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443
FXAK68 PAFC 211257
AFDAFC

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

.ANALYSIS AND UPPER LEVELS...

A mainly zonal flow aloft extends from the Bering into southwest
Alaska this morning, turning more southwesterly for Southcentral.
Closer to the surface, infra-red satellite imagery shows a low
moving 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, with the
nighttime microphysics imagery showing low stratus and/or fog
hanging just west of Anchorage in the cook Inlet, and also across
portions of the Copper Basin and Bristol bay regions.

&&

.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 to lurk just west of the airfield. Model
guidance has been split on whether this will make it into the
terminal. For now, plan to cover this with a tempo group and see
how things unfold. Light winds will prevail through the period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An upper level short wave trough will slide across Southcentral
from southwest to northeast today. Some showers and considerable
clouds will linger over the area. Tonight through Wednesday night
a long wave trough will slowly dig into southern Alaska. This will
bring increasing offshore winds and usher in a drier air mass.
Some showers could still occur in the Copper River basin and along
the coast from Prince William Sound eastward. Temperatures should
also start to cool on Wednesday and Wednesday night as the long
wave trough slowly slides east into Southcentral.

&&

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

Snow showers continue to linger across the Kuskokwim Delta area
this morning. While showers will eventually taper off, lingering
moisture in the area will keep low level stratus and patchy fog
around through Tuesday morning. By midweek, a broad upper level
trough arrives over Southwest. This will initiate a plunge in
temperatures across Southwest for the rest of the week as the
trough deepens and pulls down plenty of cold Arctic air from the
northwest. With a building ridge in place over the eastern Bering
Wednesday night into early Thursday, northerly winds are expected
to increase (especially along the coast and usual gaps and passes
along the southern Alaska Peninsula) through Thursday night. This
translates into a generally clear, but cold end to the work week
for Southwest Alaska.

&&

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

An unstable airmass over the Bering continues to allow for showery
conditions this morning. Scattered snow showers are expected to
continue over most of the northern Bering through early Wednesday.
The next storm arrives over the western Aleutians Wednesday
morning, bringing southeasterly gales. Snow will start initially
with a quick change to rain as warmer air moves in. As the low
lifts northward, the front spreads across the Bering through
Thursday. Expect high end gale force winds with this next system.
Storm force gusts are possible and will likely be concentrated
over the western Bering. This low will eventually move northward
as a blocking ridge remains in place over the eastern Bering
through the end of the week.

&&

.MARINE (Days 3 through 5: Thursday through Saturday)...

Gulf of Alaska:

A low over the eastern Gulf Thursday will turn westward and move
to near Middleton Island by Friday morning. The low will stall out
and remain nearly stationary through Saturday morning, then
dissipate or get absorbed into a new low moving into the eastern
Gulf Saturday evening. This will keep gales ongoing continuously
across the Barren Islands and through bays and passes southwest
of Kodiak Island through Friday. The gales may weaken to small
craft winds on Saturday, depending on where the low is at that
point and where the next low moves into the eastern Gulf.
Significant forecast uncertainty persists on 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 through
Friday. Seas of 20 to 35 ft are expected over a large area of the
central and western Bering and Aleutians through Friday night. By
Saturday the low will move to near Kamchatka, which will result
in gradually diminishing winds over the western Bering on
Saturday. Strengthening high pressure over the eastern Bering will
cause gale force offshore winds to develop Friday night through
Saturday from the Southwest Alaska coast. The gales will extend
south through the bays and passes of the Alaska Peninsula and
eastern Aleutians by Saturday evening.

&&

.LONG TERM FORECAST (Days 3 through 7: Thursday through Monday)...

The stagnant weather pattern intensifying on Thursday will persist
through the weekend, and potentially last even longer. A strong
and nearly stationary low centered south of the western Aleutians
will keep gale force winds and rain ongoing across the central and
western Aleutians through at least Friday night. On Saturday, the
low moves off to the Kamchatka Peninsula, but another North
Pacific low may move in to the same area to replace the old one
into Monday. There is significant model uncertainty about where
the new low tracks, however.

Meanwhile, across the eastern Bering and mainland Alaska, high
pressure will strengthen with time through at least Saturday.
After that there is some model disagreement as to how quickly the
high pressure moves westward into the Bering. The GFS is much
faster moving the high westward than the foreign models. Either
way, the high will support increasingly cold air flooding
southward on strong winds across Southwest Alaska, the eastern
Bering, and the gaps and passes of Southcentral from Saturday
through Monday. This will be a favorable pattern for another round
of wind chill hazards in these same areas.

Across the Gulf, the weather will remain unsettled as a low over
the eastern Gulf on Thursday moves to near Middleton Island on
Friday morning, then parks itself there and spins moisture, very
likely in the form of snow given the cold air around, into
coastal areas. Since the low will be relatively weak and hardly
moving, any snow that falls is not expected to become heavy, but
it looks to persist in some areas through Saturday. There is
considerable uncertainty on who gets what amount of snow and when,
so stay tuned for future forecast updates. By Monday, with the
high gradually drifting west over the Bering, the storm track may
begin to shift westward along with it into the Gulf and
Southcentral, which, if it occurs, would bring back more
seasonably cold temperatures, and much greater threats for snow
into Southcentral.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Heavy Freezing Spray Warning 160 185
         Gale Warning 176 177 178
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...JW