Small Craft Advisory 769, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
528 AM AKDT Wed Sep 6 2023

.ANALYSIS AND UPPER LEVELS...

A cool, deep upper level low is continuing to make easterly
headway into the center of the Alaska Mainland this morning,
inducing strongly cyclonic flow over all but the far western
reaches of the outlook area. A basal shortwave trough on the south
side of the low is now beginning to become negatively-tilted as it
crosses from Bristol Bay over the Alaska Range into Southcentral,
This shortwave is being accompanied by a 95 kt jet streak situated
just upstream over the Southwest coastline, helping the trough to
dig southeast as it enters the greater Cook Inlet region.

At the surface, a cold front stemming from the large area of low
pressure drifting into the northern Mainland has now crossed over
the Alaska Peninsula into the central Gulf, with extensive cold
air advection ongoing behind the front across much of the eastern
Bering Sea and Southwest. Farther west, portions of this same
frontal axis have nearly stalled out just off to the north of the
Aleutian Chain, with widespread low stratus and mist still
lingering along the central/western Aleutians. Farther north, much
of the Bering has nearly cleared out as cool, dry air continues
to stream in from the northwest. A showery regime is now in place
across most of the southern Mainland as cool air wraps in around
the southern side of the low.

&&

.MODEL DISCUSSION...

Models remain in good agreement on the large scale through
Thursday, but agreement begins to deteriorate towards the end of
the week. There remains a good consensus that the closed low now
sitting over the Interior will continue to drift south towards the
western Gulf and AKPen through Friday, but the exact placement of
the center and smaller embedded waves becomes increasingly unclear
by Friday morning. This spread compounds even further towards
early Saturday. The differences in low placement over the Gulf
result in varying trajectories for a much stronger system moving
east across the Bering Sea, with the GFS and NAM in recent runs
trying to hold on to a much slower and farther north track for
the Bering low.

&&

.AVIATION...
PANC...Predominantly VFR conditions with Turnagain Arm winds
bending into the airport. Winds are expected to decrease this
evening. Although precipitation potential is lower, ceilings could
decrease to MVFR this afternoon should a shower past through the
terminal.

&&

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

A few showers linger through the Matanuska and Susitna Valley
this morning, training under southwesterly flow up Cook Inlet.
Additionally winds are gusty through Turnagain Arm and the Copper
River. Winds should become increasingly gusty through Cook Inlet
and Kodiak later in the morning. Going into this afternoon, winds
will become more westerly as the upper trough sinks into the
Southcentral and helping to advect cooler air. Clear skies are
likely through the Cook Inlet area, Kodiak Island, and the lee of
the Kenai Mountains mainly due to the passage of a shortwave, but
also from some terrain induced subsidence.

These winds should come to an end Wednesday evening as a lee-side
trough forms near Montague Island. Looking at the weather through
Friday, expect days to look fairly similar to one another as the
trough parks overhead. Steadier precipitation is most likely over
the eastern Sound, northern Susitna and portions of the Copper
Basin due to southerly flow, while the Cook Inlet areas and the
southern Mat-Su will have showers more confined to the mountains
and occasional sprinkles or light rain in lower elevations. These
may become more pronounced at times as small perturbations move
over the mountains. As advertised, snow levels are expected to
drop as low as 2800 ft, with a couple inches of accumulation
expected at elevation. Isabel Pass may even see some accumulation
or slush on the road Thursday morning. Kodiak island, and even the
western Kenai will be on the sunnier side of things due to
westerly and eventually northerly flow helping to keep the area
dry.

Cool surface temperatures are likely on Thursday and Friday
morning with many locations dipping into the mid 30s. A few spots
could even be near enough freezing. Plan ahead for any frost
sensitive plants. Low temperatures should come up again for this
weekend to 40 to 45 degrees for most locations.

&&

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

A cool, winter-like pattern is taking shape for most of Southwest
as a large upper low continues to churn in place over the Mainland
and drift slowly south through the end of the week. A cold air
mass will continue to wrap into this low from the Arctic down into
the eastern Bering Sea through Friday morning, pushing
temperatures all the way down into the mid to upper 30s for lows
both tonight and late Thursday night. Multiple shortwaves embedded
in the low will continue to drive southeast across Southwest, and
this along with plenty of very cool air aloft will continue to
support rounds of scattered rain showers and even snow showers at
higher elevations. Temperatures will finally begin to rebound back
to more seasonable levels on Friday into early Saturday as the low
continues to move south and shifts flow to a more northeasterly
orientation, allowing slightly warmer air to filter back in from
the Interior.

Meanwhile, most of the Bering and Aleutians will continue to enjoy
a break in the active pattern as a stout upper ridge moves
overhead through Thursday morning. Later on Thursday, a warm front
arcing ahead of a Kamchatka low will lift northeast into the
western Aleutians with a band of steady rain and south to
southeast winds up to gale force. Rain and gusty south to
southeast winds will expand into the central Aleutians and out to
the Pribilofs on Friday as the low and it's frontal boundaries
curve east into the western Bering. A new low will likely form at
the frontal triple point in the southern Bering early on Saturday,
but forecast uncertainty for this evolution is still fairly high
at this time. In any case, a wet and unsettled pattern will
continue through the end of the week.

&&

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

Model consistency is good, with no significant outliers. A well
developed long wave trough moves from the Bering into Central
Mainland Alaska through Tuesday. A number of short waves will be
rotating through the pattern, bringing periods of rain, occasional
heavy in the East for the extended period. Gusty winds expected
closer to the front over most of Prince William Sound into the
lower Copper River Valley, worse in channeled terrain. The rest of
Southcentral should remain under mostly cloudy skies and periods
of rain.

High pressure builds its way into the Central Bering and weakens
as it exits through Bristol Bay. Coming in behind is another well
developed low and longwave trough from Kamchatka, driving along
the Aleutians and into the Gulf of Alaska by Tuesday. This brings
another round of gusty winds, low clouds and rain across most of
Southern Alaska, with higher amounts occurring in Prince William
Sound and the low Copper River Valley.

&&

SYNOPSIS AND MODEL DISCUSSION...AS/VR
SOUTHCENTRAL ALASKA...VR/CJ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AS
LONG TERM...KZ