Gale Warning 781, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
522 AM AKST Sun Dec 18 2022

.ANALYSIS AND UPPER LEVELS...

The upper level pattern along with most surface conditions remain
very stagnant, as an Omega block ridge continues to take shape
over the western Mainland and northern Bering Sea. Arctic air is
now deeply entrenched across Southcentral, with the coldest
temperatures over interior valleys hovering between -10 and -30
Fahrenheit. A strong temperature inversion has set up underneath
the eastern side of the ridge, resulting in much warmer
conditions in the 10s to 20s at mountaintop levels. This strong
inversion is also helping to trap a stubborn layer of fog and low
stratus that has affected mostly the west side of Anchorage, as
well as portions of the western Kenai Peninsula. Cool and quiet
conditions are holding steady across Southwest as well, with
mostly calm winds under the ridge axis.

Most of the more active weather continues across the Bering, where
a large belt of easterly winds mainly in small craft range
continues to affect most of the Aleutians and southern Alaska
Peninsula. A stationary front is draped across the southwest
corner of the Bering, with warm temperatures well above freezing
allowing for mainly rain to fall with bands of showers moving
along and north of the front and across the Aleutian Chain.

&&

.MODEL DISCUSSION...

Models have come into better agreement for a series of upper
troughs expected to move south from northern and central portions
of the state into Southcentral through Tuesday. However, there is
more substantial disagreement for a small low expected to spin up
over the northern Gulf this evening, with models diverging on the
track and location of the low as it drifts west through Tuesday.
This yields substantial forecast uncertainty regarding the
prospect of light snow spreading into the Prince William Sound
region between Monday and Tuesday near and north of the low. There
are also differences appearing for a low expected to develop near
a stalled front over the central Aleutians on Tuesday. The primary
detriment to forecast confidence regarding this uncertainty will
mainly pertain to the strength and location of continued easterly
winds over the Bering Sea.

&&

.AVIATION...
PANC...Areas of fog and low stratus drifting off of the Knik Arm
will likely persist around the terminal through much of the
morning. Some improvement is possible by the early afternoon as
fog lifts back to a stratus deck, perhaps scattering out entirely
and allowing for periods of VFR conditions. A return to IFR/LIFR
conditions will again be possible tonight tonight with very little
change to the pattern in place supporting the sharp temperature
inversion that has continued to trap moisture in the lowest few
hundred feet. However, there will be a modest increase in
northerly flow along with cooling temperatures aloft, and this
could be enough to stave off fog reformation tonight into Monday
morning.

&&

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

High pressure is expected to continue to dominate most of the area
as a blocking high has developed over western Alaska. This is
bringing mostly clear skies and below normal temperatures. Patchy
fog and stratus is expected to hang around the Anchorage area,
Knik Arm, and the western Kenai Peninsula. This fog and stratus
may erode a bit during the daylight hours but is not expected to
completely dissipate and will likely linger over portions of the
area through Monday morning. By Monday afternoon northerly winds
will increase across the area and should scour out any remaining
fog and stratus. An upper level shortwave will move across
Southcentral today and will enhance the winds around Valdez and
Thompson Pass beginning this afternoon and continuing through
Monday night. Expect gusts of 40 to 50 mph across Valdez and the
Thompson Pass area. Meanwhile an area of low pressure has
developed over the northeastern Gulf and will move very slowly to
the east. However, most of precipitation with this low will stay
offshore through Monday. On Monday an upper level low will develop
over the Copper River Basin and drift south near the coast on
Tuesday. This will draw moisture from the surface low over the
northeastern Gulf into parts of eastern Prince William Sound with
light snow expected around the Cordova and Valdez areas.

&&

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

An upper-level ridge of high pressure continues to dominate
Southwest Alaska, and clear, dry, and cold conditions will persist
across the region through at least Wednesday morning. Although,
the Alaska Peninsula can expect increased chances for rain as a
result of easterly to southeasterly upslope flow. The Alaska
Peninsula will also be warmer than the interior Southwest regions
located under the ridge. The coldest temperatures will continue to
blanket the Lower Kuskokwim Valley, with maximum daytime
temperatures remaining below zero degrees F across much of the
region. A general cooling trend unfolds through as least Wednesday
morning, further dropping the already frigid temperatures, as a
result of the ridge weakening and cold air advection from the
east.

&&

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

The Bering Sea and Aleutians will continue to see more active
weather through the short-term period. A dome of high pressure
will remain nearly stagnant as it stretches form the central
Bering and Aleutians east into Southwest Alaska. An upper-level
trough is currently located over the Kamchatka Peninsula while an
attendant surface low spins just south of the central Aleutians.
Chances for rain will decrease today across the western Aleutians
and Bering as the trough and low gradually weaken in place. By
tonight, the front extending from a vertically-stacked Kamchatka
low will reach the western Aleutians. The upper low will remain
nearly stationary due to the large, stubborn ridge, and various
surface fronts will track eastward to the central Bering and
Aleutians through Wednesday morning. Rain (possibly a snow/rain
mix) and widespread easterly gale-force winds are expected to
accompany the fronts. A corridor of easterly storm-force winds may
develop over the central Bering beginning late Tuesday night.

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

A blocked, high amplitude ridge dominates over western Alaska.
Offshore flow and bitter cold temperatures prevail for inland
Alaska going into midweek. Some changes are possible with the main
concern being offshore gap flow through Valdez and the Matanuska
Valley. An upper level polar trough looks to sink southward and
elongating toward Southwest by Wednesday morning. This will
enhance the pressure and temperature gradient and help to move air
through those wind prone gaps.

While confidence remains low, model solutions are consistent in
showing an easterly wave transporting the cold, dry airmass across
the state from east to west by Thursday. Boundary layer winds
have a widespread area across Bering into Gulf of easterly winds,
potentially to storm force. The presence of this airmass and the
associated upper level wave appears to slow any progression of
surface lows north and east. There is a chance of a low moving to
the northeastern Gulf though. By the end of the week, another low
appears to descend south, similar to midweek. Forecast confidence
is low on the timing and track of the occurrence.

Farther west, an active pattern is expected as conditions appear
rather unsettled through the long term that will bring widespread
precipitation and gusty southeasterly winds to the Aleutian Chain,
Bering, and Alaska Peninsula.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 119, 127, 170, 179, 185, 412, 414.
         Heavy Freezing Spray Warning: 139, 160, 181.
FIRE WEATHER...NONE.

&&

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