Small Craft Advisory 823, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
451 AM AKDT Thu Oct 29 2020

.ANALYSIS AND UPPER LEVELS...

At the upper levels, a shortwave trough is digging into Seward
Peninsula with this trough heading towards Southcentral Alaska.
Further west, an upper ridge extends from eastern Siberia through
the northwestern portions of the Bering Sea. A deep upper low is
centered just to the south of the western Aleutians. A weak upper
trough is found across the Gulf of Alaska.

At the surface, a low remains stationary southeast of Cordova
producing rain showers along the northern Gulf coast and
spreading clouds and few scattered snow showers through the Copper
River Basin.

Southcentral continues to experience very light northerly flow
and fog/stratus have once again spread across much of the Mat- Su
Valley, Anchorage Bowl and into portions of the Kenai Peninsula.
Widespread stratus continues to overspread much of Southwest
Alaska under a very weak flow regime.

Further south and west, a fully developed surface low is
gradually moving northeast towards the Western Aleutians from the
North Pacific. Rain and gusty winds are beginning to overspread
the central Aleutians as the front associated with this low lifts
northward.

&&

.MODEL DISCUSSION...

Model agreement is reasonably good over the next couple of days.

Outflow winds will increase across coastal Southcentral today into
early Friday as a shortwave trough digs into the region bringing
some stronger cold advection along with it.

There is also good agreement on the track/strength of the storm
force low approaching the central Aleutians currently. All models
are in agreement on this storm tracking south of the Aleutian
Chain before weakening and heading southward into the North
Pacific late on Friday.

Models are starting to come into slightly better agreement on the
positioning of a new triple point low developing southeast of
Kodiak on Friday, with the general track of this low coming in a
bit further south/east versus previous runs. Confidence on the
track of this low remains low until we can see if these southward
adjustments have come to an end.

&&

.AVIATION...

PANC...Fog and low stratus persist into the early afternoon
hours, thinning out and ceilings lifting a bit for the late
afternoon/early evening. We continue to monitor the potential of
fog redevelopment Thursday evening/night, but we have moderate
confidence that increasing northerly winds will help keep the fog
at bay.

&&

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

The big story continues to be cold, cold, cold as an Arctic
airmass settles over mainland Alaska this weekend. While certainly
not unusual for late October, these type of deep cold air masses
are less common this early in the winter. Cold air advection has
been weak to this point. However, a short-wave trough dropping
out of Eastern Russia will dig across Southcentral tonight into
Friday and provide a much stronger push of cold air. Until then,
areas of fog, dense at times, will persist in the Matanuska
Valley, Anchorage, and the western Kenai Peninsula. As the colder
air arrives tonight, low level northerly winds will strengthen
and mix out the inversion and the fog. Coastal gap winds will also
ratchet up.

Meanwhile, areas of light snow will persist across the Copper
River Basin as weak cyclonic flow aloft acts on the existing low
level moisture. The arrival of the aforementioned upper short-wave
will tonight will increase the intensity of the snow just a bit.
Thus, do expect some minor snow accumulation (on the order of an
inch or so). Snow will then taper off Friday as the short-wave
exits.

Further south, a warm front extending from a deep low along the
Aleutians will reach Kodiak Island by early Friday morning.
The airmass in place will initially be cold enough to support
snow, but this will be very short-lived across the south end of
the island as warmer air quickly moves in. Kodiak City and areas
further north may see snow a little longer. The big limitation
continues to be the warm sea surface temperatures (in the mid to
upper 40s). As winds blow off the Gulf this will tend to warm
surface temperatures above freezing. Thus, continue to think
Kodiak City will see little or no snow accumulation. A triple
point low is expected to form east of the island Friday afternoon.
As this develops and tracks northeastward across the Gulf, low
level flow across Kodiak Island will back to the north to
northwest. This should allow a transition back to snow Friday
night before precipitation ends early Saturday. The overall model
trend for the track of this low has been farther south. THus, it
now appears eastern Prince William Sound and the Copper River
Basin will remain completely dry as this low passes to the south
Friday night and Saturday.

