Volcanic Ashfall in Alaska, September 25, 2023

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
424 AM AKDT Mon Sep 25 2023

.ANALYSIS AND UPPER LEVELS...

A 140 kt jet streak extends across the Central Aleutians and into
the North Pacific Ocean. With the jet stream (aka the storm track)
oriented away from Mainland Alaska, much of the southern portion
of the state has been experiencing fairly benign weather under a
ridge of high pressure. Radiational cooling under light winds and
pockets of clearer sky are leading to a chilly morning;
temperatures have dropped as low as the mid 20s in the interior,
with lows hovering in the mid 30s to low 40s nearer the coast.

More active weather is evident in the Bering Sea and along the
Aleutian Islands. Satellite imagery shows a compact, occluding low
near the Pribilof Islands. The low's front is bringing rain to the
Pribilof Islands and Eastern Aleutians, while westerly flow
wrapping along the backside of this low is contributing to gusty
winds moving through Adak and Atka. Looking to the southeast, a
front extends into the Gulf of Alaska from a broad low off the
coast of British Columbia.

&&

.MODEL DISCUSSION...

Models are in excellent synoptic scale agreement through at least
midday Wednesday. By Wednesday evening, there are minor model
disagreements with the timing and placement of precipitation as a
front moves into Southwest Alaska. Overall, good model agreement
and run-to-run consistency are leading to high forecast confidence.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

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

A tranquil Fall weather pattern will continue for the next several
days as an upper level high pressure system as well as a surface
high pressure system remain over Southcentral Alaska. Skies will
remain mostly clear with daytime temperatures rising into the
lower 50s and night time temperatures dropping into the lower to
mid 30s around Anchorage and the MatSu Valleys as well as the
Kenai Peninsula. Low temperatures will drop into the 20s in the
Copper River Basin.

Winds will remain light for most areas inland, however gusty
offshore winds will develop and increase as a low in the
Southeastern Gulf of Alaska sends an inverted trough north and
west into the Northern Gulf of Alaska late Tuesday and Wednesday.
As the trough moves north lowering surface pressure over the Gulf
will begin to support an offshore gradient focused along the north
Gulf Coast. In addition, a negatively- tilted shortwave moving
into the western Gulf on Tuesday will help reinforce a surface
trough across the northern Gulf. This will lead to a prolonged
period of low level offshore flow and gap winds through the
typically favored spots along the coast between the Kenai
Peninsula and Prince William Sound vicinity. Stronger
northwesterlies winds will develop behind the trough moving into
the Gulf near the Barren Islands and portions of Kodiak Island on
Tuesday and persist into Wednesday.

Aside from the winds over the Gulf, conditions will remain very
calm, and dry for September, with negligible precipitation chances
for most of the outlook area through Wednesday. The developing
offshore winds should also help clear out what scattered low to
mid clouds that remain over Southcentral Alaska.

&&

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

Southwest Alaska continues to see calmer and drier weather as
relatively higher pressure spreads across the region. An upper-
level trough and its surface low are located over the
central/eastern Bering this morning. Its surface front lifting
east is bringing light rain to the Pribilof Islands and eastern
Aleutians. Westerly winds up to small craft are wrapped around the
southern periphery of the low over the southern Bering. The front
will reach the tip of the Alaska Peninsula this morning, and the
entire system will weaken and track southeast into the North
Pacific this evening. A weak ridge of surface high pressure
spreads across the Bering Sea for the remainder of today, limiting
any precipitation to isolated light rain showers as winds
diminish.

On Monday night, southerly winds increase in the western
Aleutians as a Kamchatka low begins to push into the western
Bering. Winds in association with this low are expected to reach
gale force near Attu Island on Tuesday afternoon. Otherwise, winds
should be predominantly small craft as the system tracks east.
Rain along the front reaches the Pribilof Islands on Wednesday as
the low center begins to track into the northern Bering Sea.
Southwest Alaska can continue to expect quiet, partly to mostly
sunny skies, with a slight chance of isolated showers until the
low in the northern Bering brings increasing winds and rain to the
Kuskokwim Delta coast late on Wednesday.

&&

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

Forecast models agree through mid-period, but starts to diverge
in positions and strengths by Sunday. High pressure that covered
most of the Interior bringing variable cloudy skies will retreat
further towards the Canadian Border. A well developed long wave
trough extends from a low in the Arctic through the Western Bering
progresses into Mainland Alaska by the end of the period. A
series of lows will traverse the Aleutians into Southwest and
Southcentral Alaska, bringing locally heavy rains and gusty winds
with each center. The exception to this pattern will be a deeper
low moving into the Central Aleutians late Thursday, and into the
Northern Alaska Peninsula by Sunday.

As a result, chances for rain increase toward late week along the
Gulf coast, with some potential for offshore small craft winds
through gaps along the coast as well. Gale force winds pushing its
front across the Aleutians Friday. However, the exact track of
this low is fairly uncertain by the end of the period. The general
trend in the pattern suggests increasing chances of widespread,
steady rainfall over Southwest Alaska up to the North Gulf Coast
going through the weekend.

&&

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

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
611 AM AKDT Mon Sep 25 2023

AKZ181-261415-
Alaska Peninsula-
Including the cities of Cold Bay and Sand Point
611 AM AKDT Mon Sep 25 2023

...TRACE ASHFALL POSSIBLE FOR UNIMAK ISLAND AND COLD BAY...

As of 0545 AM this morning, satellite imagery indicates that an
eruption is ongoing at Shishaldin Volcano. Ash is being lofted to
the south and east of the volcano, which may lead to trace
amounts of ashfall for the communities of False Pass, King Cove,
and Cold Bay. This statement will be updated if conditions change.