Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 201437
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
537 AM AKST Sat Feb 20 2021

.ANALYSIS AND UPPER LEVELS...

A deep upper trough is now beginning to lift out of Southcentral
towards the AlCan border, although very cold, dry Arctic air
remains in place near the surface in the wake of the trough.
Temperatures by and large remain near or below zero across much of
the mainland. The cold temperatures are combining with the
northeast winds spreading away from the Arctic high centered over
the northern interior to create bitterly cold wind chills in
places, including along the Kenai Peninsula and across the Y-K
Delta. To the south, a rapidly moving low is racing just south of
Kodiak Island, where significant snowfall has been occurring over
the past several hours. The nose of a robust upper jet streak
extends over the surface low, maintaining both the low`s strength
and very fast eastward progression.

To the west, a series of occluded lows are tracking along the
Aleutians within a complex longwave trough. Another weak center
is drifting towards the Y-K Delta over the eastern Bering, and
band of light snow is moving into the northern AKPEN and Bristol
Bay with the low`s front as it begins to stall out and weaken.
This stalled front also stretches out across the Northern Bering,
remaining south of St Matthew island and allowing northeast small
craft winds and freezing spray to continue in the area.

&&

.MODEL DISCUSSION...

The most significant model related issue in the short term
continues to be centered around the potential for additional
snowfall near Kodiak. Models have been slow to latch on to the
progression of a low and supporting upper trough now passing south
of Kodiak Island, and they have not quite been keeping pace with
the very rapid movement of particularly the upper level wave.
Snowfall has been letting up in the past few hours with winds
shifting to the north around Kodiak City, and this is inconsistent
with the continued onshore flow and heavy precipitation signal all
guidance has continued through this afternoon. An additional
piece of energy moving in from the southwest this afternoon
complicates the picture even further, as it may encourage a
secondary low center to develop farther west as it approaches,
potentially prolonging the snowfall event later into the evening.
Therefore, confidence for this region over the next 24 hours
remains lower than optimal.

Aside from the continued uncertainty near the western Gulf and
around Kodiak Island, model agreement is very good with most
larger scale features of note through this weekend and into early
next week. Some disagreement persists for the evolution of the
smaller lows tracking across the Bering/Aleutians, but overall
confidence in the majority of the forecast over the next few days
remains good.

&&

.AVIATION...
PANC...VFR conditions and light north winds will continue.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Sat and
Sun)...A fairly unusual weather pattern is unfolding across parts
of Southcentral this morning. Arctic air has settled over the
northern half of the area. Meanwhile, a surface low pressure
center is skirting from the Alaska Peninsula quickly through
Kodiak and across the northern Gulf. The net result is the rare
combo of very cold temperatures across the population centers
with some breezy winds. This is driving wind chills to 20 to 30
degrees below zero in some places this morning. The coldest
conditions should abate after sunrise with some diurnal warming.

Kodiak Island continues to see bouts of snow this morning. The
first big surge moved faster than expected overnight dropping
approximately 3 inches. There will be a brief lull this morning as
winds have shifted out of the northwest. However, another surge is
starting to move towards Kodiak City and should allow for another
couple quick inches of accumulation before noon. The snow will
likely linger through the afternoon and into the evening, but
rates should decrease as the surface low tracks south and east of
the island and pulls some drier air in from the west.

As this low tracks north towards Prince William Sound tonight,
expect an uptick in gap winds near the coast (Whittier, Seward,
Thompson Pass). It will slide towards Yakutat and fall apart in
that region on Sun. As it does so, it will loft some moisture over
the Copper River Basin that will likely fall as some very dry,
light snow. Conditions will likely be a rain snow mix for Cordova.
Several more strong shortwaves will then proceed to drive across
the AK Range towards the Barren Islands late Sun into early next
week. Each one will serve to bring a reinforcing shot of Arctic
air which should result in periods of stronger winds and heavy
freezing spray over parts of the marine zones (Barren Islands,
Cook Inlet). With the exception of the Copper River Basin, the
reinforcing shots of Arctic air will continue to bring very dry
conditions too.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

A front is moving inland over the Bristol Bay region and Alaska
Peninsula this morning, spreading light snow in across the area.
Meanwhile, a cold Arctic airmass has settled in over much of
Southwest. As offshore winds remain elevated over the Kuskokwim
Delta today and tonight, a Wind Chill Advisory remains in effect
during the overnight hours as wind chill values drop as low as 45
below zero. Conditions will remain relatively quiet through Sunday
with clear and cold weather expected. By Monday, a broad low in
the Bering will push a front eastward, bringing the next chance
for precipitation.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

An active weather pattern will continue across the Bering as a
broad upper level trough remains in place. A low over the western
Aleutians will continue to track eastward, spreading snow along
the Chain as it moves into the Gulf late Sunday. In the wake of
this low, very cold northerly flow will fill in across the
northern Bering, with scattered snow showers likely tonight
through tomorrow. The next system will be a large area of low
pressure that moves in over the Bering late Sunday into early
Monday. The arrival of this low will create a more amplified
pattern, and will keep cold air in place across the Bering through
early next week.

&&

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

Bering Sea: A low weakens as it crosses into the Eastern Bering
for Wed. Models are similar, so confidence is good. Widespread
small craft winds and waves with areas of gale force winds cross
the Bering with the low and diminish Wed. Wave heights to 22 feet,
subsiding. A new low and front approaching from the west bring
widespread high end small craft winds with areas of gale force
winds for Wed. Wave heights building to 20 feet.

Gulf of Alaska: A Northern Gulf low with a front stretching
across the Gulf through Tue. Models differ somewhat, but
confidence is good. Widespread high end gale force winds and waves
south of the low over the Gulf diminish to small craft winds by
Wed.Wave heights to 22 feet, subsiding. A new front moves into the
Western Gulf Wed with widespread small craft winds spreading
eastward Wed. Wave heights building to 14 feet over the Southern
Gulf Wed.

&&

.LONG TERM FORECAST (Days 4 through 7: Monday through Thursday)...

A low in the eastern Gulf will bring light precipitation to
eastern Prince William Sound and the Copper River Basin on Sunday.
Northwesterly offshore flow looks to persist elsewhere across
much of the mainland through Monday with weak high pressure over
the interior. This will bring clearer skies and cooler
temperatures across Southwest and Southcentral Alaska. There
remains some disagreement into Tuesday as the ECMWF displaces the
upper level ridge faster than the GFS or Canadian and overspreads
snow well into the Middle Kuskokwim Valley. In any case,
precipitation will track eastward across the state Wednesday
before an upper ridge builds in on Thursday. As usual, the Bering
looks to remain active under a series of lows and fronts.
Confidence remains good regarding the overall longwave pattern,
but details are still lacking.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Wind Chill Advisory: 155.
  Winter Weather Advisory: 171.
MARINE...Gale Warning: 119,120,130,138,139,351,352,150,155,176,
 177,185.
  Heavy Freezing Spray: 121,127,130,139,141,160,180,185.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...AS
SOUTHCENTRAL ALASKA...KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MSO
MARINE/LONG TERM...MK/ED



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