Storm Warning 120, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
509 AM AKDT Tue Oct 10 2023

.ANALYSIS AND UPPER LEVELS...

The position of a longwave trough over Alaska has changed little
the past 24 hours. Pinned in place by an upper low over the Seward
Peninsula, a pair of shortwave troughs are digging around and
downstream of the trough. The most prominent shortwave is lifting
northward across Yukon into the Northwest Territory of Canada. A
second shortwave is located offshore of British Columbia and
meandering slowly southward along the coastline. The combination
of the two shortwaves will help the longwave trough begin to break
down and shift eastward through tomorrow. Upstream of the
longwave trough a weak ridge of high pressure has overspread the
southern Bering and Aleutians. Northwesterly surface winds through
the Alaska Peninsula have calmed considerably with winds well
below small craft speeds. Yesterday's upper level disturbance into
Southeast Alaska has manifested itself into a weak trough and
surface low to the southwest of Prince William Sound. Associated
precipitation has shifted from the coast into the Chugach and
Copper River Basin, as well as portions of the Mat-Su Valley. Snow
has persisted from Eureka into Glennallen and points further
north.

&&

.MODEL DISCUSSION...

Models are in good synoptic agreement with the overall pattern. A
boon to the forecast is how clustered the deterministic guidance
is with the upcoming storm force low into the Aleutians on
Wednesday morning. There is good consensus with the center of a
~950 mb low pushing across the Western Aleutians late Wednesday
evening, with a storm force front spreading into the Central
Aleutians earlier by midday Wednesday.

&&

.AVIATION...

PANC...MVFR ceilings are expected to continue through the morning
and afternoon hours. The morning upper air sounding out of PANC
shows some light snow is likely occurring above 3000 feet. As
precipitation aloft continues to advect into Anchorage from the
Susitna Valley, some light precipitation could develop down to the
surface. The best chance for precipitation of any kind is more
likely to hold off until tonight into Wednesday morning.

&&

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

Upper-level troughing across southcentral Alaska will continue to
bring cool, cloudy conditions with a chance of rain and snow. Area
observations note generally light snow falling across much of the
Copper River Basin, with webcams showing slightly heavier snow
falling across Tahneta Pass / Sheep Mountain area. Light
precipitation, primarily rain as of 5am, has made it as far west as
the Susitna Valley and Cook Inlet. This will continue to be the case
for much of the day as a shortwave trough lifts up across
southcentral Alaska and the surface/low-level continues to meander
in the Gulf near the Prince William Sound. Modest cold air advection
should allow temperatures to cool such that any precip today is
snow, with the exception of locations near sea level.

Strong, gusty northwesterly gap winds across Shelikof Strait and
Kamishak Gap will continue today, though gradually subsiding late
today into the overnight. Wednesday, widespread precipitation
focused over the Copper River Basin becomes more showery as the low
continues to weaken and linger over the northern Gulf near Prince
William Sound. The best chances for precipitation late Wednesday
into Thursday look to be generally over/near Prince William Sound.
Later on Thursday, models are advertising a surface low tracking
into the eastern Gulf. This is likely to bring another round of
precipitation to southcentral Alaska, as well as the potential for
Gale Force Winds near the Copper River Delta. Confidence in the
details are still low at this point as model spread remains
relatively high.

&&

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

The High Wind Watch has been upgraded to a High Wind Warning for
the Western and Central Aleutians and the Special Weather
Statement remains in effect for the Eastern Aleutians Wednesday
through Thursday.

A strong North Pacific low lifts northwards into the Western
Bering Sea and pushes its front eastward through Wednesday. Storm
force winds with hurricane force gusts will be prominent along
the front, impacting the Western and Central Aleutians Wednesday
into Thursday. Additionally, anticipate higher seas during
strongest winds. The front begins to lose steam as it moves
eastwards, though elevated winds (high end gale force with storm
force gusts) are likely across the Eastern Aleutians and Pribilof
Islands Wednesday night into Thursday morning. The low parks
itself west of the Pribilof Islands and deteriorates through the
end of the week.

Southwest Alaska will continue to see relatively benign conditions
over the next couple of days.

&&

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

A weakening upper level low across northern half of the state
will give way to a ridge nosing in across eastern Alaska into the
weekend. This ridge will push much of the colder air entrenched
out across the Bering as it starts to dive south. The powerful
Aleutian system that will have brought the forecast high winds and
hurricane force wind conditions will be unable to progress towards
mainland Alaska and will stall and weaken Saturday as it loses all
upper level support. Overall rain and unsettled weather will be a
mainstay through the weekend in the Aleutians and the AKPEN.

Southcentral will see moderation. Early into the weekend a storm
across the panhandle will lift north, keeping the Gulf coast and
the Copper River Basin in the cross hairs. Temperatures should
swing back into the normal range, after an early week cool down.
For the most part, no appreciable snow is expected in low areas
through the weekend. Overnight Sat/Sun, the Anchorage/MatSu could
light precipitation, mainly focused higher in the mountains. Much
of the extended forecast plays into the evolution of TS Bolaven,
which is currently in the tropics, east of Japan. Beyond the
weekend timeframe, confidence is increasing that the remnants of
this storm will move into the Gulf of Alaska. Model trends
continue to tilt this scenario towards likely.

&&

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

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
1001 AM AKDT Tue Oct 10 2023

AKZ171-110000-
Kodiak Island-
Including the city of Kodiak City
1001 AM AKDT Tue Oct 10 2023

...RESUSPENDED ASH FROM KATMAI-NOVARUPTA EXPECTED OVER KODIAK
ISLAND THROUGH THIS AFTERNOON...

Strong northwesterly winds are beginning to diminish across
Shelikof Strait this morning and will continue to decrease through
this afternoon. These winds have resuspend ash from the 1912
Katmai-Novarupta eruption. The communities of Kodiak, Karluk and
Larsen Bay are most likely to be impacted. Resuspended ash is
expected to settle out of the atmosphere as winds come down
throughout the day.

Those with respiratory ailments in these areas are encouraged to
avoid or reduce outdoor activity to avoid inhaling the ash.

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
118 PM AKDT Tue Oct 10 2023

AKZ185-111330-
Eastern Aleutians-
Including the cities of Unalaska and Nikolski
118 PM AKDT Tue Oct 10 2023

...HIGH WINDS POSSIBLE FOR THE EASTERN ALEUTIANS THURSDAY MORNING
THROUGH THURSDAY AFTERNOON...

A strong North Pacific low lifts into the Western Bering Sea by
Wednesday. The front will push eastward across the Bering to the
Eastern Aleutians by late Wednesday night into early Thursday
morning. South winds of 30 to 45 mph with gusts up 65 mph will be
possible (30%-50% chance) Thursday morning into the afternoon
hours. Winds should begin to diminish Thursday during the evening
hours as the storm slowly weakens.

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
152 PM AKDT Tue Oct 10 2023

AKZ195-111300-
Pribilof Islands-
Including the city of Saint Paul
152 PM AKDT Tue Oct 10 2023

...A STRONG LOW WILL MOVE INTO THE BERING BRINGING HIGH WINDS TO
THE PRIBILOF ISLANDS...

A North Pacific low is going to rapidly intensify as it moves
north. The surface low will be north of Adak Thursday morning with
a surface pressure of 952 mb. Southeastly winds of 30 to 45 mph
with gusts up 65 mph will be possible Thursday morning to Friday
morning for St. Paul and St. George. Winds will become more
easterly late Thursday into Friday.