Storm Warning 125, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
414 PM AKST Fri Nov 17 2023

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

Key Takeaways:

1) The coldest air of the season will arrive tonight and persist
through the weekend. Expect overnight lows ranging from 10 to 15
above zero along the coast, from 10 below zero to 5 above zero
from Cook Inlet to the Mat-Su Valleys, and 25 below to 10 below
zero for the northern Susitna Valley and Copper River Basin.

3) Strong northerly gap winds will develop along the coast from
Seward to Whittier and Valdez through Sunday. Expect blowing snow
along the coast of Prince William Sound and Turnagain and
Thompson Passes. Blizzard conditions are expected for Whittier and
Thompson Pass through Saturday.

A broad, longwave trough axis extending south through the Susitna
Valley and anchoring upper-level low over the eastern Kenai
Peninsula continue to slowly lift to the north and east this
afternoon. Areas of snow showers persist from Eagle River south
across the western Kenai Peninsula, due to a now weakening meso-
low that moved onshore north of Kenai this morning. Snow showers
are also persisting across the northern Gulf, moving north to
Cordova and Glennallen due to a surface low near Middleton
Island.

These snow showers will slowly taper off from west to east, with
snow showers lingering over the eastern Copper River Basin through
Saturday as the upper-level low exits.

As the low moves east, a ridge will build over interior
Southcentral while a surface low remains in the northern Gulf.
The result will be a rapid development of northerly gap winds for
mountain passes and gaps along the coast. These wind swill peak
by early Saturday afternoon for Seward and Whittier, with the peak
of winds arriving late Saturday evening for Valdez and Thompson
Pass. The combination of recent snow from Whittier to Valdez will
result in widespread blowing snow through Saturday with blizzard
conditions expected for Whittier and Thompson Pass.

Temperatures will also continue to fall as 850 mb temperatures
fall to near -20C through the weekend. This will result in the
coldest temperatures so far this season.

Winds and coastal precipitation will likely return by Monday as
frontal system moves into the northern Gulf.

TM

&&

.SHORT TERM FORECAST SOUTHWEST/BERING SEA/ALEUTIANS (Days 1
through 3)...

Nearly the entire region will continue to see fairly benign
conditions for about the next day or two. Widespread ocean effect
snow showers generated by cold air moving over the comparatively
warm sea surface will continue to drift into the Aleutians and
Alaska Peninsula, as well as along parts of the Southwest coast at
times. This convective activity will be further aggravated as an
upper level shortwave crosses the Bering from northwest to
southeast. Within the upper shortwave are two embedded shortwaves
that have developed on satellite imagery. These will continue
move southeast, bringing snow showers past the Pribilofs this
evening, Kuskokwim Delta overnight tonight, then into the central
and eastern Aleutian Chain with lingering showers for the
Kuskokwim Mountains, due to upslope flow for Saturday morning.
Some snow showers could make it into Bristol Bay for early
Saturday morning.

Low temperatures will likely be the coldest early on Saturday,
dipping into the single digits above/below zero away from the
immediate coastline and especially in the Bristol Bay to
Kuskokwim Valley and western Aleutian range. The Kuskokwim Delta
and coasts could see temperatures in the teens and twenties. In
general, daytime highs will be 10 to 15 degrees warmer.
Temperatures will gradually increase again Tuesday.

Attention from there shifts to the next notable weather maker
arriving out in the western Bering Sea this weekend. By Saturday
night, a deep 940s mb low moving east of Kamchatka will send a
very strong front into the western Bering Sea and Aleutians.
Sustained winds well into storm force range will move first into
Shemya and Attu, then progress east with the front towards the
central Bering and out to Adak on Sunday as the core of strongest
winds weakens slightly. Gusts to hurricane force are likely to
affect the far western Aleutians and Bering as the front approaches
between Saturday night and Sunday, including in Shemya. Confidence
decreases as the front moves to the central Aleutians due to the
timing becoming less certain and the placement of the core of the
highest wind speeds potentially moving more north of the islands
and being more in the open Bering. An additional blast of winds
will ramp back up again over the southwest corner of the Bering,
to include Attu and Shemya, late on Sunday as the parent low
approaches from the south. A lot of warmer air and precipitable
water is associated with this low and rainfall could be heavily
at times.

AS/rux

&&

.LONG TERM FORECAST (Days 4 through 7: Tuesday through Friday)...

High confidence in active weather across Southern Alaska and the
Bering Sea for the long-term, though confidence remains low in the
finer details at this time due to model uncertainty. Here are the
hazards being monitored for this time period:

* Heavy precipitation along Eastern Kenai Peninsula and Prince
  William Sound from Tuesday through Thursday
* High winds across much of Southcentral Alaska from Monday into
  Tuesday

Tuesday begins with a 960-970 mb low moving north into Southcentral
Alaska. Some differences persist with timing and low position,
but there remains a consistent signal for sustained winds at storm
force (>= 50 kt) extending across much of the northern Gulf
coast, from Cordova in the east to Kodiak Island in the west.
Gusts could reach as high as hurricane force (>= 65 kt). In
addition to strong winds along the front, a tightening surface
pressure gradient between a 1020-1030 mb interior Alaska high and
this incoming low could contribute to gusty gap winds prior to the
arrival of this front, likely through Matanuska Valley and
Thompson Pass. These bear watching, especially as residual cold
air in the Copper River Basin could lead to a bora-type wind
event. Speaking of cold air, some uncertainty remains with
precipitation type as abundant moisture streams in with this
front. Will temperatures stay below freezing as this weekend's
cold air takes longer to scour out? Or could warm air advection
lead to mixed precipitation? Overall, while the position of the
low will have some effect on precipitation amounts, there seems
to be a decent downsloping signature, suggesting that much of the
precipitation will be focused along the north Gulf coast mountains
with not much making it inland into the primary Southcentral
population corridor.

Overall, the general trend for the long-term will include a warm-
up as the aforementioned Southcentral low moves in from the south
and an occluded low moves towards Western Alaska from the Bering
Sea. This warm-up will be further enhanced with the movement of
yet another low towards the forecast area by Tuesday. A North
Pacific low (with current guidance pointing towards minimum sea
level pressure of 940-950 mb) moves along the Aleutian Chain from
Tuesday through Friday. Model discrepancies with the positioning
of this low means that wind directions and speeds are difficult to
pinpoint at this time. However, operational guidance is pointing
at sustained winds of at least storm force (and perhaps even
hurricane force) for the Bering Sea and Aleutian Islands. It's
too early yet to delve into the forecast details. However, the
outlook is for strong winds along the Bering and Aleutians, and
for elevated winds (up to gale force) and abundant moisture to
sweep across Mainland Alaska along the low's front. Precipitation
type, again, will be something to watch as this front further
warms temperatures.

KC

&&

.AVIATION...

PANC...VFR ceilings and visibility will continue through the
forecast period. Winds will remain light and mainly from the
north.

CC

&&