A much stronger trough will dig southward across the state Friday
night through Saturday, bringing a surge of even colder air, with
850 mb temperatures on the order of -16 to -20 degrees Celsius.
So not only will temperatures continue to drop this weekend, but
most areas will see some wind and it will feel quite blustery.
This cold air outbreak looks even more impressive than previous
forecasts, so have strengthened winds across the region, but
especially along the coast. With the upper trough settling right
over Southcentral, upper level forcing is far from ideal and do
NOT expect to reach high wind criteria (75 mph).

&&

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

An upper level ridge will continue to build into Southwest Alaska
throughout the day today, keeping cold and clearing conditions for
most of the region. However, an upper level wave will bring a
chance for the Kuskokwim Valley for the early part of the day.
Colder air will continue to filter into Southwest Alaska
throughout the week, keeping high temperatures at most locations
at or below freezing today and then dropping into the teens and
20s through the weekend. Winds and offshore flow will increase on
Friday as a low over the Aleutians moves eastward. The associated
frontal boundary will bring increased chances for precipitation
to the AKPEN on Friday through Saturday. This low system should
stay far enough south to keep dry conditions across the remainder
of Southwest Alaska through the weekend.

&&

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

Water vapor imagery shows a front associated with a North Pacific
low system beginning to move over the central Aleutians this
morning. Storm force winds and widespread rain will accompany this
system as it lifts northward today. The center of the low will
move just south of Adak by this afternoon and then continue on an
eastward track staying just south of the Chain through Friday
afternoon, when the system moves back into the North Pacific. The
storm force over the central Bering this morning will move into
the eastern Bering overnight tonight into Friday, before weakening
to gales late in the day as the system tracks southward.

Another system approaches the western Aleutians and Bering
Saturday afternoon, which will likely stay at gale force level.

&&

.MARINE (Days 3 through 5)...

Northwesterly gales are likely across the bays and passes of the
eastern Aleutians and southern coastal AKPEN late Saturday into
Sunday. Winds and waves over this area will generally diminish
through the day Sunday; however, some gales are still possible
southwest of Kodiak Island into Monday. A gale-force front skirts
the western Aleutians late Saturday and the central Aleutians
SUnday into Monday before before moving south.

For the Gulf, northwesterly gales are likely Sunday morning
into Monday across the northern Gulf, especially through bays and
passes and out over the Barren Islands.

&&

.LONG TERM FORECAST (Days 4 through 7)...

The main weather story moving through the weekend and into next
week will be the intrusion of arctic air across the Southern
Mainland in the wake of a vigorous surface low moving from the
western Gulf to the Alaska Panhandle. The core of the coldest air
looks to arrive on Monday as the polar upper-level low settles
over Southcentral. 850 mb temperatures (~4500ft AGL) look to
plummet to -18 to -20C across interior Southwest and over the
Anchorage Bowl and Kenai Peninsula. The Copper River Basin could
see 850 mb temperatures fall to -22 to -25C. This would translate
to highs struggling to climb into the teens to lower 20s for most
locations not immediately along the northern Gulf coast by early
next week. Daytime highs for parts of the Copper River Basin
during this time could remain near zero. Needless to say, low
temperatures would likely be in the single digits for most areas
and well below zero for the Copper River Basin and normally colder
locations like sheltered valleys.

Gusty outflow winds will also remain a potential along the
immediate coast through next week due to the build up of colder
air inland and a lingering gradient between an interior ridge and
trough along the coast. Models, however, are indicating that one
or more compact, meso-lows could spin up at the surface over the
northern Gulf due to the interaction of the upper-level dynamics
and the increased surface winds and thermal gradient. If this does
happen, there could be additional cloud cover that could help
moderate temperatures nearer the coast and also bring about an
increased chance for snow showers through early next week.

Farther to the west, a ridge of high pressure will build over the
western Aleutians and expand east by next week. Multiple
shortwaves diving south between the ridge and the aforementioned
polar low to the east will allow for clouds, snow showers, and
gusty northerly winds to persist from the central Bering to the
Southwest Alaska coast.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Dense Fog Advisory: 101 111 121.
MARINE...Storm Warning: 170 173-175 177.
         Gale Warning: 132 138 150 160 165 171 172 176 178-180.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BG
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...TM