Storm Warning 51, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
441 PM AKST Sat Feb 18 2023

.ANALYSIS AND UPPER LEVELS...

The southwesterly jet continues to be positioned along the Alaska
Peninsula and stretching into the western Gulf of Alaska. The
position of this jet is not expected to deviate much from this
location for the next 24 hours. The main surface low feature for
Southern mainland Alaska is right into Bristol Bay. Snow has
switched to rain at King Salmon this morning. Precipitation has
since stopped with temperatures remaining in the mid 30s.
Temperatures and precipitation remain cold enough for snow in
Iliamna.

The triple point of the occlusion is appearing near
Kodiak Island in the western Gulf. Gale force, southwesterly winds
along the southern edge of the low along the Alaska Peninsula.
Windy conditions to a lesser extent into Shelikof Strait. Storm
force gusts are occasionally reported.

&&

.MODEL DISCUSSION...

Evolution in the triple point and the movement of the weakening
low over Bristol Bay and coinciding triple point low into Prince
William Sound is going to play a role in the timing of snowfall
across Southcentral, but snow accumulations are expected to be
light. Further west, there is fair agreement in the smaller low
features in the Bering Sea. The magnitude of localized features
have less agreement, but impacts should be minor. Looking further
to Monday night, the low moving into the Central Aleutians appears
to be in relatively better agreement, however, the core of those
winds have slight differences. The low central on the Canadian and
ECMWF appear to be the bigger outliers with a more westward
location.

&&

.AVIATION...

PANC...MVFR conditions with light winds and light snow will
continue through this evening with light accumulations expected.
Periods of heavier snow bands could drop visibility to below 2SM
at times. A southwesterly wind shift that develops tonight could
raise the potential for areas along Cook Inlet to develop lower
(IFR) cloud ceilings and/or fog over the terminal late tonight
through Sunday morning.

&&

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

A weak low in Prince William Sound will track eastward tonight and
weaken. Light snow will continue this evening, tapering off from
west to east with the exiting low. Higher amounts are possible in
the higher elevations of eastern Prince William Sound also as a
result of onshore, upslope flow. Though, at lower elevations
temperatures warm enough to lead to a mix with or complete
changeover to rain, limiting any accumulation. Snow will end over
most of the region by Sunday morning. Back behind the low, cold
air advection and tight pressure gradients will sustain
southwesterly gales across the western Gulf/Kodiak Island waters
through tonight.

On Sunday, high pressure begins to build over Western Alaska,
which will direct colder air on northwesterly flow across
Southcentral. This will cause the typical gap winds to develop
along the coast. Temperatures will also gradually trend colder,
while skies trend clearer. However, light surface winds will
likely allow fog or low stratus to form in the wake of today's
snow in the Cook Inlet region, from Kenai/Soldotna to Anchorage
and into the southern Mat-Su. Gap winds along the coast will
continue to increase going into Monday as high pressure
strengthens to the west. As the surface ridge axis shifts east
Monday night, these winds will diminish with the decreasing
pressure gradient. Warmer air will begin to work in from the
western side of the ridge, as a broad low in the Bering Sea pushes
its front into Southwest Alaska. This will contribute to a slight
chance of precipitation over Kodiak Island and the western Gulf
along the eastern periphery of the front going into Tuesday.

&&

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

A low pressure system currently situated over Bristol Bay will
quickly push inland this afternoon. Most of the precipitation in
the form of snow will be concentrated over the eastern corridor of
Bristol Bay. About another inch of snowfall will be located
between Port Alsworth and King Salmon tonight. Snow is expected to
end early Sunday morning as the aforementioned low weakens and
exits the region. Heading into early next week, some weak
shortwaves will bring occasional light snow showers for the
Southwest mainland from Sunday night to early Monday. Otherwise,
quieter weather is in store as a ridge of high pressure builds
from the eastern Bering Sea and moves overhead.

By next week, an active weather pattern gets underway as a series
of strong storm systems moves across the Bering Sea and impacting
western Alaska. There is still some uncertainty with the timing
and track of these systems, so stay tuned for further updates.

