215
FXAK68 PAFC 160108
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
508 PM AKDT Sun Sep 15 2024

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

Discussion:

Much of the cloud cover was able to clear out of the Cook Inlet and
portions of the Mat-Su Valley and Copper River Basin by mid-morning
as showers pushed off to the north. Gusty southwesterly flow and
minimal cloud cover in these areas allowed temperatures to warm into
low to mid 50s by this afternoon. However, cloud cover is starting
to build back over the Inlet and Susitna Valley in response to a
shortwave trough rounding the base of the parent low now centered
over Norton Sound. This stacked system will send several shortwaves
across Southwest and Southcentral Alaska over the next few days
before pushing into central Alaska and weakening. This first wave is
has already kicked some scattered showers across Bristol Bay and the
northern AKPen, and should see an uptick in showers along the
southern Alaska Range, and potentially up towards the Talkeetna`s,
as it lifts northwards this evening. Should also see showers start
to develop over the Copper Basin as lift from the aforementioned
trough nears the Basin by tonight. While models are showing about
100-200 J/kg of CAPE along the Talkeetna Mountains, clearing was
minimal and short-lived...so while chances of an isolated
thunderstorm are not nil, it does look like a low probability of it
happening. Another shortwave trough, not far behind the previously
mentioned shortwave, is becoming more defined in satellite imagery
as it moves into southern Bristol Bay. This trough will move over
the AKPen this evening before passing over Kodiak Island late
tonight...kicking up some showers as it does before moving out into
the Gulf early Monday morning.

Monday will see generally more quiet weather with drier conditions.
The exception will be along the Alaska Range and northern Copper
Basin as the upper low moves inland, and along the Gulf coast where
scattered showers will continue as the southern shortwave trough
treks across the northern Gulf. Cloud cover will become more
important Monday night into Tuesday morning as cooler, drier air
filters into Southcentral. For areas that clear, light winds and
weak ridging will allow for good radiational cooling overnight and
Tuesday morning temperatures may feel a bit chilly as temperatures
drop into the 30s to lower 40s for most areas in Southcentral. The
Copper River Basin will likely see the coldest morning temperatures
with lows looking to bottom out in the low 30s. Increasing mid
and upper level cloud cover, moving in from the south, should keep
temperatures from dropping too low for areas along the Cook Inlet
up through the Mat-Su Valley, but timing of this will factor
heavily into how low temperatures drop. One thing to note, is that
we are now reaching that time of year when we start seeing our
first temperatures of 32 degrees for the upcoming cold season. Now
is a good time to start thinking about how to protect those
cherished warm season plants that are still outside as we start to
increase our chances reaching low temperatures of 32 degrees or
lower.

- PP

&&


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

Key Points:

* A River Flood Advisory for the Lower Kuskokwim River
  communities has been allowed to expire. Water levels have peaked
  and should continue to trend down. A Coastal Flood Advisory for
  the Kuskokwim Delta coastal communities continues through 1AM
  AKDT tonight.

* A stronger storm system remains forecast to track across the
  Aleutians into the Southern Bering Sea Monday afternoon then
  track toward Southwest AK through Tuesday. There remains high
  uncertainty in the exact track of this storm and locations most
  at risk of coastal storm surge flooding.

Discussion:

An upper low sits over Norton Sound this afternoon with a broad
area of small craft winds spread across the eastern Bering. This
low is also maintaining showers across the Kuskokwim Delta into
the Kuskokwim Mountains. Precipitation is expected to trend down
through the overnight hours tonight, though light rain will still
be possible in some areas through Monday morning. High pressure
behind the departing upper low and the next incoming system to the
west will see most precipitation come to an end across Southwest
Alaska by midday on Monday.

The main focus for the next few days will encompass a deepening
low pressure system over the western Bering on Monday. A deep low
over Kamchatka is expected to dig into the Bering while
interacting with a vigorous, lead-shortwave trough emerging from
the North Pacific into the southern Bering. The end result will be
the development of a stronger upper low over the southern Bering
through Tuesday, which then tracks into Southwest Alaska.
Forecast models continue to trend toward a stronger low solution
with widespread gales across much of the Bering and a swath of
storm force winds on the south of the low. This swath of stronger
winds ultimately gets directed into Bristol Bay on Tuesday with
building seas of 15 to 20 feet possible.

There remains continued uncertainty with the exact track of the
low as models have various discrepancies. Have leaned the forecast
toward the EC and Canadian, which are reasonably aligned. The GFS,
however, continues to be the southern outlier, though consistently
south. The NAM is more of a middle ground solution but does
ultimately trend the lower further south. The one area of higher
confidence remains in the winds. Regardless of the northern or
southern track, model continue to bring the strongest winds into
Bristol Bay. Questions that need to be resolved, however, are the
placement of the low and duration of winds. A further north and
deeper solution of the low would extend the duration of winds into
Bristol Bay and possibly necessitate some flood products. A more
southerly track, such as the GFS solution, would still bring very
strong winds into Bristol Bay, but for a shorter duration as the
GFS solution is more progressive with the low.

Finally, based on the orientation of the low as it occludes in a
bent-back fashion, the potential increases for the development of
a sting jet as cold air is drawn into the backside of the low.
This sting jet could bring storm force winds into the Pribilof
Islands late Monday, but uncertainty is too high in where the
sting jet will ultimately develop. Keep an eye on the forecast as
forecast details become more clear over the next 24 hours.


&&


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

By the start of Thursday morning a building ridge over the
eastern Bering Sea appears to settle in. The resiliency of ridges
is usually a point of contention in the model solutions, however
agreement is currently strong regarding a fairly strong ridge
remaining over the area, warding several low pressure systems off
south of the Chain into the early portions of the weekend. Model
uncertainty is much higher to the east in Southcentral and the
Gulf of Alaska. By Friday morning, the GFS solution pulls a fairly
strong low pressure system up from the northern Pacific into the
Central Gulf, spreading wind and rain across the coast, whereas
the Euro and Canadian models keep a mild trough in the northern
Pacific, with Southcentral staying clear. By Sunday morning,
solutions comes back into better agreement with the ridging over
the eastern Bering weakening as a strong low pressure system fills
over the western Aleutians and a new low over the the Gulf of
Alaska. A broad troughing pattern looks to resume thereafter,
which will keep shower chances in play for both Southwest and
Southcentral Alaska through the remainder of the long term period.

-CL

&&

.AVIATION...

PANC...Overall, conditions will be quite persistent at the
terminal, with gusty southerly winds due to low level instability
and flow into a low tracking across Interior Alaska. There will
be some fluctuations in strength of wind gusts, but generally
remaining between 25 and 35 kts through at least 18Z Monday.
Winds will finally slacken after 18Z as the low over the Interior
dissipates and a short-wave ridge approaches from the west.

Meanwhile, a series of weak short-waves will track overhead
through Monday, leading to occasional showers in the area. The
mean atmospheric flow will be out of the southwest, favoring the
front range Chugach for showers, though can`t rule out a shower
or two at the terminal. Ceilings should remain VFR, though expect
lowering of the ceilings with one particular short-wave coming
through Monday morning. For now, have indicated low end VFR
ceilings, but expect there will be some MVFR ceilings in the
vicinity and perhaps even very briefly at the terminal.

&&


$$