Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 301421
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
521 AM AKST Wed Nov 30 2022

.ANALYSIS AND UPPER LEVELS...

A strong, longwave ridge remains firmly in place over most of
Mainland Alaska and across most of the Bering Sea this morning.
The ridge axis is beginning to nudge farther east and weaken
slightly in amplitude, and this is allowing upper cirrus to spill
around the ridge farther into Southcentral compared to yesterday.
The presence of even this thin, upper level deck is having a
noticeable impact on surface temperatures by preventing the
surface from efficiently radiating heat out into space.
As a result, temperatures across Southcentral are a few degrees
warmer this morning compared to 24 hours ago, mainly in the single
digits above/below zero Fahrenheit. The one exception is over the
Copper River Basin, where skies remained clearer for longer
overnight and allowed temperatures to fall into the -10s to -20s.

Out to the west, temperatures across Southwest are mostly in line
with Southcentral, with much a much warmer air mass in the 40s
continuing to spread across the Bering and Aleutians underneath
and just upstream from the upper ridging aloft. The next key
player in the forecast for much of the outlook area today and
tonight is now approaching the western coastline as well, with a
potent upper trough now pushing east past St Lawrence Island and
into the Norton Sound and Y-K Delta. At the surface, a front
extends from the shortwave`s attendant surface low near the Bering
Strait down across the western Aleutians, with mainly rain along
the front beginning to reach the Pribilof Islands. This is the
same system set to bring a variety of impacts to the southern
Mainland today, with a short period of mixed precipitation pushing
past Southwest, then light snowfall across Southcentral this
afternoon.

&&

.MODEL DISCUSSION...

Synoptic agreement among models remains well above average through
the end of this week, supporting continued high forecast
confidence. However, differences have persisted in some of the
finer scale details with the progressive shortwave crossing into
Southcentral this afternoon and evening. Models are struggling to
ascertain how quickly the very dry air mass in place will saturate
down to ground level as snowfall begins aloft, and in terms
resolving how much moisture will be able to advect east-
northeast across the Alaska Range ahead of the trough passage.
This is leading to slightly lower confidence for snowfall amounts
associated with this system across Southcentral through this
evening.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist until later
this afternoon. Starting between about 3 PM and 5 PM, ceilings
will drop and snowfall will begin quickly, resulting in a sudden
dip to IFR to MVFR conditions. This snowfall may be a bit
`squally` in nature, with rapid fluctuations in visibility
possible along with slightly increased and erratic winds as bands
of snow move past the terminal. Conditions should rapidly improve
back to mainly VFR as snow ends this evening. Winds will also pick
up out of the north tonight as the snow and lower clouds clear
out, with gusts to around 20 kts possible through Thursday
morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Wednesday through Friday night)...

The upper-level ridge providing the generally clear, dry, and
very cold conditions will crest over the region today as an upper-
level shortwave and associated surface low move to the northwest
coast of Alaska. High clouds are beginning to spill over the top
of the ridge this morning, and this trend in increasing clouds
and lowering cloud heights will continue through the first half of
the day. Temperatures will also slowly begin to warm up through
the day in response to both the increasing cloud cover and the
cold northerly flow aloft becoming more west-southwesterly.

As the aforementioned low moves inland, its front will sweep
across the western half of the state, running up against the
Alaska range. Lift ahead of the upper-level wave and moisture
advecting in from the southwesterly flow ahead of both the wave
and surface front will allow for the development of widespread
snow across Southcentral. The snow is expected to begin late
this afternoon across Cook Inlet and the Susitna Valley, spreading
east to Prince William Sound overnight and into the Copper River
Basin by early Thursday morning. The upper-level wave, while
quite strong, is fast-moving and running into a very dry airmass.
As such, it may take a while for the atmospheric column to fully
saturate and allow for snow to reach the ground. However, once
this happens there may be a brief period this evening of heavier
snowfall, especially across the Anchorage Bowl and Matanuska
Valley, allowing for a general 1 to 3 inches of snow accumulation
in just a couple of hours. Higher amounts of snow are possible in
favored upslope locations and at higher elevations as the winds
aloft turn back to the northwest in the wake of the passing upper-
level wave.

