Storm Warning 188, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
432 PM AKST Fri Dec 6 2024

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

Key Points:

* Another round of freezing rain and drizzle is possible late
  tonight into early morning Saturday for Anchorage and the Mat-
  Su Valleys.

* A wintry mix of rain, freezing rain, and snow is possible along
  the Seward Highway corridor from Turnagain Pass to Moose Pass
  Saturday due to cooler temperatures and snow levels dropping to
  around 1,000 ft.

* Despite a brief, dry and cold day Saturday, the unusually warm
  and wet pattern will continue to bring a wash, rinse, and repeat
  cycle for Southcentral into the week ahead. The next system
  arriving Sunday through Tuesday.

Discussion:

Weak, southeasterly flow across Southcentral is leading to
showers for the Kenai Mountains and Prince William Sound. Upslope
flow against terrain is the key feature allowing precipitation on
the windward side of the mountains and for the leeside to be
fairly dry. Precipitation has redeveloped along the Alaska Range
of the Susitna Valley. Much of this precipitation is running into
drier air nearer the surface and is not reaching the ground for
most of western Kenai Peninsula, Anchorage, and MatSu Valley. A
second wave, along with the remnants of the surface front, may
bring another round of very light freezing rain to these same
areas later tonight into early Saturday morning.

For the eastern Kenai Peninsula, this second wave will also
advect slightly cooler air both at the surface and in the mid-
levels, helping to drive snow levels down to around 1,000 ft.
Although there is a definite trend in cooler temperatures for the
eastern Kenai Peninsula, the extent of colder air is still
uncertain. This is leading to a tricky precipitation type forecast
starting this evening along the Seward Highway corridor,
specifically whether any rain and/or freezing rain changes over to
snow.

Looking ahead, a shortwave ridge could bring another round of
patchy fog for Saturday, though confidence at this time is low.
Cloud cover could stunt development. Yet another system is
scheduled toward the end of the weekend, reaching Kodiak on Sunday
morning. Gale force winds are expected along the front and ample
moisture looks to be associated with this system. Again, warm
southeasterly air looks to advect into Southcentral. Recent model
runs have slowed the progression slightly and added a slightly
southerly component to the flow ahead of the front. Confidence in
the timing of its passage could change, though Sunday to Sunday
afternoon appears likely for Kodiak and Kenai Peninsula, spreading
across Southcentral Sunday afternoon and night. Warm air advection
is also likely to raise snow levels to around 2000 feet or higher. Midlevel
temperatures also vary and precipitation type challenges seem
moderately likely. Though, this system appears to have more
energy involved and precipitation changes could switch more
quickly, especially for areas susceptible to southeasterly and
warmer onshore flow. If precipitation rates for areas close to
freezing fall heavy enough, temperatures could cool to freezing
and fall as snow. This is a challenging forecast ahead, please
stay winter weather prepared and check for updates to the
forecast.

-TM/Rux

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

A col in the Central Bering is flanked by two weak low pressure
systems, with one currently south of the western and central
Aleutians and the other just north of Nunivak Island. A chance for
brief showers across the Southwest mainland through Saturday
remains as the trough over the region continues to weaken and
meander slowly northward out of the area. In the Bering, the
surface low just south of Amchitka Island is producing small craft
winds and light rain in the western and central Aleutians. This
low will continue to slowly shift eastward before beginning to
dissipate and becoming absorbed by a northward moving North
Pacific low. A ridge building over the Alaska Peninsula and
southwest Alaska Saturday will allow for calmer and drier
conditions. This ridging and break in the weather will be brief
and is quickly pushed out Sunday morning into Sunday afternoon by
the aforementioned North Pacific low that will continue to head
northwards into the Bering by Sunday night.

This North Pacific low will send bands of precipitation across
Southwest Alaska, the Eastern Aleutians, and the Alaska Peninsula.
The Bering could see a brief period of mixed rain/snow before
warm air advection occurs and transitions the precipitation fully
to rain. Gale force winds are expected accompany this low and will
set up on the southern coasts of the Aleutians and the Alaska
Peninsula as well as in Bristol Bay. Locations such as Cold Bay
are expected to see a period of strong winds with gusts to 50-60
mph possible by late Saturday night/Sunday morning.

On the mainland, a brief period of a rain/snow mix is possible in
Bristol Bay Sunday morning, but a quick transition to plain rain
is expected given the strong downsloping southeasterly winds
quickly warming the region to above freezing. The strength of the
southeasterly downsloping will limit the chances for precipitation
for much of the Bristol Bay region, particularly for the King
Salmon area. As the low lifts northward Sunday morning into
Monday, a favorable pressure gradient will increase the winds
through Kamishak Gap into portions of Bristol Bay with wind gusts
up to 40-50 mph. In the Kuskokwim Delta, mainly snow is expected,
but a small possibility of a short period of snow mixed with
freezing rain is possible in the Bethel region Sunday afternoon
before quickly transitioning to rain for any precipitation that
continues to fall. However, this is highly dependent on how fast
and how warm the temperatures can rise and how much downsloping
the area experiences once the winds turn southeasterly. Snow mixed
with freezing rain also remains possible for an area
approximately from Aniak to Sleetmute through Monday. Regarding
the warm-up expected with this system, the high temperatures for
most of Southwest will be near 25 degrees F Saturday, whereas
Sunday's high temperatures are expected to be near 35 degrees,
with a trend towards warmer temperatures possible heading into the
workweek.

-JH/CL

&&

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

Operational models continue to show negatively tilted 500 mb
troughing across the Aleutians into the Gulf with its associated
upper low moving across the Aleutians. Main weather story is a
deep frontal occluded low tracking the Aleutians. Sufficient upper
level forcing will support rain/mixed precip (locally heavy)
along the coast with snow farther inland. There is some degree of
uncertainty that needs fine tuning along the coast where it is
possible these areas could see more snow than expected. These
areas include Alaska Range, Eastern Kenai Peninsula and across
Prince William Sound to the Canadian border. Pressure gradient
tightens through the period in which wind gusts will felt up to
gale force from Kodiak Island extending northeastward to Prince
William Sound. Overall impacts should remain confined closer to
the coast and away from the northern mtns. The relatively warm
southerly flow complicates precip types during the period,
especially just inland from the coast.

&&.AVIATION...

PANC...VFR conditions and light northerly winds will prevail. Cannot
completely rule out a chance of freezing rain this evening, but
confidence and coverage remain much too low to include in TAF at
this time.

&&

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
148 AM AKST Sat Dec 7 2024

AKZ126-071600-
Western Prince William Sound-
Including the cities of Whittier, Seward, and Moose Pass
148 AM AKST Sat Dec 7 2024

...AREAS OF LIGHT FREEZING RAIN FROM MOOSE PASS TO SEWARD THROUGH
EARLY SATURDAY MORNING...

A wintry mix of rain, freezing rain, and snow continues across
the Seward Highway corridor. From Moose Pass to Seward,
temperatures overnight have dropped to freezing or slightly below
freezing, allowing some of the precipitation to fall as freezing
rain in these locations.

A light glaze of ice is possible for these locations before
temperatures warm to above freezing by mid- morning as the
precipitation comes to an end. Travel may be difficult in these
areas. Motorists are urged to slow down and use caution when
traveling.