Brisk Wind Advisory 64, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
514 AM AKDT Sun Apr 7 2024

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Today through Tuesday night)...

A messy longwave upper trough now extends across much of the
southern Mainland out into the Gulf this morning. Two smaller
features are visible on satellite near Southcentral, including a
stacked and weakening low over the eastern Gulf and a nearly
stalled out upper trough drifting over the Susitna Valley. Areas
of showers are spinning around the Gulf low, at times moving into
the coastline and affecting parts of Prince William Sound and the
eastern Kenai Peninsula. Areas of patchy low stratus/fog have
developed in the wake of a few snow showers that skirted across
the Mat-Su and Anchorage areas yesterday evening. However, the
patches of fog should dissipate within a few hours after sunrise
as heating of the day commences. A few snow showers could return
to the Mat-Su, and perhaps even Anchorage, near the upper trough
still hanging around later this afternoon and evening. However,
this shower activity will likely not be as widespread compared to
yesterday evening.

On Monday, the weak Gulf low will make landfall near Yakutat and
dissipate as it does so. A few rain/snow showers could push past
Valdez and Cordova as the low shears apart, but this system will
otherwise be unimpactful to the region. Meanwhile, a much more
potent system will barrel south from Southwest across the AKPen
into Kodiak Island as a very deep and cold upper level low.
Substantial cold air advection will begin as the low enters from
the north, particularly from Kamishak Bay south along the AKPen
to Shelikof Strait. Gusty northwest winds to gale force range will
rapidly pick up out of gaps and passes along this corridor as
cold air rushes in from the northwest between Monday and Tuesday,
with gusts up to storm force range not out of the question. By
Monday night, a surface low will form and quickly deepen under or
just ahead of the main upper level low, likely just off to the
northeast of Kodiak. South to southeast flow will also strengthen
northeast of the low, allowing snow showers enhanced by upslope
flow to overspread much of the Gulf coastline from the southern
Kenai Peninsula out to the Prince William Sound through Tuesday
morning.

On Tuesday, the strong upper level low will nearly stall out over
the western Gulf as lobes of vorticity in the upper levels wobble
north a bit into Southcentral. By this time, cold air will begin
to wrap all the way around the low, ushering a colder air mass
into the Kenai Peninsula up into the Mat-Su from the southeast.
Very cold temperatures aloft (below -40C at 500 mb) coupled with
diurnal heating at the surface will yield a very unstable
atmospheric profile developing near the low towards the afternoon
on Tuesday. This will help fuel vigorous bands of convective snow
showers streaming into the coast, especially along the eastern
Kenai Peninsula and across the Prince William Sound. Some of this
activity will likely spill over the mountains into interior
Southcentral from Tuesday afternoon into Tuesday night,
potentially affecting parts of the Mat Valley, Anchorage and the
eastern parts of the Kenai Peninsula at times. Snowfall
accumulation could vary wildly depending on exactly where bands of
showers train over the same areas, with the best chance for
several inches of accumulation on Tuesday focused near Seward,
Whittier, Valdez and Cordova.

-AS

&&

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

The forecast through the short-term remains largely on track. A
potent shortwave trough begins diving south from the Arctic in the
wake of the departing low, bringing a shift towards colder
temperatures. The impact from this shortwave trough will be most
strongly felt in Southwest Alaska, the Alaska Peninsula, the
Eastern Aleutians, and the marine waters surrounding these areas.
Cold air surges southwards from Sunday night through Tuesday
morning, dropping air temperatures as low as the single digits
for much of Southwest Alaska and as low as the teens for southern
Bristol Bay and the Alaska Peninsula. This strong cold air
advection, coupled with robust upper level vorticity, will lead
to gusty winds. Wind chills drop below zero and as low as -25
from Monday night into Tuesday.

Showery conditions will continue along the Alaska Peninsula down
to the Eastern Aleutians. It will be mostly a rain/snow mix as
temperatures rise above freezing this afternoon. Temperatures will
drop around 10-12 degrees tonight, promoting a transition to all
snow. Cold arctic air will continue to pull southward through at
least Tuesday, so anticipate snow showers and colder temperatures
to continue through the early portions of this week for most of
Southwest Alaska and the Aleutians. Northerly winds will pick up
across the eastern Bering Sea and Alaska Peninsula, enhancing gap
winds south of the AKPen by Monday morning, with wind gust
reaching as high as 50 to 60 mph and creating areas of extreme
freezing spray along the ice edge. There may be bouts of reduced
visibilities due to blowing snow at times, primarily during
periods of snow showers.

The next system is expected to push into the western Bering Sea on
Tuesday and moves into Southwest Alaska by Wednesday. There is
still some uncertainty regarding how quickly this low travels and
how strong it will be. Model guidance over the past couple days
have showed the front moving progressively faster. Currently,
there is a decent chance for areas of storm force winds in the
Bering Sea, as well as some heavy precipitation along the
Aleutians and Southwest Alaska, primarily along the AKPen and
Southwest Coast. This system will continue to be monitored over
the next few days.

&&

.LONG TERM FORECAST (Days 4 through 7) Wednesday through
Saturday...

The long term period will start off with below normal temperatures
across Southwest and Southcentral Alaska on Wednesday. An arctic
trough, having dug into the Gulf of Alaska, will shift into the
eastern Gulf through Thursday morning. Showery conditions, including
the potential for heavy snow, will exist along the Gulf coast and
coastal mountains. At the same time, a Bering Sea low will be in
the process of pushing a broad front into the coast of Southwest
Alaska Wednesday night with increasing gusty winds. Showery
conditions along the front, initially a mix of rain and snow,
should quickly transition to all snow as cooler air moves in
behind the front from the west. Southcentral looks to dry out and
remain mostly quiet from Thursday into the start of the weekend.
The amplification of the Bering low, however, will favor
widespread gusty and showery conditions, as well as potential
for snow lifting into the coast of Southwest Alaska by Saturday.

-BL

&&

.AVIATION...

PANC...VFR conditions and light north to northwest winds will
persist. Some areas of low, patchy fog could affect the terminal
with brief visibility reductions to IFR conditions possible, but
widespread fog and low stratus is not anticipated this morning. A
couple stray snow showers could also drift near the terminal this
afternoon, but they will likely be much more isolated than what we
saw yesterday evening.

&&