Gale Warning 535, NWS Anchorage, AK

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

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

A negatively tilted trough and strong surface low is currently
tracking northeast from the Pribilof Islands and over the Y-K
Delta. Over Southcentral, a ridge of high pressure is being
shunted to the east, with increasing cloudcover and precipitation
moving its way over Cook Inlet and the Mat-Su Valleys. With the
strong pressure gradient between the Bering low and a coastal
ridge, strong gap winds are beginning to develop through Turnagain
Arm, Knik Valley, and the Copper Valley. These will continue to
strengthen, peaking in intensity between midnight and 3am as an
upper level shortwave moves overhead. The strong southeasterlies
in the lower levels will dry the lowest levels of the atmosphere,
which in many cases mitigates the rain, but in this case the
airmass is warm and wet enough that light rain on the magnitude of
0.1 to 0.35 inches is expected for the typically downsloped areas.

On Friday winds will slowly taper off and switch to the south as
the low moves inland to the north and the shortwave moves into the
interior. Light rain will linger for the northern Susitna Valley
and southwesterly upslope locations, but otherwise the subsidence
behind the shortwave should lead to a sunny day for many areas.
For Saturday, another deep low out west will act to build heights
over Southcentral once again, leading to more calm conditions as
upper level clouds slowly move overhead and to the east. By
Sunday, expect similar conditions to today, with strong winds
developing through the mountain gaps as another coastal ridge
builds and a deep low moves north through the western Interior.

-Jones

&&

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

Confidence remains high for wet and windy conditions across much
of Southwest Alaska and the Bering Sea in the coming days. A
Coastal Flood Advisory is now in effect along the Kuskokwim Delta
coast, from Kuskokwim Bay and west, through Sunday night. This
Coastal Flood Advisory covers the tail end of today's storm and
the entirety of this weekend's storm, which moves in by Friday
evening.

Diving into the details: an occluding low is currently located
near Saint Matthew Island. While the low is weakening, it's still
strong enough to pack quite a punch. Confidence is high that
southwesterly winds wrapping back around the low will be quite
strong. As such, sustained winds across Kuskokwim Delta from
tonight through tomorrow morning are expected to be as high as
35-45 mph, with gusts up to 50-60 mph. These strong winds could
contribute to a coastal storm surge, which was the impetus for
issuing the Coastal Flood Advisory. Otherwise, expect rain to
gradually taper off from west to east through tomorrow morning.

A second low pressure system tracks into the western Bering Sea and
strengthens on Friday, pushing its front to Southwest Alaska
Friday evening. Models are coming into better agreement with the
evolution of this low, which allows for a clearer picture of its
impacts and lending greater confidence to the forecast. The low
looks to draw more moisture into Southwest Alaska than today's
low, but upper level dynamics appear weaker. The end result of
this is a similar amount of precipitation or slightly less
(roughly around an inch) for much of Southwest Alaska as the low
lingers through Saturday night. A more significant difference
which requires further monitoring is that the track of the low
will make for a more prolonged, southerly oriented fetch, even
though winds are forecast to be of similar intensity to today's
system or slightly weaker. This wind direction (and duration) is
more conducive to elevated coastal water levels along the
Southwest coast, so with surge guidance suggesting similar water
levels to the first storm, there is greater confidence that these
could be reached on Saturday or Sunday. Mariners should also
beware of higher seas, with 15-20 ft seas likely near the
Pribilof Islands and in the offshore marine zones. The low lifts
into Western Alaska for Sunday and a ridge extends across the
Bering Sea for the start of next week, creating a period of
relatively calm and dry conditions before the arrival of a third,
similar storm next week.

Circling back, here's a few things to keep in mind when it comes
to the Coastal Flood Advisory for the Kuskokwim Delta. First, the
timing of the highest water levels will depend on the tide cycle
and may not occur at the same time as the strongest winds. Expect
the highest water levels to coincide with high tide, with water
levels likely dropping with low tide. Secondly, higher-than-usual
coastal water levels could persist through several tide cycles. As
such, be aware that even if water levels are receding during low
tides, they could rise again (and perhaps to even greater levels)
at the next high tide. Finally, while confidence is high in the
winds producing this surge and coastal flood risk, the actual
impacts are difficult to ascertain. Observations are limited along
the Kuskokwim Delta coast line, which makes it difficult to
compare this to past events and get a sense of what will unfold.
Topography and bathymetry will also play a role in actual water
levels and impacts. One other thing to consider will be the effect
of the moderate to heavy rain that we saw with today's storm, as
well as more rain incoming with this weekend's storm. Confidence
is low, but it's possible that the influx of all this rain could
worsen flooding. All told, use caution and carefully monitor both
local conditions and the forecast in the coming days, especially
as the forecast could shift as models better hone in on the
incoming storm systems.

-Quesada/Chen

&&

.LONG TERM FORECAST (Days 4 through 7/Sunday through Wednesday)...

On the Alaska Weather map, an amplified and energetic upper level
flow pattern supports multiple rounds of significant surface
weather across the region through midweek. The GFS and ECMWF
serves as a good compromise on system development through Monday,
with an ensemble blend taking over through Wednesday. The main
upper level closed low in the North-central Bering weakens and
moves into the Chukchi Sea for Monday. Gusty near gale force winds
expand over the Central and Eastern Bering into the Kuskokwim
Delta through early Monday morning, diminishing. The remaining
trough moves across the Mainland and across the Alaska Peninsula,
Kodiak Island and Southcentral Alaska through Tuesday, with
locally heavy rains over Western Alaska Sunday. The front
continues locally heavy rains across the Southcentral Interior for
Monday. The remnants of the former Tropical Storm Ampil sweeps
over the Western Aleutians Monday night and up the Western Bering
through Wednesday. An upper level trough extending across Russia
lends support and helps keep the track to the West. Locally heavy
rains accompanying this system spread over the Central Aleutians
and Bering through Tuesday. Areas of gusty near gale force winds
and Pacific side gale force gusts moves from the Western Aleutians
to St Lawrence Island, diminishing Wednesday.

- Kutz

&&

.AVIATION...

PANC...Southeasterly winds out of Turnagain Arm are expected to
persist into Friday afternoon and then begin to subside. The rain
moving over the terminal overnight is not expected to lower
ceilings and visibilities too much. Most likely limitation is for
the ceiling to lower near or to MVFR levels after midnight into
the early morning hours. Otherwise VFR conditions are expected.

&&