Winter Storm Warning 5, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
455 PM AKST Sun Nov 3 2024

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

Most of Southcentral is in a brief break of active weather, with
an exception being the Western Kenai Peninsula which remains under
a Winter Weather Advisory until 9pm AKST this evening for Homer
north along the Sterling Highway including Ninilchik. Snow
accumulations of 3 to 6 inches are expected with higher amounts
along the Homer Bluff and Diamond Ridge. A weak, but broad wave
will shift northward tonight, overspreading areas of light snow
across Southcentral, including but not limited to, Anchorage, the
Susitna Valley, and the Copper River Valley. Snowfall over
Anchorage is expected to be light and brief, bringing 1 to 2
inches to the city and areas of hillside. The Susitna Valley will
see a slightly more prolonged period of snow, with a shortwave
lingering snowfall over portions of the Parks Highway near
Talkeetna into early Monday morning. Monday will see a return to
benign conditions across Southcentral, however yet another shift
towards active weather will come by late Monday evening.

A robust front will enter the region from the Southwest, moving
over Kodiak Island before midnight Tuesday. This system is
supported by a strong upper level jet, which will amplify the wind
produced, as well as a deep moisture tap from the Pacific, which
will similarly amplify the precipitation potential of the system.
Heavy rainfall and strong winds are expected to begin late Monday
night over Kodiak, persisting into late Tuesday morning as the
front exits and moves over the Kenai Peninsula. Offshore in our
marine areas, storm-force winds are expected off the eastern coast
of Kodiak as well as over the Barren Islands and east of the
Kenai Peninsula.

Early Tuesday morning the front will begin to move over the Kenai
Peninsula and spread heavy precipitation across the area. While
there may be a transition towards a wet snow later in the morning
in places such as Seward, Whittier, and Portage, the precipitation
associated with this system is largely anticipated to comprise of
snow. Due to the winds and snowfall expected, several statements
have been produced for the inclement weather Tuesday and Tuesday
evening. A Winter Storm Watch has been issued for the Eastern
Kenai Peninsula along portions of the Seward Highway, specifically
for heavy snow with accumulations of 10 to 18 inches possible at
higher elevations such as Turnagain Pass and and Summit Lake.
Another Winter Storm Watch has been issued for Thompson Pass, for
potential Blizzard conditions, winds gusting to 45 mph or more,
and snow accumulations of a foot or more possible. A High Wind
Watch has been issued for both eastern and western Turnagain Arm,
with the eastern sector showing potential for wind gusts up to 75
mph near Portage, and the western sector showing potential for
winds up to 70 mph for the higher elevations of Anchorage Hillside
including Potter Marsh and Bear Valley. Please see mentioned
products for more information.

Downsloping will mitigate the heavy precipitation for the Cook
Inlet areas and southern Mat-Su, but there is strong upper level
support so some rain and snow will likely still fall. Following
the relatively quick passage of the front, exiting to the north,
pressure gradient will relax some weakening the winds across the
area, and precipitation will be mainly upslope on southern facing
mountain ranges. A snow to rain transition is expected for any
areas of lingering precipitation as the front pushes north.

-CL

&&

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

Discussion:

Just a few areas of light snow still ongoing across Southwest Alaska
and the interior Bristol Bay area this afternoon as weak troughing
lingers over the region. Shortwave ridging moving in tonight will
finally push the trough north with precipitation ending. This will
be a brief respite as the storm-force low is still on track to
deepen as it moves out of the North Pacific and across the Central
and Eastern Aleutians, then tracking north of the Alaska Peninsula
towards Cape Newenham and Kuskokwim Bay. Models have finally
converged and are in good agreement with both the timing and track
of this low. A slight shift from yesterday's track now has the
surface low crossing over to the Bering side between Akutan and
False Pass and upper low tracking over near Dutch Harbor/Unalaska
with the system becoming more stacked as it nears Cape Newenham. The
low will bottom out around 950 mb just before crossing the Aleutians
and southern AKPen Monday night. Winds will quickly strengthen as
the system deepens and storm-force winds are expected to reach the
Eastern Aleutians early Monday morning, spreading into the
southern/eastern Bering, Pribilof Islands and the Alaska Peninsula
by Monday night. In addition, strong north to northwesterly winds
will also develop on the backside of the system so portions of the
Central and Eastern Aleutians will see a somewhat prolonged
period of strong winds from Monday morning through Tuesday night.
Kamishak Gap winds, as well and winds through the bays and passes
will also a period of gales to storm-force winds as the initial
front lifts through.

