002
FXAK68 PAFC 210118
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
518 PM AKDT Thu Jun 20 2024

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

Key Messages:

* Warmer temperatures and an increase in thunderstorm activity
  coming to Southcentral this weekend.

* Gusty offshore winds in Resurrection Bay and Passage Canal
  Friday night through Saturday morning.

* Gusty northerly winds across the northern Susitna Valley and
  northern Copper River Basin on Friday night through Saturday.

Looking at satellite imagery, the key weather features are a
vertically stacked low in the northeast Pacific tracking into the
Gulf of Alaska and an east-west oriented upper trough lifting
northward from the Gulf into Southcentral. Winds and rain are on
the increase across the Gulf and Kodiak Island ahead of the low.
Extensive cloud cover with the upper trough is leading to a cooler
day along coastal areas of Southcentral from the Kenai Peninsula
to Prince William Sound. However, there is very little in the way
of precipitation, with just some patchy light rain along the
immediate coastline. Meanwhile sunshine over interior Southcentral
has led to increased instability ahead of the upper trough. As
a result scattered showers and isolated thunderstorms have
developed (as of mid afternoon) near the Wrangell Mountains,
Alaska Range, and Talkeetna Mountains and are tracking roughly
from ESE to WNW. The bulk of showers and thunderstorms will exit
toward the Alaska Range this evening

On Friday, the main upper trough from today will weaken and rotate
northwestward toward Interior Alaska. Atmospheric flow over
Southcentral will back around to the northeast in response to the
Gulf low progressing eastward. Clouds will diminish along the
coast, while the northerly component to low level flow results in
warming temperatures and establishment of a thermal trough over
interior Southcentral. This will result in an uptick in showers
and thunderstorms tomorrow afternoon and evening, though the bulk
of them will remain inland. Storm motion will generally be from
northeast to southwest.

Continuing to Saturday, an upper level ridge will strengthen
somewhere over the Alaska Interior. Model guidance varies on the
center of the ridge, with most guidance over the western Interior.
This will bring even strong northerly flow to Southcentral Friday
night through Saturday, leading to additional warming and
destabilization of the atmosphere. Locally gusty offshore winds
are expected along the coast and along the Alaska Range into the
Susitna Valley and Copper River Basin, contributing to very warm
and dry conditions region-wide (probably not quite as hot as last
Saturday, but not too far away). Showers and thunderstorms will
form inland and track toward the coast the late afternoon and
evening hours, potentially including Anchorage once again.

While Sunday continues to look warm with afternoon/evening showers
and thunderstorms, there is quite a bit of model spread in
location of upper ridging and troughs. Thus, it`s difficult to
nail down the preferred areas for convection and overall storm
motion.

&&


.SHORT TERM FORECAST SOUTHWEST ALASKA, THE BERING SEA/ALEUTIANS
(Days 1 through 3: This afternoon through Sunday afternoon)...

The big picture of the forecast remains largely unchanged, though
there have been major updates to the precipitation and
thunderstorm forecast for Southwest Alaska as we continue to
assess more data. Expect thunderstorm chances to persist through
the weekend for Southwest Alaska, along with a general warming
and drying trend. For the Bering Sea and Aleutian Islands, most
areas will see light winds less than 25 kt as the ridge moves east
across the Bering Sea, and incoming lows remain on the periphery
of the forecast area.

Diving into the details... the big eye-catcher will be the
numerous showers and isolated to scattered thunderstorms moving
across Southwest Alaska this afternoon and evening. While the
forecast had been plagued by poor model agreement, models have
steadily come into better agreement today, especially with the
weak shortwave moving overhead that`s helping to spur on this
convection. We`ll be keeping an eye on this convection through
the overnight hours, but based on how the convection has developed
so far, showers and thunderstorms could be moving west a bit more
quickly than initially anticipated. This means that areas west of
where thunderstorms are currently forecast, like Bethel, could
see some lightning today and tonight. Storm motion will be east
to west today, so if you see lightning and thunderstorms
approaching from the east, be sure to stay weather aware as they
could be moving into your location.

Thunderstorm chances continue through the weekend, though forecast
confidence decreases appreciably for Saturday and Sunday
afternoon. Even for tomorrow afternoon, there is some forecast
uncertainty based on how storms perform today. If clouds linger
from today`s storms, that could affect daytime heating for
tomorrow`s storms. Either way, expect storms to generally move
from north to south both Friday and Saturday afternoons as
steering flow aloft becomes more northerly. For mariners in
Bristol Bay, this means that showers and thunderstorms could move
off land and onto coastal waters, presenting a marine hazard. Stay
tuned as we continue to refine this aspect of the forecast.
Otherwise, shower coverage gradually diminishes this weekend as
the upper level ridge moves in. Afternoon and evening highs will
climb into the 70s under this ridge, with the potential for
temperatures to even approach 80.

-KC

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

A number of changes come to light on the Alaska Weather map
through the forecast period. The upper level low over the Russian
Far East curves towards the Sea of Okhotsk and dissipates late
Tuesday. Its extended trough forms another low over the Western
Aleutians and Bering for Wednesday, and moving to the Central
Bering for Thursday. A third low in the Eastern North Pacific
exits along the Canadian coast for Thursday. A shortwave ridge
over the Eastern Aleutians and Bering moves over Southcentral
Alaska and reinforces an Interior ridge through Thursday. Model
agreement in the geographic broad brush is good, but becomes
somewhat chaotic in the finer details. ECMWF/GFS are closest in
the details, the Canadian model runs low and slow.

A well developed low and front spreads locally heavy rain and
gusty surface winds less than gale force across the Aleutians and
Bering, impacting the Pribilofs and Eastern Aleutians for midday
Tuesday, and into Western Alaska, the Alaska Peninsula and Kodiak
Island through Thursday. Surface thermal troughs support some
convection across the Southcentral Interior South of the Alaska
Range. Areas closer to the coast will see isolated to scattered
showers through Thursday.

-Kutz

&&

.AVIATION...

PANC...Light and variable winds will become stronger out of the
southeast by this evening as the winds out of the Turnagain Arm
strengthen. Winds will eventually abate during the overnight
hours. Showers and perhaps and isolated thunderstorm may develop
in the general area (Talkeetnas, Chugach Mtns) on Friday, but
will likely remain out of ANC. Otherwise, VFR conditions will
prevail throughout the TAF period.

&&


$$