a ridge of high pressure
extends along the edge of the Western Aleutians into the North
Pacific. The amplified jet, in conjunction with the North Pacific
high pressure, the low over Southwest Alaska, and a surge of cold
continental air being pulled from the Interior, will produce
strong, storm force winds along the eastern Bering, Alaska
Peninsula, and areas north and south of the AKPen Tuesday night
through Wednesday night. Increased wave heights produced from
strong winds increase the possibility for high surf potentials
into Bristol Bay Tuesday night.

A warm front will produce back and forth transitions between
rain, snow, and a rain/snow mix today and tomorrow. There will be
a period of snow tonight, becoming mixed precipitation before
quickly switching over to rain Tuesday morning. Rain will be the
dominant precipitation type on Tuesday for the Kuskokwim Delta and
Bristol Bay, while the a good chunk of the lower Kuskokwim Vally
holds on to mixed precipitation. Snow will still be expected along
the higher elevations. No significant accumulations are expected,
1-2 inches are possible from Bethel to Koliganek over night
tonight, with less than an inch along the immediate coast. As well
as, 1-2 inches possible in the lower Kuskokwim Valley on Tuesday.
A colder airmass then wraps around the backside of the low
Wednesday, and the main precipitation type will become primarily
snow across the whole of Southwest. Additionally, cold northerly
air spreads across the region, resulting in a drop in overall
temperatures, especially in the Kuskokwim Valley, where overnight
temperatures may drop into the low teens Wednesday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

Gale force winds and areas of heavy rain move over the Western
Aleutians into the Bering today south of a developing low
pressure system. A strong jet extends across the southern Bering,
driving the low eastward towards Southwest Alaska. As the jet
amplifies, the low shifts south, tracking along the Southwest
Coast towards Kodiak Island. A swath of cold air from the Interior
wraps around the backside of the low. Northerly winds on the
western edge of the low, along the eastern Bering crossing over
the AKPen, will increase to storm force.

Seas are expected to increase to 20 feet by late tonight in the
southern Bering and waters south of the Aleutians, and then up to
25 feet in the eastern Bering Tuesday evening.

&&

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

Colder than normal temperatures are expected to be the main
feature across southern mainland Alaska for the extended period.
Low temperatures look to drop into the 10s for inland locations as
a low in the Gulf of Alaska draws cold air from eastern Russia
and interior Alaska southward on Thursday. This will help to
entrench a persistent, quasi-stationary longwave trough over the
state into the weekend. The Gulf low winds down through Friday
night, gradually decreasing the pressure gradient driving
north/northwesterly gap winds out of bays and passes along the
Gulf coast.

Beyond these large-scale trends, there is large spread in model
guidance after Thursday, and confidence in any particular solution
coming to fruition is low. However, general trends in guidance
indicate a southerly-biased storm track with a colder airmass
extending across the Aleutian Islands into the North Pacific and
the Gulf of Alaska. The most recent model runs suggest that this
could materialize as a near gale force low tracking from the
western Bering Sea/Aleutians on Thursday along the Aleutian Chain
to the southern Gulf of Alaska for Friday night/Saturday, where
it would reinvigorate gap winds and draw more cold air southward.
This solution is generally agreed upon by the deterministic GFS
and EC at this time. In this pattern, southern mainland Alaska
should expect to remain mostly dry, while the focus for
precipitation will be along the Aleutian Islands and Alaska
Peninsula, where it will fall primarily as rain.

By Sunday, there are hints of another weak low in the North
Pacific approaching the central/eastern Aleutians. However,
confidence with regard to this feature is very low. There will be
some potential for precipitation to overspread the Copper River
Basin on Sunday as the low in the Gulf moves inland over
Southeastern Alaska, but this is also a low confidence scenario.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 121
MARINE...Gale Warning 130 132 150 155 160 165 170-181
185 352 411-414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MF
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KM
LONG TERM...CQ