The other impact from this system will be precipitation amounts and
type. Enough cold air remains in place across the Alaska Peninsula
and Southwest Alaska that precipitation should initially start out
as snow with a transition to a wintry mix and/or all rain as warmer
temperatures move in behind the warm front. The Aleutian Islands
and southern end of the Alaska Peninsula will likely see more rain
or a rain-snow mix with only the higher elevations seeing all
snow. The northern Alaska Peninsula and Bristol Bay area will see
precipitation begin as snow will a transition to rain-snow mix or
all rain. As southeasterly to easterly winds increase over the
Alaska Range, downsloping should keep the bulk of the
precipitation on the western side of Bristol Bay (generally along
a line from Dillingham to Koliganek and west). Sufficient moisture
with this system will result in decent snowfall rates as the
initial precipitation band moves in and up to 5 inches of
snowfall is possible for lower elevations before warmer
temperatures nudge in. The combination of strong winds and snow
will result in blowing snow concerns for this area and as such a
Winter Storm Watch has been issued for Western Interior Bristol
Bay from Monday night through Tuesday morning. A Winter Storm
Watch has also been issued for Western Kuskokwim Delta and Nunivak
Island as this region is less likely to see temperatures warm
enough for a changeover from snow to rain. Gusts up to 50 mph and
modest snowfall amounts will create blowing snow and likely
blizzard conditions from late Monday night through Tuesday
afternoon. The Pribilofs are an area of uncertainty forecast-wise
as temperatures should stay in the mid 30s but dynamical cooling
will likely result in snow developing. Models also have the
deformation band right over the islands so temperatures and
snowfall rates will be a big factor in snow accumulations.

- PP

&&

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

A large low associated with the remnants of Kong-rey will remain
stationary just off the Kuskokwim Delta coast through the long
range period. Precipitation will be mostly in the form of snow
with a rain/snow mix in the coastal areas. Cold air advection will
slowly filter into Southwest Alaska through the period, which
will make any precipitation solidly snowfall in the Kuskokwim
Delta and Bristol Bay regions as the low remains in place. There
is some potential for coastal flooding in the Kuskokwim Delta
coastline near Kongiganak and Kipnuk on Thursday due to the high
tide and the low. However, with wind speeds of 25 to 30 mph, the
odds of flooding occurring is on the lower side.

A broad low will move into the Gulf of Alaska by Thursday. Gusty
gale force winds will affect Southcentral coastlines, especially
near Kodiak Island, the Barren Islands, and Kamishak Bay before
weakening on Friday. There is uncertainty with the track so the
winds may be weaker if the low takes an easterly track.
Precipitation across the Southern mainland coastline is expected
with this low. The coasts will see mostly rain with higher
elevations seeing mostly snowfall. Inland areas will be mainly dry
but if they receive any precipitation, it will be in the form of
light snow. Another broad low will impact Southern Alaska on
Sunday. There is even more uncertainty with the low, but models
show that it may be more impactful than Thursday's low. It will
bring strong gale force winds for the marine regions of Southcentral.
It will also allow for more heavy rains across the Gulf coastline
and snowfall further inland and at higher elevations. The end of
the period brings us back to the West, where another low passes
south of the Aleutian Islands in the north Pacific. Uncertainty
arises but this low may impact the Gulf in the future.

-JR

&&

.AVIATION...

PANC...Periods of light snow will continue through early Monday
morning as an upper trough lifts onshore of Southcentral. It is a
fairly complex set-up in the upper levels with an upper low
forming in the vicinity of Prince William Sound, stopping the
northward progression of the trough and creating some uncertainty
in where snowfall will persist. The end result will be some
variations in snowfall intensity and perhaps even some breaks in
snow which will lead to some ups and downs in ceiling/vis. Largely
expect MVFR conditions to prevail, but with light winds and a
moist environment, an IFR ceiling could develop as snow tapers off
Monday morning and linger through the morning hours. A slight
increase in low level flow should lead to improving conditions
Monday afternoon.

&&