Gale Warning 650, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
544 AM AKST Fri Nov 24 2023

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

A large vertically stacked low centered near Unalaska/Dutch Harbor
is nearly stationary, while numerous short-waves circumnavigate
the low center. Of note in water vapor satellite imagery, one
distinct short-wave is rotating from the northeast Gulf into
interior Southwest Alaska. A second stronger short-wave is
tracking northward across the Gulf toward Prince William Sound
and the north Gulf coast. At the surface, a cold occluded front
over the western Gulf is tracking north and east toward
Southcentral, with strengthening east to southeast winds ahead
of it - especially the typical barrier jet area from Hinchinbrook
Entrance to Cape Suckling. Widespread rain is falling along the
coastal zone, moderate to heavy in places - especially across the
Kenai Peninsula where strong southeasterly upslope winds are in
place. Downslope flow is leading to a downslope hole in the lee
of the Kenai and Chugach Mountains, though this is beginning to
fill in as the short-wave and surface front lift northward and the
strength of downslope flow begins to weaken.

The big story for today is the continued threat for freezing rain
from Anchorage north to the Mat-Su as well as in the Copper River
Basin. Surface temperatures have been slow to rise, with many of
these areas hovering near the freezing mark. Even where air
temperatures have inched above the freezing mark, rain can still
freeze to cold roadways and sidewalks. Rain can also flow along
ice and snow covered surfaces and pose similar hazards to freezing
rain. Therefore, Winter Weather Advisories are in effect for all
of these areas for freezing rain and light ice accumulation.
Temperatures will slowly rise through the day, reaching the upper
30s for much of the region. As a result, the threat of freezing
rain will diminish this afternoon.

The primary short-wave will exit northward tonight, but additional
weaker short-waves will move up from the Gulf, maintaining the
overall wet pattern. The upper levels will cool a bit behind the
cold occlusion, with snow levels dropping from 3000 to 4500 feet
down to 2000 to 3000 feet. Rain with above freezing surface
temperatures will persist for lower elevations. Southeasterly
winds will weaken behind the front, but there will be enough
southeast flow to limit precipitation in the lee of the mountains.

The upper trough extending south from the low center will amplify
over the North Pacific, causing amplification of the downstream
ridge and development of an atmospheric river (AR) from the
Pacific north to the Gulf of Alaska. This AR will reach the
northeast Gulf coast Saturday, likely clipping Cordova and
southeastern Prince William Sound but directly pointed further
east toward Yakutat. At the same time, one or more surface lows
will form over the Pacific and track across the central Gulf this
weekend. Model guidance is struggling with the track and strength
of these, which is common for this pattern. The track of these
lows will determine how much of Southcentral is impacted by
precipitation from the low and AR. Right now, the most likely
areas to see precipitation include eastern Prince William Sound
and the Copper River Basin.

Meanwhile, the upper level low will open up into a trough and
progress eastward through the weekend, leading to a drying trend
for Kodiak Island, the Kenai Peninsula, Anchorage, and the Mat-Su.
This will be short-lived, as progressive flow aloft brings in
another upper trough and surface frontal system Sunday night into
Monday. Colder air will accompany the trough, leading to a good
likelihood that precipitation falls in the form of snow.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA AND BERING SEA/ALEUTIANS
(Days 1 through 3: This morning through Monday morning)...

A weakening low south of Unalaska Island brings wet and windy
conditions to the Bering Sea/Aleutian Islands and precipitation-
type concerns to Southwest Alaska. From Friday through Monday
morning, a front moving west to east across the Bering Sea brings
a shift towards more active weather.

As weak upper level shortwaves rotate around the low and into
Southwest Alaska, any precipitation that falls could potentially
do so as freezing rain. Freezing rain could remain in the forecast
for the Lower Kuskokwim Valley through Saturday morning as
forecast high temperatures hover just around freezing. Surface
temperatures are very close to reaching the freezing point; if
they do so, we could see a transition towards plain rain/snow by
the afternoon.

The next big weather system, a Kamchatka low, sweeps its front
across the Bering Sea, reaching the Western Alaska coast by Sunday
afternoon. Confidence is high that this will be a gale force
front, with localized areas of embedded storm force winds. The
minor challenge with this front will be identifying where triple
point lows develop along the occluding front, as these developing
lows will lead to increased winds in their immediate vicinity.
Otherwise, model agreement is good with this front, leading to
relatively high forecast confidence from Friday evening through
Sunday afternoon.

ER

&&

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

The workweek opens with a well developed upper level low exiting
the Russian Far East into the Northern Bering. Its long wave
trough rotates across the Aleutians and Bering into the Mainland
and Gulf of Alaska by the end of the period. Several short waves
will ripple through the flow over the Aleutians, bringing periods
of heavier precipitation and gusty winds. This pattern will
displace the warmer temperatures over most of the state as the
trough moves across the Southern half of Alaska by Thursday.
Strong winds are expected as this system progresses Eastward. Some
gale force winds spread over the Aleutians and Bering for Sunday,
but diminish as the main trough approaches the Alaska Peninsula
by Tuesday. A closed low develops South of Dutch Harbor on Monday,
but its stronger winds will not come to bear until the low moves
towards the Alaska Peninsula. Gale force gap winds are expected
over the Alaska Peninsula and around Kodiak Island for Wednesday
before diminishing Thursday over the Gulf. Locally heavy
precipitation will occur along the North Gulf Coast and move into
Southeast Alaska by Thursday. A strong trough brings gale force
winds to the Western Aleutians and Bering for Wednesday and
Thursday.

Kutz

&&

.AVIATION...

PANC...Light rainfall will affect the terminal through this
afternoon. Though it is likely to remain above freezing,
temperatures may briefly dip below freezing and result in a light
glaze of ice on surfaces. The highest rain chances/heaviest rain
is expected with the passage of a front, most likely between 9 am
to 3 pm. During this timeframe, ceilings may drop to MVFR.

Winds will be light out of the north until the passage of the
front, at which point they are expected to become southeasterly
with the Turnagain Arm wind just grazing the airport. Once these
southeasterly winds develop, temperatures will remain above
freezing. Scattered light rain showers then continue to be
possible into the overnight hours and through Saturday morning,
though ceilings and visibility return to VFR.

CQ

&&