Small Craft Advisory 855, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
517 AM AKDT Mon Oct 2 2023

.ANALYSIS AND UPPER LEVELS...

The neutrally-tilted axis of an upper-level trough is tracking
from Southwest Alaska into Southcentral this morning. An attendant
surface cold front is moving across the Alaska Range. A weak
surface low centered over western Prince William Sound is lifting
north ahead of the main trough, with precipitation spreading
across most of Southcentral. Rain is falling near the surface with
snow falling at elevations above 2,500 ft. The northern portions
of the Copper River Basin, along with Eureka, are seeing a
transition from rain to snow as temperatures fall to 32 degrees.
Further west over the Bering Sea and Aleutians, benign weather
conditions prevail under the influence of a high pressure ridge.
The pressure gradient between the aforementioned trough and ridge
are promoting gusty northwesterly gap winds along the Alaska
Peninsula and through the Barren Islands.

&&

.MODEL DISCUSSION...

Models remain in good agreement with the overall synoptic pattern
over the next several days. Minor differences still exist in the
timing of a trough tracking across Southcentral today. The NAM
remains the fastest solution while the GFS remains the slowest.
Thus, the GFS suggests that precipitation tapers off slowest
across Southcentral late tonight. The next system of note is an
upper-level low and surface front that enter the western Bering
and Aleutians starting Tuesday. Models currently have a good
handle on the timing and evolution of said features.

&&

.AVIATION...
PANC...While VFR ceilings and visibility are expected to persist
for most of the day today, a period of MVFR ceilings and vis are
possible through this morning as rain continues across the
Anchorage bowl. Light winds will give way to stronger northerly
winds this afternoon and evening. Precipitation begins to taper
off late tonight and early Tuesday morning with clearer and drier
conditions as a frontal system departs to the east.

&&

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

A potent upper-level arctic trough and attendant surface cold
front continue to move east over the Alaska Range this morning.
The resultant southerly flow ahead of both features is drawing up
a weaker upper-level shortwave and surface low over Prince
William Sound this morning north toward interior Southcentral
Alaska. The result is widespread rain from Cook Inlet to Prince
William Sound and the Copper River Basin. Snow levels are
beginning to drop as colder air slowly filters over the region
from west to east, with levels between 2,500 and 3,000 ft as of 4
AM.

As the arctic trough and cold front continue eastward, widespread
cold air advection will occur over Southcentral. Snow levels will
drop to around 2,000 ft through Monday morning and continue to
fall to around 1,000 ft by Monday night. 1-2 inches of snow
remains possible for the Upper Hillside of Anchorage and higher
elevations of Eagle River. 1 to 3 inches of snow is also possible
along portions of the Glenn Highway west of Glennallen and along
the Tok Cutoff from Chistochina to Mentasta Lake. 3 to 6 inches of
snow is possible for Hatcher Pass and across the far northern
portions of the Copper River Basin, especially from Paxson north
to Isabel Pass. Gusty northerly winds through Isabel Pass are also
anticipated which could result in areas of blowing snow north of
Paxson for Monday afternoon and evening.

Lower elevations will see rain, which will come to an end from
west to east as the front moves through. Expect clearer and drier
conditions in the wake of the front, with low temperatures early
Tuesday morning dropping well below freezing for much of
Southcentral. Sheltered valleys and interior locations could see
low temperatures drop into the teens through midweek.

&&

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

Key Messages:

-High Chance of Gale force winds for the western Aleutians on Tuesday

Discussion:

A broad ridge of high pressure is slowly rolling into the central
Bering this morning. Scattered sky cover, from low stratus
trapped beneath the ridge, spans much of the Bering. What remains
of a trough over the Western Alaska Range is gradually shifting
eastward. Southwest Alaska will benefit the most from the
departure of the trough to the east and the approach of high
pressure from the west, as generally clear skies are forecast
through Tuesday morning. However, northern portions of Southwest,
from Bethel eastward into the Lower Kuskokwim Valley, will see a
weak front dip into the region with light scattered showers on
Tuesday. A few snowflakes may also fall, though no accumulations
are expected.

In the short term, the most notable weather will stem from a
strengthening pressure gradient over the Eastern Aleutians and
lower Alaska Peninsula. North to northeast wind gusts to 35 knots
will be possible into tonight, especially out of the bays and
passes. High pressure pushes into the eastern Bering by Tuesday
morning, which will see previous northeasterly winds become
easterly. This will see inland winds diminish and only small craft
winds hugging the coast of the Eastern Aleutians.

A much windier day is in store for the Western Aleutians on
Tuesday, as a large front moves into the western Bering. Gale
force, southeasterly winds and higher gusts are forecast before
gradually tapering off Tuesday night. Adak and Atka may experience
a brief round of gales early Wednesday morning though the front
lifts northward and dissipates by Wednesday afternoon.

&&

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

The long term forecast period will be defined by one main feature:
a low in the North Pacific becoming centered near the Alaska
Peninsula, pushing a ridge, and its accompanying drier weather,
out of the way on Thursday. The low anchors itself in the longwave
trough axis, which slowly shifts eastward through the forecast
period, keeping generally unsettled weather over southern mainland
Alaska through the weekend. The low pushes a warm/occluded front
across the Alaska Peninsula and the Gulf of Alaska beginning on
Thursday, spreading precipitation across much of southern mainland
Alaska. At the same time, northeasterly winds will be drawn from
Interior Alaska out across the Kuskokwim Delta, which look to
approach gale force. Precipitation will be heaviest along the
coast, but lighter precipitation will spread inland, north of the
coastal mountains through late week. The best chances to remain
dry through Friday will be north and west of the leading edge of
the front, in the Kuskokwim Delta. Meanwhile, a low in the western
Bering Sea weakens into an elongated trough as some of its energy
is absorbed into the broad low centered over the Alaska
Peninsula.

Then, the front over southern mainland Alaska shifts west by
Saturday, which moves the focus of precipitation from
Southcentral (which begins to dry out) over to the Southwest
coast. Warm southerly flow on the east side of the low will mean
that most precipitation is expected to be rain, with snow levels
rising back into the 3000 to 5000 foot range. Precipitation may
shift out over the eastern Bering Sea by Sunday, though increasing
model spread leads to lower confidence in the location of
precipitation.

There are some indications that a gale force low approaches the
western Aleutians for the weekend, but there is significant track
uncertainty at this lead time. Most model solutions keep the low
out over the North Pacific, but there is a chance for light to
moderate rain and gusty easterly winds through the western/central
Aleutians Saturday or Sunday.

&&

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

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
1002 AM AKDT Mon Oct 2 2023

AKZ171-031200-
Kodiak Island-
Including the city of Kodiak City
1002 AM AKDT Mon Oct 2 2023

...RESUSPENDED ASH FROM KATMAI-NOVARUPTA EXPECTED OVER KODIAK
ISLAND THROUGH MONDAY NIGHT...

Strong northwesterly winds up to 40 mph have developed across
Shelikof Strait. These winds are resuspending ash from the 1912
Katmai-Novarupta eruption. The villages of Karluk and Larsen Bay
are most likely to be impacted. Ash is currently north of Akhiok,
but ash may mix south to the area.

Those with respiratory ailments in these areas are encouraged to
avoid or reduce outdoor activity to avoid inhaling the ash through
late this evening. The winds will diminish early Tuesday morning,
which should cause the ash to settle out of the atmosphere.