Gale Warning 742, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
503 AM AKST Wed Nov 29 2023

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

A high amplitude negatively tilted trough extends from the
eastern Bering Sea southeastward into the northeast Pacific. A
corresponding high amplitude ridge is positioned downstream over
western Canada and up along the AlCAN border. A deepening low out
ahead of the trough is tracking northward into the Gulf of Alaska
this morning, gradually being pulled a little toward the west as
the trough continues to dig and become more negatively tilted. To
the north of the low, upper level deformation and low level
upslope and convergence are producing a narrow band of moderate
precipitation from the western Susitna Valley/Western Alaska Range
southward to Kodiak Island. Short-waves ejecting out of the
trough in the Gulf combined with low level instability are
producing showers along the north Gulf coast/Prince William Sound
and the eastern Kenai Peninsula. Some of these showers trained
over the same areas overnight, producing as much as 8" of snow in
the interior Kenai based on SNOTEL data. Widespread gusty
southeast winds which affected much of Southcentral yesterday are
finally starting to diminish as flow begins to back toward the
east to northeast in response to the low in the Gulf. Where the
wind is still blowing, temperatures are quite balmy - in the upper
30s to around 40. Temperatures are beginning to fall through the
30s as the winds diminish.

The big forecast challenge for today is the track of the low
across the Gulf today and then onshore of Southcentral tonight
through Thursday. There is still a fairly large spread in model
guidance, with hi-res guidance generally bringing the low track
further north and west this afternoon through tonight compared to
the global models. This greatly affects the marine forecast for
the Gulf offshore and coastal waters. However, the low position
also affects the northward progression of the leading frontal
system and the amount of precipitation and wind pushing onshore of
Southcentral today through tonight. For the sake of consistency
and desire not to chase any particular solution, have made little
change to the existing forecast which leans toward the more
northern solution with stronger winds and heavier precipitation -
especially across the eastern Kenai Peninsula. Where there is
agreement is that easterly upslope flow will strengthen ahead of
the low and front and upper level short-waves ejecting out of the
trough will drive northwestward from the Gulf and across the Kenai
Peninsula. When combined with and remaining low level
instability, this suggest a high chance of a period of moderate to
heavy precipitation this afternoon and evening. The heaviest
precipitation will be along western Prince William Sound, but some
of that will extend inland toward Turnagain Pass and east
Turnagain Arm areas. If the more southerly and easterly low track
wins out, low level flow will ultimately back toward the northeast
causing precipitation over the interior Kenai to diminish in
intensity sometime this evening. Precipitation will be primarily
in the form of rain near sea level, with snow levels rising to
around 1000 feet by this evening.

There may be some light spill-over precipitation, particularly
for the western Kenai Peninsula, but most areas inland from the
coastal mountains will stay dry. Eastern Prince William Sound
(Cordova/Valdez areas) will initially stay drier due to the strong
low level easterly flow, but steadier precipitation will move
onshore with the front tonight. As we head to Thursday, forecast
confidence takes a big drop, as guidance is really struggling with
where and when the low will ultimately track onshore. Keep an eye
out for forecast updates, as the forecast for Thursday and
Thursday night will likely change once we have a better idea of
where the low will track. The entire upper trough will lift north
and east in a much weakened form, bringing a chance of
precipitation to inland areas, but if anything does fall it will
likely be brief and light in nature.

A new trough will enter the Gulf from the Aleutians Thursday,
with potential for multiple lows to spin up along the leading edge
of the trough. Model guidance is really struggling with run-to-
run consistency Thursday through Friday. The latest trends
indicate a large well-formed low over crossing the southern Gulf
and perhaps producing some precipitation for Kodiak and the Gulf
coast. The upper flow looks quite zonal and progressive with the
arriving trough, so this favors confining the greatest impacts to
the Gulf and Kodiak, with cooler and quieter conditions across
Southcentral Friday on into the weekend.

&&

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

An upper-level trough will exit the Bering today and continue to
flatten across Southwest Alaska on Thursday while high pressure
lingers at the surface. Light winds and generally drier conditions
are expected for Southwest today through the end of the week.
Areas of fog and low stratus are likely to form during the evening
and overnight hours for various Southwest locations.

Further to the west, a strong front reaches the western Aleutians
this morning and will continue lifting east across the Bering and
rest of the Aleutians through the week. Steady precipitation and
gale force winds will accompany the front. The low itself will
track over the western Aleutians by early Thursday morning. West
to southwesterly storm force winds along the southern periphery of
the low are expected to reach the Pacific marine waters just
south of the western Aleutians late Wednesday night and early
Thursday morning. Models are indicating one or more additional
lows developing along the front as it extends east towards the
eastern Aleutians late Wednesday night. Depending on the location
of these lows, isolated areas with storm force gusts could be
possible, but honing in on their location is challenging at the
moment due to a large model spread. Nonetheless, the Bering and
Aleutians can expect to see generally active weather through at
least the end of the week with this system as it lingers over the
Bering.

-AF

&&

.LONG TERM FORECAST (Days 4 through 7)...

The long-term forecast period begins Saturday morning with fair
model agreement depicting a broad upper-level trough stretched
across the Bering Sea into the Gulf of Alaska, creating several
surface low pressure centers in the southeastern Bering and
northern Gulf. These lows will draw westerly gales across the
Aleutian Chain and easterly gales across the northern Gulf coast
into Prince William Sound. Generally a showery, wet pattern will
be in place across all of Southern Alaska, with areas such as the
lower Kuskokwim Valley, and Copper River Basin seeing the driest
conditions. Looking into Sunday model uncertainty remains fair,
with most solutions sinking the low in the Southern Bering across
the Aleutian Chain, into the northern-most Pacific, south of
Adak. By Monday morning, model disagreements become severe with
some solutions keeping a low in the Southern Gulf of Alaska while
keeping coastal Southcentral in a light rain and breezy pattern.
While other solutions take the low southeast, pushing into
Juneau's region before advancing northward towards Southcentral.
By Monday afternoon, models seem to converge on a solution
bringing a strong low pressure system towards the Southcentral
coastline. Heavy rains and strong winds appear to move in, however
timing the system's arrival is still debated. At latest, Tuesday
afternoon will likely be rainy and windy in the Prince William
Sound region. Overall, a wet pattern for the Aleutians and
Southcentral regions, looking colder and drier from mid to late
week for Southwest Alaska, and cooler and wetter for Southcentral.

CL

&&

.AVIATION...

PANC...Winds are expected to shift northerly around 15Z. They
will diminish this morning before picking back up this afternoon.
Otherwise, VFR conditions should continue through today.

KM

&&