Storm Warning 118, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
518 AM AKDT Wed Oct 4 2023

.ANALYSIS AND UPPER LEVELS...

High pressure stretches across the eastern Bering Sea and over
southern Alaska through this evening. Two low pressure systems are
moving into the region, one dropping out of Kamchatka this morning
and another moving out of the North Pacific towards the southern
Alaska Peninsula through Thursday morning. High pressure ridging
will continue to get shunted eastward through the end of the week
as a series of troughs rotate around the North Pacific low.

&&

.MODEL DISCUSSION...

Models are in good overall synoptic agreement through the end of
the week. There is some discrepancies later on Friday with the
North Pacific low as it crosses the Alaska Peninsula and moves
into the Bering Sea. The main difference is with low placement,
which ultimately results in uncertainty regarding precipitation
placement, but have come into better agreement with the low
strength.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. A steadier
north wind will develop by this evening and continue into
Thursday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3;
Today through Friday evening)...

Mostly clear skies this morning will slowly give way to clouds by
afternoon / evening as a frontal system lifts into the Gulf from
the North Pacific. Expect high end gales and widespread rain out
ahead of the front across the coastal waters. A small area of
storm-force winds is likely for Thursday morning across the Barren
Islands as the front begins to move to the Southcentral coast.
Widespread precipitation will move across Southcentral, first
for Kodiak Island later today, then along the immediate coast by
late Wednesday night, moving over the Susitna Valley and Copper
River Basin by early morning Thursday. While most locations will
remain warm enough for rain, colder air in place over the Copper
River Basin will likely result in snow. A mix of rain/snow may
also fall initially for parts of the Susitna Valley, especially
areas around Willow and Skwentna, before a changeover to all rain.
Accumulating snow should remain confined to the Copper River
Basin, where 1 to 3 inches could fall. 2 to 4 inches of snow are
are possible for Thompson Pass Thursday morning.

This is a complex storm system, with the surface front stalling
and weakening along the coast as its attendant upper-level
continues to lift into the interior of the state. As this wave
moves north, the steadier precipitation is expected to diminish in
coverage, especially for interior locations and areas in the lee
of the coastal mountains. A secondary upper-level wave is then
expected to travel along a north-south oriented cold front over
the Gulf and move over Southcentral late Thursday evening. This
feature will help drive the front on shore and provide additional
moisture and lift to initiate a second round of precipitation for
most of Southcentral into Friday. Warmer air will also accompany
these features as they move north with a stronger southerly flow
aloft developing. The advection of warmer air should allow any
lingering snow in the Copper River Basin to transition to rain by
late Thursday or early Friday morning.

Winds will increase out of the Matanuska Valley later today
through Thursday morning in advance of the first system, helping
keep overnight lows tomorrow morning above the freezing mark from
Palmer to Anchorage. Winds will also be on the increase for Kenai
and Homer. Southeasterly Turnagain Arm winds will also increase
by early Thursday morning as the front draws closer to the coast.

&&

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

The majority of Southwest Alaska will wake up to mostly clear
skies and cool, crisp morning temperatures on Wednesday with only
the Copper River Delta and western portions of the Lower Kuskokwim
Valley experiencing patchy fog. Taking a look at the bigger
picture on satellite and one can see where all the cloud cover and
rain is presently. A large western Bering low sits to the north
of the Western Aleutians. Expansive cloud cover is blanketing all
but the southeast Bering with cloud cover. The low's front extends
southward and across the Central Aleutians where southerly winds
are gusting to gale force. Through the remainder of today the
front and associated winds will lift northward and back toward
Kamchatka through tomorrow. The bigger weather feature that will
affect Southwest the next few days is a large North Pacific low
currently 380 nm south of Sand Point. Forecast models have been
consistent with bringing the low northward along with a broad warm
front and plentiful moisture. Timing of the front is such that
the Eastern Aleutians and lower Alaska Peninsula will see
increasing showers and elevated easterly winds by this afternoon.
The front will move into Southwest later tonight and continue
northward through the day on Thursday. Despite previously cooler
temperatures at or below freezing across Southwest, a quick warm
up will minimize any measurable snow for all but the higher
elevations. The majority of Southwest will likely see a 1-2 hour
period of a rain/snow mix if any snow at all. Otherwise, the last
one to two days of clear weather will quickly become replaced by
overcast skies and showery conditions across Southwest into the
start of the weekend.

&&

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

A well developed upper level longwave trough extends across the
Bering into the Gulf of Alaska through Tuesday. Several model
defined embedded surface low centers will transit through and
around the trough on various tracks, bringing unsettled weather
across the region. This pattern is expected to continue through
the end of the forecast period.

Saturday opens with a low centered over the Alaska Peninsula, and
is accompanied by a front spreading precipitation across much of
southern mainland Alaska. Rainfall will be heaviest along the
Eastern Kenai Peninsula Coast all the way over to the Copper River
Delta and Canadian Border with lesser amounts, but possibly mixed
rain and snow inland, and tapering off through Tuesday.

Northeasterly winds drawn from Interior Alaska through the
Kuskokwim Delta will result in gusty offshore winds increasing to
near gale force for Kuskokwim Bay and parts of Bristol Bay and gap
winds off the Alaska Peninsula and Barren Islands for Sunday into
Tuesday. Gusty winds across the Pribilofs to the Aleutians Sunday
through Tuesday. A weak ridge over the Western Bering yields to a
North Pacific low center moving along the Pacific side of the
Western Aleutians for the end of the period.

&&

SYNOPSIS AND MODEL DISCUSSION...KM
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BL
LONG TERM...MK