Small Craft Advisory 324, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
525 PM AKDT Thu Apr 13 2023

.ANALYSIS AND UPPER LEVELS...

A large upper level low circulation is centered over mainland
Alaska, anchored by a deep cold upper low over northern Alaska.
This is all embedded within the larger long-wave trough which
encompasses nearly all of the Alaska region from the Bering
Sea/Aleutians to mainland Alaska and the Gulf of Alaska. A broad
short-wave trough is digging southeastward across Southwest Alaska
and the Aleutians, leading to showers. More significantly, an
Arctic front is following close behind this, bringing colder air
and gusty winds to the region. The Kuskokwim Delta and Kuskokwim
Valley are already firmly entrenched in the cold air mass, with
temperatures in single digits to teens, roughly 20 degrees or
more below normal highs for this time of year. Snow showers are
beginning to develop over the open waters of the Bering Sea as the
much colder air moves over the relatively warm waters. A separate
smaller short- wave is lifting from the Gulf northward toward
Southcentral. A weak surface low has also formed over the
northwestern Gulf and is tracking slowly northward. Steady
precipitation ahead of these features is headed toward the north
Gulf coast and Prince William Sound. For the rest of Southcentral,
there are areas of largely unorganized snow showers as weak lift
from nearby weak short-waves combines with an unstable airmass in
place.

&&

.MODEL DISCUSSION...

Models are struggling with location of precipitation over
Southcentral, with multiple weak features impacting the region
through Friday. Otherwise, solutions are fairly similar for
handling of key weather features over the next few days.

&&

.AVIATION...

PANC...Showers will remain in the vicinity through Friday
afternoon as a series of weak short-wave rotates across the
region. The terminal will likely see an occasional snow shower,
with potential for a brief period of MVFR conditions. It will be
very difficult to nail down the best opportunities for showers
and reduced ceiling/vis, so will rely heavily on radar trends and
adjust the TAF accordingly.

&&

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

A large, upper-level low continues to be centered over Interior
Alaska this afternoon. As it begins to track south and west along
the west coast of Alaska tonight, it will cause a weak northern
Gulf low/shortwave combo to lift northward into Southcentral,
supporting precipitation chances along the north Gulf coast,
Kenai Peninsula, Anchorage Bowl, and Mat-Su Valleys. The heaviest
precipitation can be expected along the coastal mountains,
increasing in intensity this evening, and culminating in
widespread snow and rain showers along the coast by midnight. Rain
will be likely at low elevations along the coast. A band of snow
showers over the Kenai Peninsula and Anchorage Bowl this afternoon
will begin to shift slowly westward, away from the population
centers, as low- level easterly flow develops this evening. Behind
that band, snow will continue to be possible for the Kenai
Peninsula and Anchorage Bowl through Friday morning as the
lifting shortwave spreads some showers inland past the coastal
mountains. Snow accumulations are expected to be minimal with this
feature inland of the coastal mountains, but may reach as much as
one inch for localized areas, particularly on the northern Kenai
Peninsula, through Friday.

A ridge of high pressure nudges in from the east beginning Friday,
which will contribute to warmer daytime temperatures. While skies
may not be fully clear, cloud cover will thin out and allow for
meaningful solar heating for highs to return to the 40s. A North
Pacific low then pushes its front westward across the Gulf on
Friday night. Stronger northerly flow across the mainland in
response to the strengthening pressure gradient will aid in drying
out inland areas. The front sweeps south toward Kodiak Island by
Saturday morning. Around an inch of snow accumulation can be
expected for Kodiak City, with low level northwesterly flow caused
by cold air filtering across the Aleutian Range expected to keep
temperatures low enough for snow despite a broader- scale push of
warmer air from off the Gulf. The front weakens and pushes south
of the island by Saturday afternoon.

Temperatures continue to warm into the mid 40s, near normal, for
most inland locations on Saturday. Most precipitation chances
remain away from land with weak offshore flow, but a weak upper-
level disturbance tracking toward the Copper River Basin lifting
out of Southeast Alaska may increase chances for scattered showers
beginning Saturday afternoon. This disturbance pushes slowly
westward and spreads chances for showers across mainland Southcentral
and lowers temperatures for Sunday.

&&

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

A longwave upper-level trough spreads across all of Alaska and the
Bering Sea. A surface low in the Gulf of Alaska coupled with deep
north-northwesterly flow is promoting showers and windy
conditions along the Alaska Peninsula. An upper low currently
located over the North Slope will drop southward across Southwest
Alaska on Friday and then exit into the western Gulf sometime
Saturday evening. As such, unsettled conditions will persist
across the Alaska Peninsula into early Sunday. Chances for
precipitation increase across inland Southwest beginning late
Friday. The southward digging low and associated Arctic front will
help pull cold air across the entire region through Saturday
afternoon which will maintain strong northwesterly winds through
the gaps and passes of the Alaska Peninsula. In addition,
persistent upslope flow along the Alaska Peninsula will promote
heavier snowfall there and coupled with strong winds, areas of
blowing snow are possible. Snow will taper off by Sunday afternoon
with the exit of the upper shortwave, and winds will remain
relatively elevated through the short term forecast.

&&

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

A large longwave upper-level trough encompasses all of the Alaska
region and the Bering Sea/Aleutians. High pressure aloft builds
down to the surface over the western Bering and Aleutians.
Multiple upper shortwaves upstream of the ridge will track
southeastward across the Bering on Friday and Saturday. An Arctic
front associated with the aforementioned trough will continue to
pull down cold air across much of the Bering and Aleutians through
much of the short term forecast. Snow showers and stronger winds
have already started developing across the open waters of the
eastern Bering and across the eastern Aleutians. Winds and widespread
snow showers are expected to blanket much of the eastern domain
through Saturday and into Sunday as the much colder Arctic air
over runs the relatively warmer waters. Precipitation will taper
off for most of Sunday as the upper shortwaves enter into the
North Pacific and the aforementioned high pressure ridge builds
eastward across the Bering and Aleutians.

&&

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

Looking into the extended forecast, an amplified shortwave trough
extending from the polar regions moves moves eastward across
mainland Alaska while also continuing to dig southward into the
Gulf. This will keep portions of the Mat-Su Valleys and the Copper
River Basin unsettled through the day on Monday. Conditions for
the eastern domain will become mostly quiet and benign Tuesday
through the Thursday as ridging attempts to build overhead. Out
west, conditions remain mostly benign until Tuesday when a
Kamchatka low begins to move eastward. This low will also send its
front eastward Tuesday through Thursday as well while sweeping the
Aleutian Chain. A big push of warm southerly flow is anticipated
ahead of this front as rain looks to be the main precipitation-
type with this system. Winds accompanying this front look to be
high-end small craft with possible pockets of low-end gale-force;
especially for the western/central Bering Sea and western/central
Aleutian Chain.

&&

 $$

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...CQ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AF
LONG TERM...DAN