Gale Warning 833, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
541 AM AKST Mon Dec 19 2022

.ANALYSIS AND UPPER LEVELS...

The large scale picture is still about the same compared to the
past couple days, with an Omega block ridge still extending from
the western Gulf up into the Chukchi Sea. A deep North Pacific
low east of Kamchatka is helping to push a broad frontal zone
extending across the Bering Sea and Aleutians slightly farther
north, with bands of mainly rain continuing to stretch along the
front past the central/eastern Aleutians and just to the west of
the Pribilofs. Strong southeasterly flow is continuing to drag
warm, moist air into much of the Bering, with surface temperatures
along most of the Aleutian Chain and still mostly in the 40s.

Out across the southern Mainland, the main story continues to be
the bitterly cold temperatures and mostly clear skies under Arctic
high pressure. Almost all lower elevations away from the coast are
now experiencing sub-zero temperatures, with the coldest
temperatures in the -30s to -40s observed across the Susitna
Valley and Copper River Basin. A shortwave trough moving into
Southcentral from the north is supporting an enhancement of north
to northeast upper level flow as well, which is helping enhance
cold drainage winds flowing out of the Copper River Basin through
Thompson Pass and Valdez, as well as through parts of the
Matanuska Valley. This is causing some rapid fluctuation in
temperatures for places such as Wasilla at times, as winds
intermittently pick up and mix down warmer air above ground level.
Some patchy low fog/stratus has managed to persist under the low
level temperature inversion still in place from the Knik Arm and
Anchorage down into the western Kenai Peninsula.

&&

.MODEL DISCUSSION...

Model agreement is fair at the synoptic level for the first half
of the week, but there is still much up in the air with the exact
details for a few key features. A mesoscale low that has developed
in the Gulf quickly becomes subject to the first area of forecast
uncertainty, with models still not in agreement for the
trajectory of this low as it drifts generally northwest through
Tuesday. The NAM and Canadian models are attempting to accelerate
the low north into the Prince William Sound on Tuesday morning,
while the ECMWF and GFS stall out the center near Middleton
Island. This is most notably impacting the forecast for snowfall
across the western Kenai Peninsula and Prince William Sound
region. The scenario where the low accelerates north would likely
result in much higher chances for snow farther west into the Kenai
Peninsula. There is also disagreement for the track and strength
of a low expected to form along the front stretching across the
Bering from Tuesday into Wednesday, resulting in forecast
uncertainty for the strength and placement of stronger winds near
the low/front.

&&

.AVIATION...
PANC...Areas of thin, low stratus and patchy fog have held on
mainly towards the east side of Anchorage this morning. There is
still some potential for this to drift over the terminal and
occasionally drop ceiling/visibility to to IFR range. However, low
level northerly flow is likely to increase slightly through this
afternoon, and this should help mix out any low stratus still in
place later today.

&&

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

An upper level ridge continues to bring mostly clear skies and
below normal temperatures to the area. Patchy areas of fog and low
stratus remain possible today through Tuesday morning across the
Knik and Turnagain Arms, Anchorage, as well as the western Kenai
Peninsula. However, an upper level low will develop over the
Copper River Basin and meander near Prince William Sound through
Tuesday. At the surface, a couple of compact lows will move
across the northern Gulf very slowly to the west. These features
will produce areas of light snow near Cordova late today and into
Tuesday with a few inches of snow possible. There is model
uncertainty on how strong and where these lows will travel which
makes for a difficult forecast through Wednesday. Still, most
models keep the snow near coastal locations around Prince William
Sound and Kodiak Island while inland areas of Southcentral remain
dry with continued below normal temperatures. On Wednesday a ridge
will move into the Gulf from the south and will push the upper
level low inland and cause it to weaken which will also weaken the
surface lows across the northern Gulf. The Mat Valley winds will
blow near Palmer and Wasilla which will increase temperatures.
Additionally, gap winds will increase a bit for Valdez and
Thompson Pass which could bring some areas of blowing and drifting
snow, but overall visibility should remain favorable with little
if any impact.

