Storm Warning 52, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
535 AM AKST Sun Feb 19 2023

.ANALYSIS AND UPPER LEVELS...

Out west, upper-level shortwaves and a low spinning near the
Western Bering promote snow showers for the Western and Central
Aleutians and Bering. Out east, lingering snow showers slowly
taper off along the mountain ranges as a low dissipates near the
Southeast coast and multiple upper-level shortwaves shift to the
east. As high pressure builds into the area from the north and
west, areas of fog and low status are being observed this morning
across much of Southcentral, including the Mat-Su Valleys, Cook
Inlet, parts of Anchorage, and the western Kenai Peninsula.
Overnight low temperatures along the coast ranged from the 20s to
30s, while locations further inland generally saw temperatures
down into the teens and single digits.

&&

.MODEL DISCUSSION...
Models are in overall good synoptic agreement for both the east
and west domains into the middle of the week. There is great
agreement for an upper-level ridge to build east across the
Southern Mainland beginning later today into Monday, while
various low pressure systems continue to traverse the western
domain. There are slight differences in the strength and placement
of an upper-level shortwave advertised to track across the
Western and Central Aleutians tonight, however impacts should be
minor. Models are slowly coming into better agreement with a
strong North Pacific low that lifts into the Bering on Monday,
with the low center likely tracking near Amchitka Monday night.
The GFS/NAM both show the core of strongest winds (sustained storm
force) tracking across the Central Aleutians on Tuesday morning.
Another North Pacific low is expected to enter the western domain
for Wednesday. Although model agreement begins to diverge around
this time, there is general consensus for the low center to track
near Shemya on Wednesday morning and spread at least high end
gales along its southern periphery.

&&

.AVIATION...
PANC...Conditions are generally expected to remain VFR with light
northerly winds through the TAF period. However, low stratus and
vicinity fog could drop conditions as low as LIFR at times.

&&

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

Drier and marginally cooler conditions are on tap for Southcentral
today through Tuesday as a ridge builds in from the north and
west. Low clouds will cover much of the region to start today, but
increasing offshore flow and mostly sunny skies above the low
clouds will help erode some of them. As is typical in this
pattern, winds will be lighter in the Cook Inlet region, so some
low clouds will likely hold in and remain around through at least
the next couple days. Also expect localized areas of fog where
there is clearing. Temperature inversions will strengthen over the
next couple days, so expect fog will become more widespread
tonight and then again Monday night. The most likely areas to see
fog include the western Kenai Peninsula, Anchorage, and the
southern Mat-Su. There should be some improvement each afternoon
as the sun works on burning it off.

Cold air will advect across the region on the leading edge of the
upstream ridge tonight through Monday. This will lead to strong
and gusty northwesterlies across the western Gulf and Kodiak
Island beginning this evening, then strengthening gap winds along
the north Gulf coast and Prince William Sound tonight through
Monday. Winds will diminish Tuesday as a high amplitude ridge
builds overhead. Very progressive upper level flow will quickly
push the ridge eastward, with chances for snow returning by
Tuesday night.

&&

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

A fairly benign pattern for Southwest Alaska as a weak ridge has
begun to develop over the region. This will keep winds calm and
showery snowfall for the remainder of today. A weak shortwave will
swing into the Kuskokwim Delta tonight into Monday morning,
bringing the chance for light snow showers and more steady onshore
flow. This weak shortwave will permeate across the rest of
Southwest Alaska Monday, dissipating by Monday night. Monday
night into Tuesday morning, a low will be advancing northward
quickly in the Bering however is expected to keep mostly west of
Southwest Alaska. Nunivak Island and Mekoryuk, as well as parts of
the western Kuskokwim Delta, such as Kipnuk and Chefornak, will
see light to moderate snow showers into Tuesday morning. A front
along the eastern edge of the low looks to lag behind the low and
push into Southwest Alaska throughout the day Tuesday. This wave
will bring snowfall throughout the region Tuesday and Tuesday
night. Details on the orientation of the wind are still uncertain,
which may dictate if the gale-force flow is along the shoreline,
or pushing inland. The risk for coastal surge currently seems
unlikely, however stay tuned for the latest forecast to be
prepared for any changes.

&&

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

An aging low pressure system in the western Bering continues to
drive south and southwesterly flow as well as the mildly unsettled
conditions currently found across the Chain and Pribilofs. Several
transient shortwaves will perpetuate the semi-active pattern in
the Aleutians today, before a weak low system enters the
Western/Central Aleutians tonight bringing mild precipitation and
southerly flow. The main concern is a trailing, intense North
Pacific low that aims to enter the Bering across the Chain Monday
afternoon. The corridor of strongest winds associated with this
system on Monday currently appears to be oriented north-south,
moving over Atka late Monday evening, with Adak on the western
periphery. This corridor of storm-force winds stretches from just
east of Amchitka to just west of Nikolski throughout Monday night.
In addition to these strong winds, moderate to heavy snow
transitioning to rainfall Monday through Monday night is expected
for this corridor as well. Hurricane-force gusts are also possible
near the tail-end of Monday night as pressure gradients tighten
and cold air from the Bering begins to get entrained in the system.
Tuesday morning the low will lift into the Central Bering, bringing
the storm-force winds, near hurricane-force gusts, and the
moderate rain/snow mix to the Pribilofs. By Tuesday night, the
frontal boundary of another North Pacific low appears along the
Western Aleutians. This system does not currently appear to be as
strong as the system on Tuesday, however gale-force winds and
additional rain/snow mix appear likely. Stay tuned to our latest
forecast for details on all of these developing systems.

&&

.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...Storm Warning: 176.
 Gale Warning: 130, 131, 138, 172-175, 177, 351, 352.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AF
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CL
LONG TERM...AS

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
1252 PM AKST Sun Feb 19 2023

AKZ152-155-161-210300-
Lower Kuskokwim Valley-Kuskokwim Delta-Bristol Bay-
Including the cities of Aniak, Sleetmute, Bethel, Nunivak Island,
King Salmon, Dillingham, Naknek, and Pilot Point
1252 PM AKST Sun Feb 19 2023

...WARM AND WET CONDITIONS TO SOUTHWEST ALASKA MIDWEEK...

A series of storms moving from the Bering Sea to 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. Peak winds will move through the
Kuskokwim Delta at 30 to 40 mph, with gusts to 45 mph are expected
Tuesday. Breezy conditions will continue through Wednesday and
Thursday, but to a lesser magnitude. The precipitation type from
snow to rain, along with the onset timing, as temperatures quickly
shift above freezing remains uncertain. Freezing rain and rain
freezing to already cold surfaces cannot be ruled out for some
locations either. With warmer air continuing to filter through the
Southwest Alaska with each successive storm, the transition
period for precipitation type should be brief. This forecast will
continue to evolve, so please stay tuned at weather.gov/afc as we
refine the details.