As the wave departs and colder air advects back over the region,
expect a return of strong offshore winds through mountain passes
and along the coast. Winds could gust as high as 50 mph for places
like Seward and Whittier beginning Thursday morning. Wind gusts
as high as 75 mph are possible through the day Thursday for
Thompson Pass and Valdez. Areas of blowing snow are also possible
for Thompson Pass, Valdez, and Portage Valley at the onset of
these winds due to the snow from the previous event being lofted
in the gusty winds. These winds will taper off on Friday as the
cold air advection shuts off.



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

A frontal system will move southeastward across Southwest Alaska
today. This frontal system will bring a mix of wintry
precipitation to the region. Freezing rain is expected across much
of the Kuskokwim Delta and Bristol Bay. Winter Weather Advisories
are in effect for both the Kuskokwim Delta and Bristol Bay for
this morning and afternoon for trace amounts of ice accumulation.
Colder temperatures across the Kuskokwim Valley will result in a
mix of snow and sleet. A quieter weather pattern will then
persist for Thursday and Friday across Southwest Alaska.

&&

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

An unsettled but benign pattern continues across the
Bering/Aleutians through Friday evening. Winds will generally
remain below gale force with no high seas expected. The next low
is expected to move over the western Bering/Aleutians and bring
gale force winds to the area. Forecast confidence is low at this
time with respect to the placement and intensity of this low
pressure system. We will continue to monitor this low and will
keep you well advised.

&&

.LONG TERM FORECAST (Days 4 through 7: Saturday through Tuesday)...

The long term is all about the high amplitude upper level ridge
drifting over southern Alaska this weekend and then the way it
breaks down next week. The short summary for the long term days
is that there will be a generally sunny weekend with more mild
temperatures than currently around southern mainland Alaska.
Increasing clouds are likely for the early part of next week for
southwest Alaska and we will have to see about Southcentral.

Now for the details: As with the warm ridge a week or two ago, the
ridge will bring a strong inversion over southern Alaska with
warm temperatures aloft and colder temperatures at the surface.
What will differ with this ridge is that it should not last quite
as long though there will be a day or two with quite warm
temperatures aloft. Saturday should see this inversion entrenched
over southern Alaska with 850 mb temperatures from 0 to +4
Celsius from the Kuskokwim Delta eastward to the Kenai Peninsula
and north Gulf Coast. Most surface temperatures at sea level
should be well below freezing at the same time for these areas. By
Sunday, the ridge will amplify into the interior of the state as
it squeezes eastward resulting in these warm temperatures aloft to
move north and eastward for Sunday. The ridge moving eastward is
due to a strong low moving into the western and central Bering Sea
Saturday night which will keep the storm track over the Bering Sea
for at least the first half of next week. Monday and Tuesday is
where the larger uncertainty really starts to creep in as much
colder air aloft from the west associated with the upper level
part of the Bering Sea low moves into Southern Alaska. The big
question is whether this will displace the upper level ridge all
the way into Canada or whether the ridge holds firm over the AlCan
border area. The first solution that breaks down the ridge will
usher in the chance for light precipitation over both Southwest
and Southcentral Alaska Monday or Tuesday. If the ridge holds firm
it may allow more warmth and moisture transport from the south
which could bring heavy precipitation to the Alaska Peninsula to
Kodiak Island, though it would keep inland parts of Southcentral
with less clouds and no precipitation. This is still about a week
out so there is time for this to show its hand as to which pattern
will set up over the region.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory: 155,161.
         Flood Advisory: 145.
         High Wind Watch: 131.
MARINE...Storm Warning: 130,131.
         Gale Warning: 120,121,127,129,132,136-138,150,351,352.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...AS
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ED
LONG TERM...EZ


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