Gale Warning 534, NWS Anchorage, AK

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

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Saturday night)...

Key Messages:

*The forecast is on track for rain and gusty winds to spread into
 Southcentral from west to east this afternoon through tonight.

*Rain will taper off Friday with some sunshine likely in the
 afternoon.

*There is some uncertainty in the forecast for the weekend,
 particularly with time of arrival of another front from the
 Bering Sea. The areas most likely to see rain late Saturday into
 Sunday include Kodiak Island, Kenai Peninsula, Anchorage, and
 the Mat-Su.

Satellite imagery shows a large multi-centered low tracking
northward across the central Bering Sea this morning, with a
building short-wave ridge out ahead of it progressing north and
east into mainland Alaska. An atmospheric river, with total
precipitable water values of 1.5" or greater is streaming
northward out ahead of the low, then wrapping back into the low
center. Weather across Southcentral is quiet out ahead of this
storm system. A weak short-wave trough riding over top of the
ridge has brought some clouds to the Susitna Valley. Low clouds
can be found along the north Gulf coast and Prince William Sound.
In between, mostly clear skies prevail.

The forecast for Southcentral is on track, with very little change
made to the forecast from yesterday. There are still some minor
model differences with time of arrival of precipitation, but
overall forecast confidence in precipitation and wind forecasts
has increased. A slightly further west low track means what looked
like a potential borderline high wind scenario for the Anchorage
Hillside now looks safely below the High Wind Warning threshold
of 75 mph. Meanwhile, in the upper levels the short-wave trough
looks very similar, with just a glancing blow to Southcentral -
particularly the western half of Southcentral. The NAM looks to
be an outlier with a higher amplitude/stronger short-wave and the
earliest arrival. Bottom line, this looks like a quick shot of
rain for most areas, with the bulk of rain falling late this
afternoon through the overnight hours, then tapering off Friday
morning as the upper level short-wave quickly exits to the north
and east. Similarly, winds will ramp up this afternoon and
evening, then peak overnight before diminishing through the day
Friday.

Another storm system will take shape along the Aleutians Friday,
leading to another amplifying ridge downstream. The overall ridge
looks to be much higher amplitude and stronger than today's
ridge and it will build northward across Southcentral Saturday.
This will serve to slow the eastward progression of the frontal
system and atmospheric river. No surprise, models are struggling
with arrival of precipitation in Southcentral this weekend. What
had looked like a chance of rain arriving Saturday now looks like
it may not get here until Saturday night or Sunday. Similar to
the current system, it looks like it will be a glancing blow for
Southcentral, with a relatively quick moving shot of rain and
wind. With the low track even further west, gap winds will not
be nearly as strong as with today's storm system.

-SEB

&&

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

An elongated low lifting across the central Bering Sea has begun
to push its front onshore of Southwest Alaska this morning.
Moderate to heavy rain is forecast along the Southwest coast over
the next several hours as the strongest forcing aloft wrings out
abundant moisture sourced from the tropical Western Pacific, with
precipitable water values in excess of 1.5 inches, over Southwest
Alaska. At the same time, southeasterly winds near gale force
will gust into the 45-55 mph range across Kuskokwim Bay. Since
this is forecast to occur during high tide, there is potential for
water levels to reach 1 to 5 feet above the normal highest tide
line along the northern Kuskokwim Bay coastline, between Kipnuk
and Apokak Creek. Factors that complicate this forecast include
the southeasterly wind direction which limits the fetch across the
water and a relatively low astronomical tide, which could result
in water levels peaking in only the low end of the range later
this morning.

As the front pushes inland, downsloping will allow for some
degree of drying near the surface and will limit rainfall amounts
through the day. In total, one to one and a half inch of rainfall
is forecast along and near the coast from Dillingham to Toksook
Bay, with inland locations, particularly the Lower Kuskokwim
Valley and eastern portions of Bristol Bay seeing totals closer to
half an inch. As the low lifts into Western Alaska Thursday
night, rain will taper to showers and winds will shift out of the
southwest. Pressure rises behind the exiting low will enhance
these winds, leading to another round of wind gusts approaching 55
mph along the Kuskokwim Delta coast, with gusts of 45-50 mph for
the inland delta.

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.

-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...Gusty southeasterly winds will develop around midday and
then steadily strengthen through the afternoon and evening hours
in response to an approaching front and low tracking to the west
coast of Alaska. There will be some fluctuations in strength
during the event. Pressure gradients would suggest a peak around
09Z at which point the Turnagain Arm jet will be strongest.
However, the core of the jet will be solidly over Fire Island and
the northern Inlet. The jet will bend into Anchorage between 12Z
and 18Z Friday, so that could be the timing of strongest winds. In
any case, gusty winds will linger through much of the day friday,
though they will diminish with time.

There will be a period of light rain as an upper level short-wave
passes through late this afternoon and evening. However, strong
(but shallow) southeasterly flow across the Kenai and Chugach
Mountains should keep ceilings/vis primarily in the VFR category.
There could be a brief period of heavier rain right as short-wave
comes through (sometime between 06Z and 12Z) in which the ceiling
and/or vis temporarily drops to MVFR.

&&