Gale Warning 368, NWS Anchorage, AK

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

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

A low pressure system in the central Gulf of Alaska that has been
the source of active weather for Southcentral the last few days
has shifted south, granting the Southcentral coastline a reprieve
in precipitation this afternoon and evening. Interior locations
are enjoying a break in the clouds and some clear skies this
afternoon due to weak ridging over the area. The clearing skies
have allowed for the first signs of convective activity over the
Mat-Su valleys, late this afternoon. Given the favorable pattern
and local conditions expected to be repetitious in the morning,
areas of fog may develop again Monday morning but should similarly
dissipate by late Monday morning into the afternoon.

By Monday morning, the low in the Gulf will shift back into our
area, making landfall east of Kayak Island. Model agreement is
poor on how far east of Kayak Island the low pushes. Regardless,
the system is expected to send easterly waves back into our area.
These waves will begin to approach Cordova/Valdez by Monday
afternoon, returning the snowfall pattern to the area. Interior
areas such as Anchorage and Mat-Su will likely be sheltered from
these initial easterly waves, potentially providing us conditions
for another clear day Monday.

By Tuesday morning, an upper-level Arctic Trough will dive into
Southwest Alaska, building a low pressure system that will cross
the Alaska Peninsula near the Barren Island. This low will be
broad due to the stout upper-level support and will envelop the
low to the northeast, amplifying the conditions across coastal
Southcentral. As such, a return to widespread snowfall will come
to the coast from Seward to Cordova. 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.

-CL

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA, BERING SEA, AND ALEUTIAN
ISLANDS (Days 1 through 3: This afternoon through Wednesday)...

Few changes were made to the forecast through Monday evening. From
Monday night onwards, the forecast was updated to better represent
a quick-moving front that crosses the Bering Sea through Wednesday.
Here are the primary hazards we're monitoring:

* Heavy to extreme freezing spray in Bristol Bay and along the
  Bering side of the Alaska Peninsula through Tuesday.

* Brief periods of blowing snow and significantly reduced
  visibility as scattered snow showers move across the Bering side
  of the Alaska Peninsula from Monday into Tuesday.

* Potential for blowing snow and significantly reduced visibility
  as Wednesday's front moves across the Pribilof Islands,
  Kuskokwim Delta/Nunivak Island, and the Western Capes of Bristol
  Bay.

Diving into the details...quieter weather closes out the weekend
as yesterday's low departs into the North Pacific. A few weaker
shortwaves keep gustier winds and occasional snow showers for the
Bering Sea, Aleutian Chain, and Southwest Alaska through tonight.

By late tonight, a potent shortwave trough dives south from the
Arctic, 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 tonight 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.

Strong cold air advection, coupled with robust upper level vorticity,
will lead to gusty winds as we head into the work week. These
northerly winds will be especially enhanced by terrain gaps on the
Alaska Peninsula, allowing gusts to reach as high as 50 to 70 mph
for marine waters on the Pacific side of the Peninsula. The
combination of frigid air temperatures, decreasing sea surface
temperatures, and sustained winds to gale force will lead to broad
areas of heavy to extreme freezing spray through Tuesday. On
land, wind chills drop below zero and dip as low as -25 from
Monday night into Tuesday.

There is still potential for blowing snow and significant
visibility reductions along the Alaska Peninsula from Monday
afternoon into Tuesday morning. However, current thinking is that
periods of lower visibility should be relatively brief as snow
showers are not expected to be persistent. The period of greater
concern for blowing snow, instead, will be on Wednesday as a low
pushes its front west to east across the Bering Sea. As the
front's gusty winds and abundant moisture move towards the
Southwest Alaska coastline, lingering cold temperatures from the
Arctic surge could provide favorable conditions for blowing snow.
A blizzard is not out of the question, but finer details such as
these will need to wait until models get a better handle at the
synoptic scale. While details such as the strength and track of
Wednesday's low are still somewhat uncertain, there is high
confidence that sustained winds to gale force will overspread the
Aleutian Islands and much of the Bering Sea from late Tuesday
through Thursday.

-KC

&&

.LONG TERM FORECAST (Days 4 through 7) Thursday through
Sunday...

Long term models are in relatively good agreement with the
amplification of a large upper low over the Bering on Thursday.
Several shortwaves digging around the low will bring it across
Southwest and into Southcentral into early Friday. A cold air mass
will remain in place across the Bering with gusty conditions
persisting along the Aleutian Chain and the potential for a winter
storm to push across the Kuskokwim Delta, the Lower Kuskokwim
Valley, and the Greater Bristol Bay regions during the same time
frame. This system will be the primary focus and concern during
the long term period as cold air over the Bering wraps around the
low into the Gulf and across Southcentral as well. With Anchorage
still in the running to have the snowiest winter on record,
Thursday could prove fruitful with a round of accumulating snow,
with widespread snow showers spreading across Southcentral. Due
to the nature of the expected airmass, lapse rates will be quite
steep, further supporting the notion of numerous snow showers
spilling over from Southwest Alaska into Southcentral. The
Canadian forecast model is the most bullish with the strength
of the trough moving into interior Alaska, with the GFS not far
behind. Still, it remains just far enough out in time that
forecast confidence will remain low until further run-to-run
consistency within model guidance is observed.

BL

&&

.AVIATION...

PANC...VFR conditions and light north to northwest winds will
persist into the overnight hours. Areas of low, patchy fog could
impact the terminal again early Monday morning before sunrise.
Unlike Sunday morning, the fog and low stratus should not be
nearly as dense or widespread, and should be shorter lived.

&&