816
FXAK68 PAFC 170029
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
429 PM AKDT Tue Jul 16 2024

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Tuesday evening through
Friday)...

A cold upper-level trough continues to spin over the Mainland.
There is actually a large/deep parent system spinning over the
Pole that connects to a weaker center over the west-central
Interior. The whole trough extends well into the Bering Sea, and
Gulf of Alaska. The jet stream influence remains south of the
Aleutians, but rounds the base of the trough and is currently
pointing at the Panhandle. The setup leaves Southcentral Alaska
within the trough which will be the controlling feature for active
weather in the next few days.

For sensible weather, a weakly unstable air mass on the downstream
side of the trough is producing widespread rain showers across
most of the area. The Copper River Basin is the exception, though
some showers are forming along the Alaska Range. Expect shower
coverage to decrease over night tonight from west to east as the
trough pushes through the area. Showers will pop back up again on
Wednesday as we remain within the broad scale trough and cold pool
aloft, but without a focusing mechanism, activity should remain
along the mountains in most areas.

Forecast uncertainty increases on Thursday as the upper low center
drops over the Kuskokwim Valley. The position of the system should
allow for both forcing and moisture to be in place for the Cook
Inlet region, but guidance is struggling with placement and amount
of precipitation. While these things are uncertain, late Wednesday
into Thursday does look more certain for periods of rain for Cook
Inlet through the Susitna Valley.

Late week looks to be drier and warmer for all areas, however, as
the upper low drops into the Gulf of Alaska with Mainland ridging,
a very low predictability easterly wave pattern sets up for late
weekend into next week. Stay tuned.

MTL

&&

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

Overall, fairly benign weather expected as weak weather systems
move across Southwest Alaska, the Bering Sea, and the Aleutian
Islands over the coming days. Expect light to moderate rain and
rain showers, with minor uncertainty in rain timing and totals.
As a ridge builds over the Western Bering/Aleutians today and
transits eastward through Thursday, rain will taper off and give
way to fog and low stratus, especially in the Bering Sea and
Aleutians.

The slowly evolving pattern is supported by an anchored upper low
currently located near the Seward Peninsula. Weak shortwaves
moving around it are currently bringing spotty rain showers to
Southwest Alaska. A more robust shortwave is expected to move
first over Nunivak Island before entering the Kuskokwim Delta
coast. The wave is expected to spread rainfall quickly inland,
bringing light rain from Kipnuk to Aniak through Wednesday
morning. Up to a quarter-of-an-inch of rainfall is possible for
this area and time. By Wednesday morning, the boundary is expected
to push across the remainder of Southwest, reaching from Bristol
Bay to Lime Village, with similar rainfall totals across mostly
the Lower Kuskokwim Valley. Rain from this system expected to
taper off by Wednesday afternoon. For the next few afternoons,
through midweek this week, there is the chance for some isolated
thunderstorms to develop along the western side of the Western
Alaska Range. Synoptic wind flow will likely keep any cells over
elevated terrain.

The Bering Sea and Aleutian Islands have increased rain chances
as a low skirts south of the Aleutian Islands and Alaska
Peninsula. As the ridge of high pressure increases in amplitude
Tuesday night into Wednesday, expect fog and low stratus to
develop along the Bering Sea and Aleutian Islands. The Pribilof
Islands will likely return to fog Wednesday and Thursday morning,
with the deck lifting in the afternoon. A likely target for some
fog development will be along the central axis of the ridge
building in the Central Aleutians Wednesday night/Thursday
morning. By Thursday, another low moves into the Western
Aleutians/Bering, bringing winds to small craft advisory speeds
and another round of rain. A front along the leading edge of the
low is expected to cross the Chain Thursday and push into the
Central Bering, reaching the Pribilofs by Friday morning.

-CL

&&


.LONG TERM FORECAST (Days 4 through 7: Saturday through Tuesday)...

Much of the long-term period will be characterized by an omega
pattern. A large upper-level low will be present in the Bering Sea
with another large upper-level low in the North Pacific in close
proximity to the Gulf. A high pressure ridge will be between
these two features. This type of pattern will keep most of the
Bering Sea, Aleutian Chain, and southern Alaska Peninsula active
as numerous shortwaves rotate around the Bering low. The ridge,
initially over Southwest Alaska Saturday and the first half of
Sunday looks to shift eastward towards Southcentral Alaska while
it weakens. As the ridge shifts eastward and weakens, this will
open the door for a frontal system to move towards Southwest
Sunday night. There are still questions on the strength of the
front and whether or not it holds together enough to deliver
precipitation to the interior locations. The coast has the best
chance at seeing rain from this system. Conditions look to remain
showery and unsettled over Southwest for Tuesday as additional
shortwaves rotate over the area from the Bering.

As for Southcentral Alaska, the North Pacific/Gulf low will help
to drag in easterly waves from Yukon Canada over the area. This is
a good pattern for interior locations of the Copper River Basin
and the Susitna Valley to receive precipitation. With riding
nudging westward from the Yukon, temperatures also look to warm
over Southcentral relative to what is being observed in the short-
term period. Depending on day-time heating and instability,
potential thunderstorm development is not out of the question
across Southcentral each afternoon Saturday through Tuesday.
However, confidence is extremely low with where exactly each
mesoscale easterly wave will track.

&&

.AVIATION...

PANC...Weak flow through the atmospheric column and persistent
cyclonic flow aloft out ahead of a closed upper low over SW AK
will maintain a fairly steady state ceiling, with just minor
fluctuations in height. VFR conditions should prevail, though
low level flow will gradually veer from SE to SW overnight
tonight and persisting through Wednesday. This will lead to
upslope flow along the Chugach and likely lead to lowering
ceilings. The bulk of lower clouds look like they will remain
banked up against the mountains, though the terminal could see
conditions temporally lower to MVFR sometime late tonight or
Wednesday morning.

Meanwhile, gusty SSE winds will die down overnight, similar to
last night. For Wednesday, the coastal ridge will be replaced
by a much broader ridge over the Gulf. As a result, W to SW
winds will prevail for Wednesday.

&&

$$