&&

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

Cold and clear conditions persist across the Southwest Mainland
as a result of a large blocking ridge encompassing much of the
western domain. Northeasterly offshore flow also continues along
the Kuskokwim Delta and Bristol Bay coasts. The Alaska Peninsula
continues to see relatively warmer temperatures, clouds, increased
chances for rain, and gusty easterly winds. The aforementioned
weather conditions will hold for much of the short-term period
while the upper-level ridge is gradually broken down by upper
troughs on either side. For Wednesday through Thursday morning, a
strong surface front lifts north to the Alaska Peninsula,
spreading clouds from south to north across the entire domain. The
heaviest precipitation will be focused along the Alaska
Peninsula, but parts of the Southwest Mainland may also see
increased chances for some precipitation. Northeasterly offshore
flow is expected to significantly increase Wednesday into Thursday
for the Kuskokwim Delta and Bristol Bay coasts with the arrival
of the front.

&&

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

The Bering Sea and Aleutians will continue to see more active
weather through the short-term period. An upper-level low
currently near the Kamchatka Peninsula will extend its front
across the Bering through Thursday morning. Multiple surface lows
are advertised to form along the front as the front itself
gradually orients east to west across the Bering by Thursday
morning. Widespread storm-force winds are expected across the
central Bering Sea on Wednesday and may extend into the eastern
Bering by Wednesday afternoon before gradually weakening on
Thursday. Temperatures should remain warm enough for the dominant
precipitation type to be rain, however a rain/snow mix is not out
of the question along the leading edge of the front. Rain will
likely transition into the wet snow across the Pribilof Islands
for Wednesday into Thursday.

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

A blocked, high amplitude ridge dominates over western Alaska
through the eastern Pacific. Offshore flow and bitter cold
temperatures prevail for inland Alaska going into midweek. The
trough sinking south across Alaska early in the week appears to
stall midweek and become elongated and stretched through the
Bering while the eastern Pacific ridge remains stubborn to
breaking down. Couple this with a large area of cyclonic flow over
the Aleutians and an enhancement of strong easterly winds look to
span across the Gulf and into the eastern Bering. The Bristol Bay
area and the Alaska Peninsula could have especially gusty winds
and a broad swath of precipitation Thursday.

Cold air drainage may continue through the rest of this week as
cold air sits over the interior. Any enhancement to the pressure
or temperature gradient could help to accelerate air through wind
prone gaps over Southcentral and near the Gulf coast.

A similar repeat in this pattern is possible going into next
weekend. A Canadian trough sinking south and stretching west into
the Bering. The challenges under this synoptic regime is whether a
low will suppress the eastern Pacific ridge enough to bring a
system to the Gulf. It appears possible at the end of the week.
There is still a lot of uncertainty associated with the synoptic
pattern during this timeframe. The track of any low east could be
blocked and progression of a low could be bumped north or west.

&&

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

&&

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

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
327 PM AKST Mon Dec 19 2022

AKZ121-220100-
Western Kenai Peninsula-
Including the cities of Kenai, Soldotna, Homer,
and Cooper Landing
327 PM AKST Mon Dec 19 2022

...Potential for ice jams on the Kenai River from Kenai Keys to
Soldotna...

Observed and forecasted abnormally cold temperatures across the
Western Kenai Peninsula through the remainder of the week will
increase the potential for freezeup ice jams along the lower
portion of the river from the Kenai Keys downstream to Soldotna. A
local observer reports the river is mostly frozen downstream of
the Soldotna Bridge, but remains mostly open upstream to the Kenai
Keys. This sets the stage for the potential of anchor ice buildup
on the river bed and hanging ice dams along the river that could
cause localized overflow flooding. Locations that have had
previous flooding issues due to ice buildup include the Big Eddy
area and the Riverside Drive/Eddy Lane Subdivision.