Gale Warning 505, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
549 PM AKDT Sat Aug 24 2024

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

A trough over the northern Alaska Peninsula will pivot slowly east
through Monday morning. The most widespread shower activity will
be tonight through early tomorrow as the upper-level low center
moves through the Bering Islands. On Sunday, a lee-side surface
low will develop over the northern Gulf, leading to more showers
for the eastern Sound and Copper Valley, as well as breezy
offshore winds through Seward, Whittier, and the Valdez Narrows.
For Mat-Su and Cook Inlet areas, sunnier skies will develop as a
brief period of ridging is able to establish.

Looking ahead to late Monday and Tuesday, a broad trough and
moderately strong upper low will move into Southwest Alaska.
Initially, south and east gap winds will develop through the
typical spots as the coastal ridge builds ahead of the front.
Southeasterly flow will lead to some downsloping as the trough
moves in, but southwesterly flow aloft and good upper level
dynamics mean another round of rain for most areas on Tuesday.

-CJ

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA, THE BERING SEA/ALEUTIANS
(Days 1 through 3: Tonight through Tuesday)...

Southwest Alaska is situated under an upper-level longwave trough
which extends across the whole of mainland Alaska this afternoon.
Cold air aloft is contributing to unstable conditions in which scattered
to numerous rain showers across the region have developed. Showery
conditions will diminish through the evening as daytime heating of
the surface decreases and more stable conditions return for tonight.
Over the central Bering Sea, high pressure prevails, producing
calm conditions and a broad area of stratocumulus. Some fog was
observed in this air mass early this morning and seems likely to
return tonight. Further west, a Kamchatka low is producing
southerly gale force winds in the westernmost periphery of the
forecast area, with a southwesterly tap of moisture bringing
moderate to heavy rain to the western Aleutians.

A progressive pattern will translate these features quickly
eastward, with the upper low over Southwest Alaska exiting into
the Gulf for Sunday and drawing cold air across the Alaska
Peninsula. As a result, northwesterly gap winds gust to 35 kt on
the south side of the Alaska Peninsula peaking tonight before
gradually diminishing through Sunday evening. The high pressure
over the Bering situates over Southwest Alaska for Sunday,
bringing calm, dry conditions with high temperatures a few degrees
warmer than today. This will be short-lived, however, as the
Kamchatka low pushes quickly into the Bering Sea, its front
sweeping through the central Aleutians and Pribilof Islands on
Sunday and reaching the Southwest coast by Sunday night.

Southeasterly winds to gale force, gusting as high as 55 mph along
the Kuskokwim Delta coast will accompany the arriving front. This
will sound similar to some of the coastal flooding scenarios from
just over a week ago, but the quick progression of the front will
mean these winds will be brief, limiting the potential for storm
surge. This will not be as significant as the recent series of
storms along the Kuskokwim Delta coast. However, there will be
some potential for water levels to rise above the normal highest
tide line by around 1 to 2 feet (with the greatest potential
focused closer to 1 foot) during high tide on Monday morning.

The front pushes east into Southwest Alaska, bringing light to
moderate rain. A shortwave driving along the Aleutians on Monday
will lift into Southwest Alaska Monday night into Tuesday,
enhancing precipitation for a second round just as the initial
front is weakening. In total, through Tuesday, Southwest Alaska
can expect half an inch to one inch of rainfall from this system,
with the greatest amounts focused along the coast. Monday through
Tuesday, another area of high pressure moves in across the
western/central Bering, bringing a return to light westerly flow
and calmer conditions as precipitation begins to taper to showers
across Southwest Alaska.

Quesada

&&

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

The start of the period towards the middle of the upcoming week is
still poised to be very progressive, with multiple systems quickly
moving across the outlook area from west to east. However, there
will likely be a marked shift in this pattern in the following
days as a much more blocked and more amplified large scale
pattern begins to take shape, especially towards next weekend.

At the start of the forecast period on Wednesday, an upper level
low/trough is progged to be crossing over the southern Mainland,
supporting a very showery regime over much of the Gulf and
Southcentral in particular. Farther west, there is also good
agreement that a more potent low will be trekking across the
western Bering Sea and Aleutians. This low will likely be strong
enough to support at least a gale force wind, possibly up to low
end storm force near the low's center if a solution on the higher end
intensity from the current model spread materializes. Strong winds
and locally moderate to heavy rainfall near this low is then
expected to shift east with the center into the eastern Aleutians
and AKPen by Thursday morning, while much of Southcentral dries
out as the upper trough in place on Wednesday lifts towards the
Yukon and allows a shortwave ridge in between the two systems of
interest to move into the southern Mainland.

From Thursday afternoon onwards, models all have the same rough
idea for the overarching pattern, but unsurprisingly, the details
begin to fall apart. Models all show the Bering low slow to a
crawl and eventually stall out somewhere close to the AKPen as the
system runs up against an amplifying longwave ridge building over
the northeast Pacific and into much of the Gulf. The largest point
of contention seems to stem from how quickly this ridge develops
and how far east the low is able to get late in the week before it
stalls out. The GFS is notably the farthest north and west for
both the track of the low and the location where it begins to
meander/stall, depicting the center making it all the way out
to the Pribilofs late on Thursday. This is in stark contrast to
other guidance that shows the low cross over to the Pacific side
of the Aleutian Chain between Thursday and Friday, then slowing or
stalling as it approaches the western Gulf from Friday into
Saturday.

There is a decent signal that a deep tropical moisture
feed will stream north around the low towards Friday and Saturday
as well, but the placement of this possible atmospheric river will
heavily depend on the uncertain low placement by this point in
time. Even with this disagreement, it does look like increasing
potential for periods of heavy rainfall with this setup along the
Gulf Coast between the AKPen and Prince William Sound towards the
end of next week.

-AS

&&

.AVIATION...

PANC...Light, generally northerly winds, will continue through
through Sunday evening as an upper level trough moves over the
region and into the Gulf of Alaska. A few rain showers may spread
across the terminal through tonight as the trough nears, then
gradually diminish by Sunday morning as drier air moves in behind
the trough. Ceilings and visibility should remain mostly VFR, but
there may be periods of MVFR...and possibly a brief period of
IFR...if a heavier shower moves over the terminal.

&&