Small Craft Advisory 16, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
449 PM AKST Wed Jan 4 2023

.ANALYSIS AND UPPER LEVELS...

A complex weather pattern remains in place across Alaska and the
North Pacific, with a long-wave trough and embedded numerous
short-waves and lows. A gale force low which crossed the northwestern
Gulf last night is now rapidly weakening as it heads inland across
interior Southwest Alaska. Pressure rises in the wake of the low
are causing strong gusty winds across the Kenai Peninsula and
northwestern Gulf, though there is a pretty rapid diminishing
trend ongoing this afternoon. Areas of snow are spreading out
ahead of the remnant low across interior Bristol Bay and northward
toward the Kuskokwim Valley. A trailing short-wave is lifting
northwestward across the Kenai Peninsula to the Susitna Valley.
Showers are lingering primarily along the eastern Kenai Peninsula
and western Susitna Valley, while strong downslope flow is keeping
the areas in between mostly dry. A separate short-wave is lifting
northward across the eastern Gulf, bringing steady precipitation
to eastern Prince William Sound northward into the southern Copper
River Basin. Precipitation type is primarily in the form of rain
along the coast near sea level, with snow at higher elevations and
inland.

Meanwhile, a series of compact lows are strung out along/near the
western to central Aleutian Islands. Satellite based wind
estimates show a tight core of strong winds around these lows,
in the small craft to gale force categories. IR satellite cloud
top temperatures are quite cold and indicative of strong upward
vertical motion and moderate to heavy precipitation. With a
uniformly cold airmass aloft (roughly -10 degrees at 850mb),
instability is a key forcing mechanism for what likely are small
scale bands of heavy snow and strong gusty winds.

In between southern Alaska and the lows along the Aleutians, the
weather is quite benign across the eastern Bering Sea, eastern
Aleutians, and southern Alaska Peninsula. Winds are generally
light, while some very weak short-waves and airmass instability
are producing partly to mostly cloudy skies with isolated to
scattered snow showers.

&&

.MODEL DISCUSSION...

Models are verifying a little better today with existing features
and are in decent agreement for about the next 24 hours. Model
spread then grows quickly across the Bering Sea, Aleutians, and
North Pacific with the track/location of increasingly stronger
lows. While there is some confidence that gale force winds will
become more widespread, confidence in which areas will experience
the strongest winds is low. There is also a fairly large spread in
solutions with the track of a low and leading warm front across
the eastern Gulf and toward Southcentral on Saturday. This greatly
affects whether/when precipitation will spread back into the
region. The best bet for the weekend is that precipitation will
affect the Prince William Sound/north Gulf coast areas.

&&

.AVIATION...
PANC...There is a chance of light snow as a short-wave tracks
overhead late tonight through Thursday morning. Although strength
of downslope flow will have greatly weakened by then, forcing is
weak, so most likely scenario is flurries with a low end VFR
ceiling. Otherwise, expect mid to high level clouds and light
winds to prevail.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
This afternoon through Friday)...

A low over Bristol Bay continues to weaken as it exits
Southcentral Alaska. Eastern Kenai Peninsula, Prince William
Sound, and Kodiak Island will continue to see showers tonight into
tomorrow morning given onshore flow and weak shortwaves moving
overhead. Precipitation type should be mostly snow, with rain at
lower elevations gradually transitioning to snow overnight.
Downsloping should keep Western Kenai Peninsula, Anchorage, and
the Mat-Su Valleys mostly dry, though a stray shower or two might
bring a quick shot of snow/rain.

Precipitation should taper off by late tomorrow morning, marking
a transition towards colder, drier weather. Temperatures will
steadily drop through Friday as cold air wraps back around the
departing low. Clearer skies and effective nighttime radiative
cooling will further promote lower temperatures. Gusty offshore
winds will also pick up by Thursday evening as pressure gradients
tighten ahead of a surface low moving north along the west coast
of North America.

As the aforementioned surface low moves into the Gulf of Alaska
late Friday night, winds and precipitation chances will increase
along the north Gulf coast. Much uncertainty remains with the
track and timing of the low and its front, with timing
differences approaching 12 hours between solutions. Stay tuned as
we continue to refine the forecast.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Tonight
through Saturday)...

A low pressure system is currently tracking westward across the
Bristol Bay region, bringing steady snowfall from Iliamna to
Dillingham. The extent of the precipitation band will reach the
coastline this evening, especially areas encompassing Goodnews Bay
to Togiak. Snow accumulation will be generally light heading into
tonight and Thursday afternoon, with 1-3 inches expected.
Otherwise, the highest snowfall will be favored for the upslope
areas along the Aleutian/Alaska Ranges and coastal mountains.
Areas of fog were added again into the forecast package as
conditions are slow to improve over the next couple of days. The
low is expected to weaken and lift out of the area Thursday
afternoon. Meanwhile, weak upper levels embedded in the southerly
flow will continue chances of snow showers across the forecast
area. By Friday, a weak ridge of high pressure builds overhead for
colder and drier conditions heading into the weekend.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3:
Tonight through Saturday)...

An elongated trough of low pressure situated over the western
Bering Sea continues to be the main weather driver. Although there
continues to be some model disagreement, multiple low centers
rotating around the aforementioned trough brings unsettled
conditions for the Bering Sea and Aleutians. A North Pacific low
currently situated south of Amchitka will bring small crafts to
gales with snow showers for the western Aleutians through early
Thursday. From Thursday afternoon to early Saturday, widespread
east to southeast small craft winds with some pockets of gales
will be over the northern Bering to central Aleutians. Seas are
expected to be 10-15 feet for these regions. Heading into the
weekend, quieter weather is expected for the eastern Bering and
Aleutians as a ridge of high pressure builds overhead.

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

There is not great agreement at the beginning of the week, with a
series of surface lows across the Bering Sea and Gulf of Alaska.
This is fairly common, as model spread tends to grow as they
progress further out in time. There is, however, better than
average agreement for a North Pacific low tracking south of the
Aleutians through mid-week. Discrepancies begin to grow again by
Wednesday afternoon when the GFS and EC continue tracking east
where as the Canadian stalls south of the Eastern Aleutians,
before merging with yet another incoming system from the west.
Active weather is likely along the Aleutians through the long
term.

High pressure sets up over eastern Siberia and across Interior
Alaska. Most of the Mainland will be on the drier side, with
around normal temperatures through mid-week. There is a low moving
into the Gulf of Alaska on Sunday that is going to rapidly
weaken, but there may be periods of snow showers for the Coastal
regions Sunday into Monday morning.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 175-177 411-413.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHCENTRAL ALASKA...KC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MF
LONG TERM...KM