632
FXAK68 PAFC 050213
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
513 PM AKST Wed Dec 4 2024

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

Key Points:

* Winter Weather Advisory for snow and freezing rain for Copper
  River Basin south of Glennallen until 1 PM AKST Thursday

* Winter Weather Advisory for rain falling on cold surfaces for
  Valdez up to Thompson Pass until 9 AM AKST Thursday

* Winter Weather Advisory for freezing rain in Anchorage and the
  Matanuska Valley from 9 PM AKST Wednesday to 3 PM AKST Thursday.

* Winter Weather Advisory for freezing Rain mixed with snow in
  the Susitna Valley from 9 PM AKST Wednesday to 3 PM AKST
  Thursday.

* Areas of mixed, wintry precipitation return to Western Kenai
  Peninsula, Anchorage Bowl, Mat-Su Valleys, and the Copper River
  Basin tonight into Thursday afternoon.

Discussion:

Currently, a lull in active weather is taking place due to brief
ridging across southwest Alaska. This will be short-lived as a
upper level shortwave trough pushes north into the region
Wednesday night. In the Anchorage region, downsloping easterly
winds will quickly shut off once a surface low associated with
the shortwave approaches from the south and precipitation will
begin to fall. This precipitation will be in the form of rain and
freezing rain in the Anchorage Bowl and the Matanuska Valley, and
any icing will be highly dependent on air and road surface
temperatures. The Susitna valley will initially see a mix of
freezing rain and snow due to colder temperatures. The Kenai
Peninsula will be mostly rain due to warmer temperatures but a
brief period of freezing rain cannot be ruled out in the northern
parts. The freezing rain will transition to rain once temperatures
warm enough Thursday afternoon before the precipitation ends due
to a return of downsloping easterly winds aloft as another low
approaches from the south. Winter Weather Advisories for the
aforementioned regions have been issued detailing the events.

Along the northern Gulf coast, including Prince William Sound,
moderate to heavy rain is expected. The Copper River Basin will
experience rapid warming due to pressure rises which will lead to
differing precipitation types, especially in Thompson Pass. First,
snow will fall, followed by a mix of freezing rain and snow, and
then freezing rain. This will all take place overnight and through
Thursday. After the precipitation ends, gusty southeasterly winds
will impact the Copper River Basin as the shortwave moves north
and a new low approaches from the south. Winter Weather Advisories
have been issued for The Copper River Basin, and Thompson Pass
due to these weather events.

It doesn`t stop there as the low that was hinted at earlier
pushes northward on Friday. This low will bring gusty gale force
winds to the coasts and usual gap regions as well as another
burst of precipitation. This low will mainly affect Kodiak Island
and Prince William Sound as Anchorage and the Kenai Peninsula will
experience downsloping. This precipitation will be mostly in the
form of rain as the warm air mass remains in place.

-JAR

$$
.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS
(Days 1 through 3: This afternoon through Saturday evening)...

Key Messages:

* Winter Weather Advisory now in effect until 12 PM AKST Thursday
  for Bristol Bay.

* Winter Weather Advisory remains in effect until 4 PM AKST
  Thursday for the Lower Kuskokwim Valley.

* Showery/unsettled conditions persist across mainland Southwest
  Friday and Saturday.

* North Pacific low lifts towards the Eastern Aleutians/Southern
  Alaska Peninsula this weekend.


Discussion:

A shortwave trough currently lifting across Bristol Bay this
afternoon is producing snow and mixed precipitation across the
region. This first round of precipitation should taper to light
scattered showers this evening before a second round of light
precipitation arrives early Thursday morning with more mixed
precipitation. For that reason, the Winter Weather Advisory for
Bristol Bay has been extended to 12PM AKST to address the freezing
rain potential with this second round of precipitation. The
Winter Weather Advisory for the Lower Kuskokwim Valley remains out
until 4PM AKST Thursday for multiple rounds of mixed precipitation.
This activity begins to taper off Thursday evening as low
pressure remains in the eastern Bering Sea through the first half
of the weekend. The result will be continued unsettled weather
across the Kuskokwim Delta and Bristol Bay Friday and Saturday.
With colder air aloft moving in, precipitation will be showery,
scattered in nature, and mostly in the form of snow.

Farther out to the west, A trough pivots southward across the
Central Bering Sea through Thursday and will help to generate rain
and snow showers across the Central and Eastern Aleutians through
Thursday afternoon. Northerly flow behind this frontal system will
help to keep scattered rain and snow showers going across the
Eastern Aleutians through late Friday night. Meanwhile, a
shortwave trough approaches the Western Aleutians Thursday night
with a series of surface low pressures spinning up south of the
area Friday morning. The result will be small-craft winds across
the Western Aleutians with pockets of gale-force gusts as well as
rain and snow showers Friday morning into Saturday morning. This
system will eventually be absorbed into a much stronger North
Pacific low which will lift towards the Eastern Aleutians and
Southern Alaska Peninsula Saturday. The gale-force front makes it
to the aforementioned areas Saturday evening with steady moderate
rain moving over the area by then.

&&
.LONG TERM FORECAST (Days 4 through 7: Sunday through Wednesday)...

A closed upper level low center South of the Central Aleutians
weakens and becomes part of an extended trough stretching across
the Bering and Alaska Peninsula into the North Pacific through the
forecast period. This pattern supports a number of shortwaves,
surface lows and fronts to track along the Southern portion of the
state through Midweek. Warm air spreads across the region into
the Interior through Wednesday with above freezing temperatures
continuing through the week. The sustained Southern airflow also
helps transport extra moisture over the AKPEN into Southwest
Alaska, and across Southcentral Alaska. Gusty winds less than gale
force and locally heavy rains are expected along the Eastern
slopes of the Alaska Range, Eastern Kenai Peninsula and across
Prince William Sound to the Canadian border. Areas of mixed
precipitation will occur further inland North of the Chugach
Mountains.

&&

.AVIATION...

PANC...Northerly surface wind will reestablish at the terminal by
early this evening and completely shut off the Turnagain Arm wind
and strong easterly flow across the mountains by late evening
allowing temperatures to begin to cool. Therefore, as
precipitation arrives (sometime between 06Z and 12Z) it will fall
as all rain. Whether it freezes at the surface or not will
heavily depend on the temperature of each specific surface. Rain
will continue through the morning hours, with quite a bit of
uncertainty on the exact end time. Some guidance shows
precipitation overnight cooling the column enough to cause
precipitation to mix with snow, but am skeptical that will occur.
The forecast ceiling/vis (bottoming out at MVFR conditions) are
based on a mostly rain forecast. If it were to turn to snow,
ceiling/vis would drop much lower.

&&


$$