Gale Warning 541, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
501 PM AKDT Sat Aug 17 2024

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: This
afternoon through Tuesday evening)...

The forecast remains on track as the upper-level ridge axis over
eastern portions of Southcentral this afternoon continues to shift
eastward and reaches the Al-Can border by this evening. This will
gradually open the way for the warm and moist air with the
atmospheric river to make its way into Southcentral from the
southwest. There won't initially be much in the way of a forcing
mechanism to produce much in the way of precipitation. Thus, there
are only minimal chances for light showers or sprinkles over
Anchorage and the Western Kenai this evening given the lack of
forcing. The one exception for more in the way of steady
precipitation will be in the Susitna Valley. Strengthening low-
level southerly upslope flow will lead to development of rain
beginning late this afternoon or evening, especially from around
Talkeetna northward.

Ultimately, a cold front will march eastward from the Bering Sea,
with widespread rain arriving to Kodiak Island, especially
southern parts of the island, and western portions of
Southcentral early Sunday morning, then spreading eastward across
the region Sunday afternoon through Sunday night. The front will
pick up speed late Sunday as an upper level short-wave rounding
the Bering low drives eastward across southern Alaska. Thus, this
will be another light rain event for most of the region. The one
exception will be the northern Susitna Valley, where 1 to 2 " of
rain are possible through Sunday night thanks to the
aforementioned strong southerly winds and upslope flow.

While there are some similarities to the pattern that brought
strong and gusty winds to Southcentral Thursday through Friday,
there are some key differences. The low track is notably farther
west and the mean flow in the atmosphere is more southerly in
the low levels and southwesterly aloft. As a result, pressure
gradients will not be as strong as the last system and the
strongest mountain level winds will be in different areas. For
instance, the southerly flow means winds along the Turnagain Arm
Mountains and hillside of Anchorage will be weaker. Southeasterly
gap winds will still develop through Turnagain Arm, the Knik River
Valley, and along the Copper River - but they will generally be
somewhat weaker. The gustiest conditions for Anchorage look to be
in the very late Saturday night/early Sunday morning through
Sunday afternoon before winds turn more southerly through Sunday
afternoon and evening. The southerly flow will favor stronger
winds in Cook Inlet and stronger and more widespread gusty winds
in both the Susitna Valley and Copper River Basin. Winds will peak
during the day Sunday with arrival of the cold front, then
diminish across Southcentral Sunday night. Cold air advection
through the Alaska Peninsula will lead to strong westerly winds in
the western Gulf and Kodiak island waters Sunday night.

The upper level short-wave will slowly exit eastward Monday
through Monday night, with some light rain showers lingering
longest in the Copper River Basin. After a string of warm nights
due to clouds and wind, expect partial clearing and much cooler
nighttime lows Monday night/Tuesday morning as a broad ridge
builds in for Tuesday.

-SEB/DAN

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA, THE BERING SEA AND THE
ALEUTIANS (Days 1 through 3)...

A Coastal Flood Advisory remains in effect through 6 pm Sunday
for the Kuskokwim Delta including the cities of Kongiganak,
Kwigillingok and Kipnuk as water levels will rise as high as 3 to
6 feet above normal highest tide impacting low lying areas along
the coast.

A Flood Warning is in effect through 6pm Monday for high winds
backing up flow along the lower Kuskokwim River and Kuskokwim
Delta is expected.

A powerful Bering Sea low continues to bring gusty southerly
winds and rainfall to the Mainland, Aleutians and Alaska Peninsula
today. As of 4pm, satellite and observations show its center was
just northwest of the Pribilof Islands. It will migrate
easterly through this evening making landfall tomorrow morning on
the Mainland north of Mekoryuk.

Wind gusts this morning ranged from 35 to 45 mph across the
Bristol Bay and Bering Sea coasts and the Aleutians though a few
higher gusts were recorded; Cold Bay recorded a gust of 62 mph
and Cape Newenham a gust of 71 mph. Persistent and strong
southerly/southwest winds will continue into tomorrow afternoon
ranging from 40 to 50 mph for coastal locations north of Togiak. A
narrow band of wind gusts ranging from 50 to 60 mph is likely to
move onshore between Kipnuk and Mekoryuk late tomorrow morning
reaching Bethel by the afternoon then weakening. South of Togiak
along the coast and in Bristol Bay, moderate southerly to
southwesterly winds are likely tomorrow morning into the afternoon
hours ranging from 35 to 45 mph. Gale force winds are expected
through tomorrow afternoon across the Bering Sea and Bristol Bay.
Rainfall amounts across the Mainland range from 0.50" to 1.0"
through tomorrow afternoon. The highest rainfall totals in the
Goodnews Bay/Togiak area. Elsewhere, rainfall totals range from
0.25" to 0.50."

Sunday evening into Monday evening is a much less active period
of time for the Mainland and Central Aleutians to Alaska
Peninsula. The remnants of Typhoon Ampil approach the western
Aleutians and Bering Sea as early as Monday evening. It is
trending to deepen as it moves more into the Bering Sea Tuesday
with a track farther west than the current and previous Bering
lows.

-DJ

&&

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

Several storms move through Southern Alaska and the Bering Sea
for the extended forecast, though model disagreement is poor for
Thursday and beyond. The first storm to move through will be the
remnants of former Tropical Storm Ampil, whose front will bring
moderate to locally heavy rain across much of Southern Alaska.
Conditions will also be breezy, especially for Southwest Alaska.
While it's a bit too early to tell, onshore flow wrapping around
Ampil on Wednesday could exacerbate any existing flood issues
along Kuskokwim Delta. For now, with Ampil's track looking to take
it further north than today's storm, winds will likely be weaker
than what we'll see this weekend. In addition, Ampil could lead to
gusty southeasterly gap winds in Southcentral Alaska, depending
on the tightness of the surface pressure gradient.

Much uncertainty arises after the passage of Ampil's remnants,
which has led to major model disagreements regarding the two storm
systems following it. Still, with the upper level pattern
becoming more zonal, chances are that conditions will trend much
closer to seasonal norms than what we've seen in recent weeks.
Regarding the storms transiting the forecast area in the extended
period, the first will move in from the west for Thursday and
perhaps extending into Friday, with major uncertainty regarding
its track and speed. The other major feature to watch for will be
the arrival of a strengthening low on Saturday. Again, much
uncertainty with track and speed. However, there seems to be
decent agreement that this low will be fairly strong, with minimum
sea level pressure dipping below 990 mb and models hinting at
gale force winds and a hefty plume of moisture. For now, it's too
early to say what might happen. But at least with next Saturday's
storm, current signals suggest it may be one to watch.

-KC

&&

.AVIATION...

PANC...VFR conditions and light winds will persist into the
evening hours. Rain and lower ceilings will gradually approach
from the west overnight tonight through Sunday morning. Ceilings
are generally expected to remain VFR, but periods of MVFR and/or
IFR are possible as rain starts to impact the terminal, though the
increasing Turnagain Arm winds may mitigate lower ceilings.
A Turnagain Arm jet will develop in response to the approaching
cold front tonight, then strengthen through the day Sunday.
Similar to the past storm, the Turnagain jet will bend into the
terminal, so it's not out of the question that wind gusts reach 35
kt again sometime on Sunday.

-SEB/PP

&&