Gale Warning 786, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
445 PM AKST Thu Dec 22 2022

.ANALYSIS AND UPPER LEVELS...

A Rex Blocking pattern currently extends across the North Pacific
with high pressure extending from Siberia to across the Alaskan
mainland and low pressure to the south of the Aleutian Islands.
This pattern is resulting in rain and strong winds for the
central and eastern Aleutians.

Closer to the surface, radar shows rain slowly moving northward
along the Alaska Peninsula.

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.MODEL DISCUSSION...

Guidance remains in good agreement with respect to the placement
and intensity of synoptic scale features at both the surface and
in the upper levels. There is some model discrepancy on the exact
track of a low in the Gulf Friday into Saturday. Therefore, there
is moderate confidence at best with respect to the Friday night
through Saturday morning snowfall forecast for Kodiak Island.

&&

.AVIATION...
PANC...VFR conditions will persist for this TAF period.
Northeasterly winds will increase tonight into Friday. Confidence
is high that winds will meet and or exceed 35 kt Friday afternoon
and continue into the evening across the air field.

&&

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

The forecast is on track for a "bora" windstorm across Southcentral
tonight through Friday night, with widespread gusty winds and
very cold air temperatures and wind chill temperatures. The Arctic
trough diving southwestward across Southcentral does look a bit
stronger than yesterday, which is reflected in the strength of low
level cold air advection and strength of the low level jet. As a
result, maximum wind gusts on Friday could be a bit higher than
the forecast. The greatest impacts from this event will come from
reduced visibilities due to blowing snow. Secondarily, areas of
drifting snow could deepen and create an additional hazard. The
worst hit area will be the Matanuska Valley where some of the
strongest winds are expected. However, gusty winds will develop
across much of Southcentral tonight and Friday, with some of the
stronger winds expected over the northern Copper River Basin
(along the Alaska Range), through Thompson Pass/Valdez, the
Susitna Valley, north to west Anchorage and the western Kenai
Peninsula from Nikiski south to Homer.

Winds will begin to diminish friday night, but is will be a slow
process as pressure gradient remain tight with a surface low
crossing the Gulf. This low will bring snow back into the
forecast, arriving in the Copper River Basin and eastern Prince
William Sound areas Friday night/Saturday morning before spreading
west to the eastern Kenai Peninsula during the day Saturday. This
system will be fairly weak in nature, so snowfall will be light.
Low level offshore flow out ahead of the low will also tend to
maintain drier near surface air, which supports the idea of
lighter snowfall.

A much larger and stronger storm system will make its way into
the Gulf Saturday night through Sunday. With a far south storm
track, the strongest winds and heaviest precipitation will
initially remain down over the Gulf through Sunday. However, an
occluded front will spread out from the low center, bringing
precipitation to Kodiak Island Sunday and onshore along the
north Gulf coast by late Sunday. The airmass out ahead of this
will start out cold enough for snow, but there likely will be a
transition to rain along the immediate coast Sunday night into
Monday as warmer air moves in with the front.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: This afternoon
through Sunday afternoon)...

The gusty conditions from this morning have quieted down along the
Alaska Peninsula (AKPEN) as pressure gradients have somewhat
relaxed. Light precipitation returns are apparent on the King
Salmon radar over Bristol Bay land and marine zones this
afternoon, but due to dry air near and at the surface, very little
is reaching the ground. Any precipitation around Bristol Bay
comes to an end tonight as a Bering Sea system continues tracking
westward near the central Aleutian Chain. The AKPEN continues with
a rain/snow mix through Friday afternoon as easterly to
southeasterly upslope flow continues. A robust shortwave ejects
westward from an upper trough located over Yukon, Canada Friday
afternoon into the evening hours. This will spawn a strong low
pressure system over the AKPEN/Extreme southeastern Bering Sea
Saturday morning. At this time, it appears that coastal Bristol
Bay and southern Kuskokwim Delta locations stand the best chance
at seeing some precipitation in the form of snow early Saturday
morning. Amounts, if any, appear to be light due to the low being
too far off shore at this time. The potential is there for more
accumulation if the low forms further eastward and closer to the
coast. The bigger story for Saturday will be in the evening and
overnight as strong gusty northeast offshore flow strengthens.
Wind-chills could be quite low as cold air advects in from the
northeast. Conditions for Christmas Day are trending to be quiet
and cold for mainland Southwest, with the southern AKPEN remaining
somewhat active with scattered rain/snow showers.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: This afternoon
through Sunday afternoon)...

Active weather will continue for the Bering Sea and Aleutian Chain
through the short-term period. The heaviest rain has tapered off
from Unalaska/Dutch Harbor with only scattered showers around the
area. A flood warning is still out for that area until Friday
morning. The low pressure responsible for all the rain over the
eastern Aleutians over the past few days continues to elongate and
weaken as it slowly drifts northwestward. The Pribilofs continue
to see scattered rain and snow showers through day on Saturday
before tapering off in the afternoon and evening as multiple
shortwaves traverse westward over the central Bering. Some
uncertainty then ensues for Saturday afternoon through the rest of
the short-term period on Christmas Day. A robust shortwave ejects
westward from an upper trough in Yukon, Canada. The general
evolution takes the shortwave over Southwest and into Bristol
Bay/Eastern Bering where a strong surface low pressure is set to
spin up around the Alaska Peninsula. At this time, the exact track
of the surface feature is not well resolved, but current
indications are that the system will pull down quite a bit of cold
air from mainland Alaska and snow would be the dominant
precipitation type with this system. Please pay attention to
additional forecast updates as details continue to be nailed down
and the evolution of this potential winter storm become clearer.

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

Looking ahead in the extended forecast, the latest model runs and
ensemble guidance are in fairly decent agreement with the
evolution and progression of the synoptic features. An upper level
wave over northeastern Alaska will retrograde southwestward into
a closed upper low. Compared to previous runs where the center of
the low was over the eastern Bering, the models now track it to
the western Bering instead. The low will remain quasi-stationary
early next week, but models significantly diverge from Tuesday
onward. Meanwhile, the upper low will push storm systems from the
Pacific into the Gulf of Alaska. While specific details still need
to ironed out, most of the coastal areas will see some wintry mix
and inland areas will stay on the drier side. A general troughing
pattern associated with the aforementioned low out west will
result in an unsettled pattern for much of the Bering Sea and
Aleutians Islands heading into late next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC - MARINE - FIRE WEATHER:
Please visit weather.gov/anchorage for the current list

&&

SYNOPSIS AND MODEL DISCUSSION...ED
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DN
LONG TERM...MF