Small Craft Advisory 52, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
534 AM AKST Tue Jan 16 2024

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Thursday night)...

Key Messages:

*  Widespread windy conditions across Southcentral Alaska tonight
   through Wednesday night, with the strongest winds affecting the
   coastal communities of Valdez, Cordova, Seward, and Whittier.

*  Dry weather continues through the end of the week and colder
   temperatures settle in.

A high amplitude short-wave ridge over Southcentral will shift
to the south and east later today as a short-wave arrives from
western Alaska. A building ridge over the Bering Sea will cause
the short-wave to dig southeastward as it crosses Southcentral
tonight, reaching the northeast Gulf of Alaska early Wednesday.
Strong subsidence with the short-wave combined with moderate
to strong cold air advection and tightening surface pressure
gradients (as a 1048 mb surface high builds over the Alaska
Interior) will lead to widespread windy conditions across the
region. The strongest winds will be at elevation across area
mountain ranges as well as through gaps in the mountains. As is
typically the case, coastal communities will see the brunt of
the wind, as they are located on/near bays and passes through
the coastal Kenai and Chugach Mountains. Areas of fog in the
Anchorage and western Kenai Peninsula this morning, will dissipate
for good by this afternoon as low level cold air advection and
stronger low level winds arrive.

On Wednesday, the upper ridge over the Bering Sea will continue
to strengthen and shift eastward over mainland Alaska. Meanwhile,
short-waves will dig southward along the AL-CAN border into the
northeast Gulf, ultimately forming a vertically stacked low over
the eastern Gulf. Even colder air will advect south across the
Copper River Basin and Prince William Sound and into the Gulf. A
northerly jet between the low/trough and ridge will strengthen
right over Southcentral. Surface pressure gradients will continue
to tighten. All of this will lead to further strengthening of
winds across the region during the day Wednesday. Of particular
concern for potential damaging winds in this scenario are areas
on the poleward side of the jet (in this case areas to the east
of the jet). This includes eastern Prince William Sound and the
Valdez/Cordova areas. Cross-sections show a significant tropopause
fold developing, with high statically stable air descending from
the tropopause down toward the surface (or mountain-top level of
the Chugach Mountains). This acts like a ceiling and can help
force stronger winds down to the surface. This has served as a
good indicator for high winds in the past. As a result, have
upgraded the High Wind Watch for Valdez/Thompson Pass to a High
Wind Warning. Cordova is not as vulnerable to the strongest
mountain gap winds, but this still looks like a strong wind event
for them. The Copper River Delta, right at the exit of a large gap
through the Chugach Mountains from the Copper River Basin may see
the strongest winds at sea level, with 100 mph not out of the
question.

The initial direction of flow is not favorable for very strong
winds in Anchorage and the Matanuska Valley. However, as the
low deepens in the Gulf Wednesday afternoon and evening, the
upper ridge will nudge eastward across eastern Alaska into the
Yukon and the mean flow over Southcentral will shift to
northeasterly. This will lead to strengthening winds across the
Matanuska Valley and southward along Knik Arm, affecting north
to west Anchorage. These winds will peak Wednesday night.

Winds will diminish region-wide on Thursday through Thursday night
as the low in the Gulf exits southward to the Pacific and the
upper ridge builds into Southcentral. Winds will linger longest
along the coastal gaps. While the low level airmass will begin to
warm, lighter winds will allow for effective radiational cooling.
Thus, surface temperatures in most locations will trend lower
Wednesday night through Thursday night.

&&

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

A decaying front extending from the central Aleutian Islands,
across the eastern Bering Sea, and into the Kuskokwim Delta this
morning will push eastward today, bringing less than an inch of
snow accumulation to the Delta and portions of the Lower Kuskokwim
Valley. While the primary precipitation type is expected to be
snow, a lack of mid and upper-level moisture accompanying the
front may lead to some periods of light freezing drizzle. Any ice
accumulation should be no more than a trace, but communities
across the Kuskokwim Delta should still be wary of possible slick
spots this morning. Patchy fog will be another concern this
morning, mainly along the eastern foothills of the Kilbuck and
Ahklun Mountains, extending along a line from Aleknagik north
through Sleetmute. This fog has been gradually eroding and should
mix out today as the front pushes eastward and increases low-level
northwesterly winds and cold air moving in aloft leads to a more
unstable atmospheric profile.

Low stratus is likely to linger over the Kuskokwim Delta today,
but clears out tonight as surface high pressure centers over
Interior Alaska while a potent shortwave trough dropping into the
Gulf of Alaska leads to increasingly northerly wind across the
region. This northerly flow will bring in a cooler, drier air mass
which will be unfavorable for fog and low stratus, meaning
Southwest Alaska will likely see a fairly sunny and clear week
ahead. For the Alaska Peninsula and the Aleutians, expect winds as
strong as gale force to develop around the periphery of the ridge
beginning today as weak shortwave troughs rotate around it,
accompanied by periods of light rain for the central and western
Aleutians through Thursday night.

&&

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

An upper-level ridge will be situated over Southern Alaska and
the Bering Sea, suppressing active weather across our region
through next week. While the high pressure is in place through the
remainder of this workweek, a clear and cool pattern will prevail
for most of Southcentral and Southwest Alaska. With the high
pressure system centered over Southwest Alaska, models are keying
in on some low pressure system originating from the Gulf of Anadyr
and the Northern Pacific to move up the western-most Bering sea
and ride the northern edge of the ridge into Western Alaska.
Pockets of energy are coming up and over the northern side of the
ridge, bringing up the winds across Southcentral and out of the
local gaps. Potentially strong and gusty winds with colder
temperatures will likely be the impact felt for Southcentral and
Southwest through the latter half of the week into the weekend.
Areas of outflow winds develop along the Alaska Peninsula and
Southcentral Coasts, with gale force gap winds through the Barren
Islands and Shelikof Strait and along the Alaska Peninsula Coast
into the Eastern Aleutians developing for Thursday and diminishing
Friday. Gaps such as Seward, Thompson Pass, and Matanuska Valley
will be feeling some gusty winds by Friday potentially diminishing
by the early weekend. Model solutions are suggesting an unusual
retrograde of the ridge, advancing the high pressure to the
northwest into the weekend. By Saturday afternoon, there is the
potential for a new low to approach coastal Southcentral from the
Gulf of Alaska, however uncertainty on this system remains high.
As the new low pressure system develops in the Gulf early next
weekend, the potential for moderate to heavy snow to return to the
Prince William Sound area by late this weekend into early next
week.

-CL

&&

.AVIATION...

PANC...Calm and variable winds will persist through tonight and
early Wednesday morning. Conditions are favorable for fog
development until winds start to increase late Wednesday morning.
Fog could move into the terminal at any time, dropping vis and
cigs as low as LIFR. Gusty northerly winds will develop by early
Wednesday afternoon, with gusts up to 45 mph possible around
midnight Wednesday.

&&