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FXAK68 PAFC 170133
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
533 PM AKDT Mon Sep 16 2024

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

Today featured a relatively nice day across Southcentral Alaska,
albeit windy at times. A weak shortwave trough rotating around an
upper-level low in the central interior of Alaska is helping to
pop off a couple of showers mostly across the Mat-Su Valleys, but
also in the vicinity of the Anchorage area as well. This shower
activity should begin to wane into this evening with the loss of
day-time heating and instability. Low temperatures tonight across
portions of the Copper River Basin look to fall to near freezing
overnight while most other locations across Southcentral see mid
30s to low 40s for lows.

Attention then turns to the next system in the form of a Bering
Sea front lifting through the Gulf for Tuesday. Rain spreads from
Kodiak Island northward to the north Gulf Coast and Prince William
Sound Tuesday morning through the afternoon. Initially, Anchorage,
the Matanuska Valley, and portions of the Western Kenai Peninsula
will enter into southeasterly cross-barrier flow late Tuesday
afternoon which will cause rain to be more sporadic in nature.
Early Tuesday afternoon, these areas could see some light over-
running precipitation with flow through a good portion of the
atmosphere still south of southwesterly before the downsloping
ensues. Southeast winds through Turnagain Arm will also develop
Tuesday and bend into Anchorage through Tuesday evening. The
strongest winds are expected to remain out over the Arm. The
Anchorage Hillside can also expect gusty southeast winds as well
and on the order of 25 to 35 mph at times Tuesday afternoon before
diminishing through Tuesday evening.

Meanwhile, steady rain, moderate to heavy at times, is likely
across the Eastern Kenai Peninsula and Prince William Sound for
Tuesday into Tuesday evening. Rain looks to fill in for Anchorage
and the Western Kenai Peninsula late Tuesday evening into
Wednesday morning as the southeasterly flow weakens and eventually
turns south to southwest. Moderate to heavy also looks to linger
across Prince William Sound as multiple upper- level waves moves
across the area aided by southerly and southeasterly upslope flow.
The system pulls away to east Wednesday afternoon as winds
through the favored gaps in terrain turn to northerly and off-
shore. This will continue to be the case through the day on
Thursday as well with quieter yet breezy weather expected. Any
showers for Thursday look to confined to the terrain along the
Talkeetna Mountains and Alaska Range as another weak upper-level
system moves through.

&&


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

The focus in the coming days will be a low currently north of
Adak, which will strengthen as it moves across the Bering Sea and
into the Alaska Peninsula for Tuesday night. Much of Southwest
Alaska, the Bering Sea, and the Aleutian Islands will see strong,
gusty winds from this low, with the potential for high surf and
coastal erosion. If you live in or will be traveling through these
areas in the coming days, please stay tuned to the forecast.
While there`s high confidence in the overall picture, finer
details such as maximum wind gusts continue to be refined. Here
are the products currently in effect:

* High Surf Advisory is in effect for Bristol Bay, primarily the
  from the Kvichak River mouth south to Port Heiden. Confidence is
  lower for areas west of the Kvichak River mouth, which includes
  Dillingham and Togiak.
* Special Weather Statement is in effect for the Pribilof Islands
  for strong winds and high seas.

Diving into the details...the low currently north of Adak will
strengthen through early tomorrow morning as it phases with a
robust upper low moving in from Kamchatka. Models came into
excellent agreement this morning, lending higher confidence to the
forecast. While there may still be changes to the forecast if the
low`s track changes, the overall strength of the system is
unlikely to change. As such, expect widespread gales across much
of the Bering Sea and Aleutian Islands, as well as the Southwest
Alaska coastline.

The strongest winds continue to be expected near the Pribilof
Islands tomorrow as cold air wraps back around the low. There is a
chance that gusts could come in at hurricane force, which is 75
mph or greater. However, the current forecast has gusts remaining
just below this threshold as confidence is low that the strongest
winds aloft will mix down to the surface. Regardless, it will be
a very windy day for Pribilof Islands and surrounding waters.

Strong winds are also expected across the Eastern Aleutians,
Alaska Peninsula, and Bristol Bay. Winds will be southwesterly
for much of Tuesday, generally parallel to the shoreline. However,
as the low moves inland near Naknek/King Salmon early Wednesday
morning, flow will become more westerly. This will focus wind and
waves directly onshore, leading to the potential for high surf and
coastal erosion. As such, a High Surf Advisory is in effect for
Bristol Bay.

While high surf/coastal flood products are not in effect
elsewhere, there still may be minor impacts. For the south-facing
shore of Kuskokwim Delta (ex: Kipnuk to Kongiganak), southeasterly
onshore flow along the front on Tuesday morning could bring a
brief period of higher water levels. However, this is expected to
be short-lived and not as impactful as this past weekend`s storm.
Similarly, the south-facing shore of Bristol Bay (from the Kvichak
River mouth to Cape Newenham) could also see higher water levels
from surge as there will be decently long duration of onshore
winds. However, these winds will be weaker than those impacting
the portion of Bristol Bay under the High Surf Advisory. Finally,
the Bering side of the Alaska Peninsula (especially Nelson Lagoon)
will also get their turn at strong winds from Tuesday evening
through early Wednesday morning, though the winds will not be
directly perpendicular to the shoreline. Overall, confidence
remains low when it comes to the potential for high surf and
coastal flooding due to lack of observations and modeling methods
that are still relatively new.

Looking ahead, northerly flow behind the departing low will usher
in cooler air, with much of Southwest Alaska seeing temperatures
in the 30s from Wednesday night through Thursday morning. This, in
combination with weak upper level shortwaves moving in behind the
low, could result in a fresh dusting of snow along area mountains
and even on lower foothills.

-Chen

&&


.LONG TERM FORECAST (Days 4 through 7: Friday through
Monday)...

There is little agreement amongst operational models by the end
of the work week on Friday. Even the ensemble means largely differ
from each other during the same time period. From a pattern
perspective, the general consensus is for a broad trough or upper
low over the Western Aleutians and western Bering to gradually
slide eastward through the beginning of next week on Monday. The
operational GFS, in particular, is more aggressive with yet
another strong low overspreading the Western and Central
Aleutians. Ensembles eventually have the low positioned over the
Gulf of Alaska by the end of the long term forecast.

As far as potential impacts, the track of the trough should keep
the threat of shortwaves over the Aleutians on Friday and
Saturday. As the general trough continues eastward, the track of
disturbances moves south of the AKPEN and eventually well south of
the Gulf coast. Therefore, moderate confidence exists in somewhat
of a quieter pattern setting in for the long term for Southwest
and Southcentral Alaska with any upper-level jet support remaining
south of mainland Alaska.

What this will do however is present the opportunity for colder
air to drop southward into both Southwest and Southcentral Alaska.
Thus, cooler temperatures are likely along with lower
precipitation chances.


-BL

&&


.AVIATION...

PANC...VFR conditions persist. Southerly winds around 25 kts
will continue to gradually diminish through this evening.
Through Tuesday morning, high confidence in a spell of fairer
conditions with no rain, lighter winds, and VFR conditions
expected. Later on Tuesday, a front will bring a return of rain
and stronger winds.

&&


$$