&&

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

A low pressure system is quickly moving inland over Southwest
Alaska. Precipitation along the central and eastern Aleutians
should end by tonight. Meanwhile, a trough extending from the
Kamchatka Peninsula to the western Bering Sea will remain anchored
for the rest of the weekend. Some weak transient lows will stream
northward, bringing a period of light snow showers and small
craft winds along the Aleutian Chain and Pribilof Islands.

The next impactful system is associated with a North Pacific low
approaching the western Aleutians early Monday. Model guidance is
slowly coming into an agreement with the low center passing near
Amchitka Island Monday night. The leading frontal boundary will
overspread the western Aleutians by Monday afternoon, bringing
gusty southerly winds, elevated seas, and heavy precipitation.
There is growing confidence for widespread gale force winds, with
localized areas of storm force winds (including Adak and Atka)
between Monday and Tuesday morning.

As the low lifts northward on Tuesday, the Pribilofs can see
increasing southerly gales and precipitation as well. Conditions
are expected to improve briefly on Tuesday followed by a second
system from Tuesday afternoon into midweek. There remains some
uncertainty with the timing and track of these systems, so stay
tuned for further updates.

&&

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

The overall pattern still looks likely to be highly amplified near
the start of the extended outlook on Wednesday. A very pronounced
and wide longwave ridge is expected to be enveloping most of
Mainland Alaska as it extends into the region from the North
Pacific. This strong ridge should effectively deflect the
prevailing storm track well to the west initially, with a series
of stronger lows and trailing fronts expected to move from
southwest to northeast across the western Bering between Wednesday
and Thursday. The 12z GFS depicts a much more potent low compared
to the other global solutions moving towards the Bering Strait,
along with a significantly more east track. This difference ties
into other downstream impacts in the GFS solution on Thursday,
with this system able to flatten out the aforementioned ridge
quite a bit faster compared to the consensus showing a weaker low
staying a little farther west over the Chukchi Sea.

In spite of the above model differences, there is a good signal
for a deep moisture tap to extend into Southwest out ahead of the
low's trailing cold front, likely resulting in a moderate
precipitation event over the region on Wednesday, with enough warm
air riding in on the long fetch of southwest flow to cause snow
to change over to rain near the coast and at lower elevations. The
heavier precipitation potential will shift east with the moisture
tap and front into Southcentral on Thursday, but it remains less
clear whether the nose of warm air aloft will reach far enough
northeast to result in issues with precipitation type by this
point. In any case, the pattern does not look favorable for a
prolonged heavy snowfall event across Southcentral, with most
models showing the moisture axis continue to sweep quickly east by
Friday as the upper ridge and jet axis begin to shift to the
southeast.

Another shift in the dominant pattern appears likely by the start
of next weekend as a longwave trough attempts to take hold over
the Mainland while a low amplitude ridge begins to build into the
Bering. This will strongly favor a return to a much colder, drier
pattern across Southcentral and Southwest as Arctic air moves
south towards the base of the trough between Friday and Saturday.
It will remain active out west in the meantime, with models and
their respective ensemble suites signaling another strong low
crossing the Bering Sea on Saturday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 120 130-132 137 138 174.
FIRE WEATHER...NONE.

&&

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

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
1132 PM AKST Sat Feb 18 2023

AKZ152-155-161-200100-
Lower Kuskokwim Valley-Kuskokwim Delta-Bristol Bay-
Including the cities of Aniak, Sleetmute, Bethel, Nunivak Island,
King Salmon, Dillingham, Naknek, and Pilot Point
1132 PM AKST Sat Feb 18 2023

...SERIES OF STORMS TO BRING WARM AND WET CONDITIONS TO SOUTHWEST
ALASKA MIDWEEK...

A series of storms moving from the Bering Sea into the Chukchi Sea
Tuesday through Thursday will bring southerly winds, warm
temperatures, and mixed precipitation to Southwest Alaska next
week. Expect temperatures to climb into the mid to upper 30s as
far north as the Lower Kuskokwim Valley as moderate southerly
winds move into the region. Precipitation type and timing remain
uncertain, but rain and freezing rain appear likely throughout the
region. The first system is expected to arrive on Tuesday,
followed shortly by a second, warmer system Wednesday. Stay tuned
to the forecast at weather.gov/afc as we refine the details in
the forecast.