Gale Warning 731, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
430 AM AKST Thu Nov 17 2022

.ANALYSIS AND UPPER LEVELS...

Quiet weather continues to rule the roost for Southcentral Alaska
as a vigorous and formidable anticyclone is centered over the
ALCAN border. The inversions remain strong, especially in the
Copper River Basin where Gulkana currently observes low stratus
and lower visibilities trapped underneath the inversion. On the
west side of this powerful ridge is where deep southerly flow
takes up residence. The southerly moisture tap originates from
about 40N 140W. Currently, southerly areas of the Alaska Peninsula
(Chignik and Sand Point) exhibit rain and cloudy conditions due
to this pattern. King Salmon radar also shows scattered showers
moving from southeast to northwest over the Bristol Bay zones.
Meanwhile, the Kuskokwim Delta and Valley experience southeasterly
offshore flow due to the pressure gradient between low pressure
in the central Bering Sea and higher pressures over interior
Alaska. Areas of showers are just now being observed on the Delta
(Kipnuk) and the Valley (Aniak) in association with a wave lifting
northward from the Bristol Bay area. Further to the west, a short
wave lifts northward over the western to central Bering as the
Pribilofs observe cloudy and misty conditions with low cloud
ceilings. Also, an upper low in the North Pacific, well west of
Attu, is sending weak shortwaves northeastward providing mostly
cloudy conditions for the western and central Aleutians.

&&

.MODEL DISCUSSION...

Models are in decent agreement, synoptically speaking, through
Friday morning before discrepancies arise for Southcentral
Alaska. The 00z NAM and ECMWF take an upper level shortwave in
association with an upper low in the Gulf of Alaska westward
around the ridge axis to the north while the 00z GFS and Canadian
models take the shortwaves more northward and plow it through the
ridge through Saturday morning. The first scenario seems more
plausible than the latter. The forecast challenge in the short
term for Southcentral will be to nail down the details of the
exact track of the shortwave and associated surface low. The track
will have impact who sees how much precipitation this weekend.
While there is uncertainty, confidence is increasing on a period
of active weather for Southcentral, especially along the coast.

Out west, guidance has a pretty good handle on synoptic features
through Sunday morning. The uncertainty and forecast challenges
will reside shortwaves rotating around a North Pacific upper low
that slowly drifts east while remaining south of the central
Aleutian Chain through the weekend. All-in-all, confidence is high
for active weather across the Chain.

&&

.AVIATION...

PANC...VFR conditions and light northerly winds will persist
through the TAF package.

&&

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

A strong upper level ridge will remain firmly in control today,
but will shift eastward Friday through Saturday, allowing for a
pattern change and return of precipitation to Southcentral. For
today, expect a similar day to Wednesday, with abundant sunshine
and near normal to slightly above normal temperatures near sea
level. If you're headed into the mountains expect toasty
temperatures in the 40s from around 1000 feet elevation on up to
5000 feet. For the Copper River Basin, low clouds and areas of fog
will persist until the steep temperature inversion in place
weakens, which will be Friday afternoon at the earliest.
Meanwhile, further south over the Gulf, the upper ridge has pushed
the bulk of rain associated with a trough over the North Pacific
off to the south and west. Moist low level upslope flow will
persist along the ast side of Kodiak Island, so intermittent light
rain or drizzle is still possible.

Beginning tonight, a leading short-wave trough over the Pacific
will lift northward up the west side of the upper ridge, crossing
the Gulf on Friday. There is some uncertainty in the track of this
short-wave, but generally expect precipitation to arrive along the
north Gulf coast/Prince William Sound region late Friday afternoon
or evening. The short-wave should take a more westerly track and
weaken as it reaches Southcentral, thanks to strong ridging to
the north over the Interior. As a result, this first batch of
precipitation looks light and generally favors the mountains of
western Prince William Sound. The retreating of the ridge will
allow the atmosphere to cool Thursday night through Friday.
Therefore, by the time precipitation reaches the coast expect
snow levels of 1000 feet or lower.

A deep low will then move into the Gulf Friday night through
Saturday, with much heavier precipitation pushing onshore along
the north Gulf coast. Once again, there is some uncertainty in the
track of the low across the Gulf, making it hard to nail down the
forecast details. At this point in time, easterly low level flow
out ahead of the low will generally favor western Prince William
Sound/eastern Kenai Peninsula for the heaviest precipitation.
Marginally warmer air will accompany this low, causing snow levels
to rise, likely to at least 2000 feet. With a deep vertically
stacked low over the Gulf, it looks like the vast majority of
precipitation will remain along the coastal zone, with dry
conditions over inland areas of Southcentral.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Today
through Saturday night)...

An impressive ridge of high pressure remains positioned over
central Mainland Alaska. The influence of this ridge has
contributed greatly to the mostly warmer and drier conditions seen
across much of Southwest the last couple of days. The next system
to enter the Southwest airspace will be a shortwave crossing the
Alaska Peninsula this morning. This wave will bring a swath of
light precipitation from Dillingham to Bethel today and tonight.
This wave will exit west back into the Bering Friday morning
leaving Southwest with quiet weather for the beginning of the
weekend. Warmer temperatures will persist today and Friday due to
a persistent flow pattern. Temperatures currently expected to
begin to drop below freezing by Friday night for the majority of
Southwest Alaska.

Freezing rain was the focus of this forecast for Southwest Alaska,
but factors such as warm temperatures and consistent warm flow
have drastically reduced the potential for freezing rain the
remainder of this week into the weekend. Currently, precipitation
associated with the aforementioned wave into Southwest is
expected to fall as plain rain.

&&

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

A stationary front remains positioned in the central Bering,
continuing to bring chances for light precipitation to the Central
Aleutians. An impressive ridge over Mainland Alaska has deflected
some energy along the Eastern Aleutians. This system will provide
consistent light rainfall through today and tonight as it slowly
shifts westward, reaching Unalaska by Friday night. The next
feature to watch will be a North Pacific low that moves toward
the Western Aleutians on Thursday night. Expect small craft to
gale force winds along the Western and Central Bering and
Aleutians, with steady rainfall throughout the region to persist
into the late weekend.

&&

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

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. The
anomalously strong upper ridge over Alaska breaks down on Sunday.
In the short term, the main storm track will stay to the south
with low pressure systems tracking northward into the Gulf of
Alaska. Although there are positional and track differences with
these features, a couple of upper level shortwaves and surface
lows brings precipitation and gusty winds along the coastal areas
of Southcentral. As the aforementioned ridge shifts east early
next week, moisture and precipitation increases further inland and
into Southwest Alaska.

Out west, a deep trough and strong upper low over eastern Russia
moves southward into the Bering Sea between Monday and Tuesday.
For the eastern Aleutians and Alaska Peninsula, the main feature
to note is the colder air advection promoting localized gusty
winds out of the bays and passes. Cold, northerly flow over the
relatively warm waters will also help generate widespread showers
over the Pribilof Islands and along the Aleutian Chain. By
midweek, a ridge will build across the western Aleutians with a
brief break of drier conditions.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 131, 138, 150, 178, 411.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...DAN
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CL
LONG TERM...MF