Small Craft Advisory 77, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
419 PM AKST Tue Jan 21 2020

.ANALYSIS AND UPPER LEVELS...

A mainly zonal flow aloft extends from the Bering into southwest
Alaska today, turning more southwesterly for Southcentral. Closer
to the surface, infra-red satellite imagery shows a low moving
on shore across eastern Prince William Sound, with a pair of meso
(small scale) lows near the southern tip of Kodiak Island, and
also near Saint Matthew Island. Showery type precipitation is seen
in various data sets with all of these features.

&&

.MODEL DISCUSSION...

The models remain in good agreement through Friday evening,
before some differences begin to show up. Even then, they remain
in fairly decent agreement aloft with the overall theme of the
synoptic features. The main differences lie with regard to where
the center of a very deep mid-level low sets up. This occurs as
high amplitude negatively tilted ridging develops from the tip of
the Alaska Peninsula to the Pribilof islands. A disturbance riding
the top of the ridge dives into southcentral or southwest Alaska,
allowing for a 490-492 dm low to develop, as very cold air dives
south.

Initially, all models agree on this developing just south of
Seward. As the pattern amplifies due to shortwave energy diving
into the base of the trough on Monday, the low repositions to
either Shelikof Straight (European), central Bristol Bay (GFS), or
north of King Salmon (Canadian Global). This dictates who gets the
coldest of the air as 850 mb temps are projected to be around -26
deg. C. These are similar values, with a similar setup, to what
was seen across southwest Alaska around New Year's Eve, when wind
chills pushed or exceeded -40 deg. F. This is also a pattern where
several rounds of cyclogenesis (low pressure development) would
occur in the Gulf of Alaska, probably just south of Prince William
Sound. Should this occur, snowfall potential would increase for
Southcentral.

&&

.AVIATION...

PANC...Stratus clouds seen on satellite imagery, likely around
1500 feet, continues around the airfield and the big question is
how long it will linger. At this time it looks like these ceilings
will vary between 1500 and 3500 ft into tomorrow with periods of
light snow.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A negatively tilted upper level trough will slowly adjust eastward
across southcentral. There are a number of embedded shortwaves in
this feature. The upper trough itself extends from NE Russia to
south of the Alaska Range and will be the impetus for ushering
Arctic or modified Arctic air into the region. Upward vertical
motion associated with the shortwaves could trigger snow showers.
But with the Arctic air settling southward anticipate increasing
offshore winds and usher in a drier air mass. While a few snow
showers could still linger over portions of the Copper River basin
and along the coast from Prince William Sound eastward later in
the forecast period.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3:Tuesday
night through Friday)...

Snow showers, low stratus and patchy fog are lingering along the
southwest coast today in association with a weak low pressure system
currently over Nunivak Island. This low will move ashore tonight,
clearing out snow showers. However some low clouds and patchy fog
will remain overnight into tomorrow morning along the coast.
Interior areas are still seeing calm weather and cold temperatures
in valleys; this is expected to continue.

A change in the pattern is expected, returning to very cold air
later this week. An area of high pressure is expected to set up
over Interior AK Wednesday into Thursday. This will create brisk
northeasterly flow over Southwest AK, bringing in very cold air.
Winds and temperatures with this flow have potential to create
wind chill advisories for southwest AK on Friday.

&&

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

High pressure currently exists over the Bering Sea, with some
low-level stratocumulus trapped under it. This high will slowly
move east over the next couple of days, allowing a gale-force low
to move into the Western Aleutians late tomorrow into Thursday.
High pressure over mainland Alaska will push very cold, northeasterly
flow into the Eastern Bering starting Thursday. This will prevent
the low from moving eastward, effectively stalling it as it encounters
this wall of cold air. However, the low maintains is strength so
expect gale-force winds to continue for the Western and Central
Bering through Friday. Meanwhile, cold air and heavy freezing
spray will be in place for the eastern Bering starting Thursday.

&&

.MARINE (Days 3 through 5: Friday through Sunday)...

Gulf of Alaska:

Weak low pressure will persist over the Gulf of Alaska Friday
through Tuesday. Strong high pressure over the mainland coupled
with the low pressure in the Gulf will produce strong outflow
winds, especially out of the Copper River Delta and out of
Kamishak Bay through the Barren Islands that should produce gale
force wind Friday and Saturday.

Bering Sea and Aleutians:

A strong and nearly stationary low centered south of the western
Aleutians will keep widespread gales with occasional embedded
storms ongoing across the central and western Bering Friday. The
Gales will likely be over the western bering Sea Saturday and
Sunday. Seas of 20 to 35 ft are expected over a large area of the
central and western Bering and Aleutians Friday. Strengthening
high pressure over the eastern Bering may cause near gale force
wind out of the Kuskokwim Delta Friday night through Sunday.
Offshore gale force wind is also possible in the Bristol Bay area
Friday through Sunday.

&&

.LONG TERM FORECAST (Days 3 through 7: Friday through Tuesday)...

The big picture will be low pressure persisting over the Gulf of
Alaska and south of the Aleutians with strong high pressure over
the eastern Bering Sea and deep cold air over the mainland. For
the mainland this translates to temperatures running around 20
degrees below average through the weekend. This setup favors the
outflow gap winds through the coastal mountains with precipitation
mainly along the coastal regions of the Gulf. Models are in good
agreement with the synoptic features through Sunday then diverge
with respect to the low pressure center location south of the
western Aleutians beginning Sunday night. However, the impact of
these differences is minor, and limited to the western Aleutians.
Where the lows end up will decide whether gales occur over the
western Aleutians or not.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 173-178
		 Heavy Freezing Spray Warning 129 130 160
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JW
SOUTHCENTRAL ALASKA...PJS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
MARINE/LONG TERM...SA