Storm Warning 126, NWS Anchorage, AK

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

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

An upper trough continues to move through interior Alaska and
will soon push across the ALCAN border. Colder temperatures at or
below freezing have infiltrated the entirety of Southcentral in
its wake, with cool, dry air lending to mostly clear skies.
Temperatures will once again plummet later tonight as a result.
Fog may pester portions of Cook Inlet up into the Knik Arm tonight
and tomorrow morning beneath the inversion. The only other
concern for tonight will entail the continuation of northerly
gap winds through the gaps and passes. Small craft winds with
gusts to gale force are forecast across Resurrection Bay and
Passage Canal. Winds out of the Copper River Delta may gust to
50 knots. Gap winds weaken somewhat tomorrow evening into early
Monday. However, a strong upper low, expected to skirt south of
the Gulf coast Monday into Tuesday, will see renewed gap winds
ratcheting up on Monday, then persisting into Tuesday. Otherwise,
mostly dry conditions prevail through Monday. Coastal regions see
precipitation chances return Monday night.

-BL

&&

.SHORT TERM FORECAST SOUTHWEST/BERING SEA/ALEUTIANS (Days 1
through 3: Today through Monday night)...

On radar, snow showers are moving into the Kuskokwim Delta and is
on track to move southeast into the Lower Kuskokwim Valley this
evening, before weakening. Otherwise, conditions will remain
colder than normal and relatively calm across Southwest Alaska
this weekend under the influence of surface high pressure.
Temperatures will remain below normal through at least Monday
under increasingly clear skies and with the current cold air mass
remaining in place.

A cumulus field of scattered ocean effect snow showers will
continue over the central and eastern Bering Sea and the
Aleutians ahead of a deep low headed for the western Bering this
weekend. The associated front is approaching the western
Aleutians this afternoon, with the cloud shield is already
covering those western Islands. The latest scatterometer pass
corresponds well with the latest forecast of gales. Lightning
observed near the low center is indicating energetic and rapid
development of this atmospheric river.

Winds are expected to increase to sustained, high end storm force
tonight for the western Aleutians and the western Bering Sea.
With the front intensifying as it crosses Shemya late tonight, a
High Wind Warning is in effect for the western Aleutians tonight
into tomorrow morning. As the front progresses northeast, the core
of those winds will likely be over the Bering Sea, but strong
winds with gusts close to 70 mph is expected near Adak. There is
a small possibility (10 to 20 percent chance) for those winds to
exceed 75 mph at Adak, but it is not expected. While winds will
decrease a bit after the initial frontal passage, another
resurgence of strong winds will likely ramp up for the western
Aleutians again Sunday afternoon. However, confidence is low for
exceeding 75 mph and therefore the High Wind Warning is only
valid for the initial frontal passage. Should the forecast track
stronger, an update to the Warning will be necessary. A lot of
warmer air and precipitable water is associated with this low and
rainfall could be heavy at times.

The low traverses the far western Bering Sea on Monday as it
tracks north, winds gradually weakening. The strongest core of
winds moves into the northern Bering and weakens below storm force
by Monday morning. The Pribilofs should expect gale force winds
Sunday afternoon through Monday afternoon. The occluded front
pushes to the Pribilof Islands and the eastern Aleutians, bringing
air warm enough on southerly flow for precipitation to fall
mostly as rain. Another low, likely storm force, looks to
approach the western Aleutians by Monday night. The somewhat
blocked pattern over the mainland will hinder the progression of
that low moving east and models have the it slowing over the
central and eastern Aleutians through midweek. This could lead to
prolonged rainfall.

rux

&&

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

There continues to be high confidence in active weather across
Southern Alaska and the Bering Sea during the extended period,
with forecast confidence hovering around average. The primary
hazard being monitored is the potential for heavy precipitation
along Eastern Kenai Peninsula and Prince William Sound from
Wednesday through Friday. Otherwise, the weather looks active
(but fairly seasonable) for this time of year.

With a low in the Gulf of Alaska moving onshore through early
Wednesday morning, the primary weather maker in the extended
period will be a low moving along the Aleutian Chain. Current
guidance continues to drop minimum sea level pressure to about
940-950 mb, increasing our confidence in the strength of this
feature. Guidance also continues to show sustained winds to storm
force around the backside of the low, as well as a short period
of sustained winds up to storm force along the low's front.
Overall, shifts in the low track look to keep storm force winds
largely south and west of the forecast area, though they still
remain within the realm of possibilities.

While the low itself will remain near the Aleutian Islands/Bering
Sea, its front will sweep out ahead of it and bring the bulk of
impacts to the Southern Mainland through Thursday. Expect winds
to gale force along this front and for abundant moisture to sweep
across the region. While the front will move rather quickly
inland, precipitation rates could be high along Western Susitna
Valley and the north Gulf coast as the front moves in, and
southerly flow behind the front could lead to persisting
precipitation along coastal mountains after the front has passed.
850 mb temperatures approach freezing as warm air advects inland
with this front, raising questions about what precipitation type
we'll see. Coastal locations are most likely to see rain, though
precipitation type remains uncertain further inland. Rain, snow,
and even freezing rain remain on the table, so stay tuned as we
continue to evaluate this aspect of the forecast.

Otherwise, model guidance continues to differ with the track of
this low as it occludes on Thursday. The long-term forecast leans
towards the scenario where ridging is stronger behind the low,
leading to this low moving across the Aleutians into the Bering
Sea. The other scenario keeps the low south of the Aleutians, with
implications for wind directions/speed along the Aleutian Chain
and precipitation in Southwest Alaska. Downstream differences
between these two scenarios worsen into Saturday, where the low
position eventually affects the location of a plume of moisture
headed for Southcentral Alaska and the track of a front moving
into the Bering Sea from the west.

-KC

&&

.AVIATION...

PANC...IFR ceilings and visibility will continue this evening and
into Sunday morning as stratus and freezing fog form over
northern Cook Inlet and intermittently drift over the air field.
Light winds mainly from the north will continue into Sunday.
Temperatures are expected to drop into the single digits during
the overnight hours.

CC

&&