Gale Warning 836, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
519 AM AKST Tue Dec 20 2022

.ANALYSIS AND UPPER LEVELS...

Another frigidly cold, clear morning is underway across nearly
all of southern Alaska as an Arctic air mass continues to pool
into valleys across both Southwest and Southcentral. Temperatures
in the -20s to -40s remain in place across portions of the Mat-Su
Valleys and Copper River Basin, with single digits above/below 0
Fahrenheit common elsewhere across the southern Mainland. An upper
level trough centered near the northern Gulf coast is helping to
erode the southern portion of the blocking ridge in place to the
west over the Bering and Chukchi Seas. This is causing the upper
level ridge to nearly pinch off into a closed circulation over the
Chukchi Sea and begin a transition from the Omega block pattern we
have seen for the past few days to more of a Rex block. A small
low continues to spin in place under the upper trough off to the
east of Middleton Island. Easterly winds up to gale force are
wrapping around the north side of the low between Prince William
Sound and Middleton Island, with some light snow intermittently
pushing into the coast near Cordova.

Out west, the overall pattern remains similarly steady-state,
with a stalled frontal axis draped along the western Aleutians and
southern Bering. A large region of easterly winds between small
craft and gale force strength continues to impact areas north of
this front, with the strongest winds extending from near Dutch
Harbor into the central and northern Bering. Bands of light to
moderate rain continue to stream across most of the Aleutians and
Pribilofs along and ahead of the front along with dense mid/upper
cloud cover.

&&

.MODEL DISCUSSION...

Overall model agreement has improved regarding some of the
pattern uncertainties over the next several days. Models now all
show the small low wound up over the northern Gulf remain stalled
out near Middleton Island through Tuesday, then quickly weaken
prior to accelerating into the coast near Prince William Sound
early Wednesday. There is still some variability in the handling
of the ridge breakdown out west, especially for a series of lows
expected to form and track along a stalled out front meandering
across the Bering. While the exact placement and track of
individual lows moving west along the front is still not entirely
clear, this does not greatly impact forecast confidence for very
strong, easterly winds to develop across the Gulf, Bering and
Southwest through Thursday.

&&

.AVIATION...
PANC...Fog and low stratus has mostly dissipated across town, but
redevelopment of fog/stratus over the Knik Arm cannot be entirely
ruled out again this morning. VFR conditions and light northerly
winds will otherwise persist outside of any periods of low stratus
that manage to move back over the terminal.

&&

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

The retreating high pressure to the west will continue to produce
northerly flow through tonight with mostly clear skies and below
normal temperatures continuing for the foreseeable future. The
exception will be southern parts of the Copper River Basin and
eastern portions of Prince William where clouds and periods of
light snow are possible as a low pressure continues to meander and
slowly move northeast through Wednesday. Snow accumulations will
be an inch or less across populated areas. There are still model
differences on where the low will move. Some models move it due
north and inland while others move it east towards the eastern
Kenai Peninsula before it dissipates. For now we have opted to
follow the easterly solutions which will spread light snow across
Prince William Sound and eastern portions of the Kenai Peninsula
tonight through Wednesday. Once again snow accumulations for most
populated areas will be a dusting to perhaps a couple of inches.

Wednesday afternoon and evening a front will move into the
southern Gulf and towards Kodiak Island. This will bring a brief
mix of rain and snow to Kodiak City Wednesday night before
changing to rain. This front will also bring a large area of gales
with some storm force gusts across a good portion of the Gulf.
This front is not expected to bring precipitation north of the
Kenai Peninsula with the interior of Southcentral remaining dry
with continued cold conditions.

&&

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

A ridge of high pressure continues to sit on the western coast of
Alaska resulting in cooler, drier, and clearer weather. However,
throughout today, the aforementioned ridge will shift westward
which will allow a trough of colder air to push into the region.
As a result, cooler temperatures are expected with day-time
highs reaching positive and negative single digits across the
coastline with negative double digits in the Kuskokwim Valley. The
exception will be the Alaska Peninsula where warm moist southerly
flow will aide in moderating temperatures and bring chances of
rain showers throughout the day. Warmer temperatures will work
their way back into the region as a whole for Wednesday as a front
associated with a North Pacific low will track across the Alaska
Peninsula through Wednesday afternoon. Precipitation will then
spread into the Bristol Bay region Wednesday night and northward
along the coast as the front traverses northward. Expect winds to
be elevated and gusty along the coastline as pressure gradients
tighten. After the aforementioned frontal passage, winds will
relax and drier conditions will return to the area through the end
of the week.

&&

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

Active weather will continue for the region through the end of the
work week. A front situated in the Bering and high pressure
located over the Mainland have created a tight pressure gradient
between the two features, resulting in a swath of southeasterly
gale force winds stretching across the northern/central Bering,
through the Pribilof Islands, and into the eastern Aleutians.
Today, a core of storm force winds will develop in the northern
Bering as the gradients continue to tighten and will extend
southward towards the Pribilofs tonight through early Wednesday
morning. Storm force winds will then develop on the Bering and
Pacific side of the Alaska Peninsula for late Wednesday into
Wednesday night as a North Pacific low and its front begin to
track into the region. Models are having difficulty resolving how
this feature will evolve and impact the region, but gusty winds,
elevated seas, and an overall 60% chance of precipitation across
the region will persist through the end of the week. Be sure to
check back in with the forecast as models get a better handle on
this feature.

$$.LONG TERM FORECAST (Days 4 through 7: Friday through Monday)...

Model consensus beyond day 3 breaks down. The general synoptic
patterns resolve reasonably, but the position and phasing of the
systems differ and will likely impact the forecast outcome. Strong
easterly winds Thursday, spanning from the Gulf through the
Alaska Peninsula to the Bering should begin to diminish for Friday
when a surface low and an accompanying front sets up in the Gulf.
This front should bring plenty of moisture. Many areas could be
cold enough for snow down to the coast and Kodiak Island.
Increased surface pressure gradient with the incoming low could
enhance gap winds through the Matanuska Valley and other northeast
and north to south terrain gaps. An upper trough over northern
Canada and Alaska will drop south through mainland Alaska and
elongate as it reaches Southcentral. This appears to form a
closed off circulation that moves west into the southern Bering
Sea Saturday and potentially phase with a low just south of the
Aleutian chain. A broad area of low pressure near the Aleutians is
expected to continue, with clusters of pressure minima that would
continue to bring nuisance activity from weak low systems that
phase and interact with one another. The initial entrance of a low
in the Gulf towards the end of the week, however, could be the
time the blocked ridge breaks down enough for active weather to
return to northern Gulf. Come Sunday night through Monday night,
another train of back to back lows from the Pacific could move
into the Gulf.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning: 150,155,185,411,412.
         Gale Warning: 119,120,130,131,132,138,139,352,160,165,
         170-172,179-181,413,414.
         Heavy Freezing Spray Warning: 139,140,160,180,181,185.
FIRE WEATHER...NONE.

&&

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