431
FXAK68 PAFC 240051
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
451 PM AKDT Mon Sep 23 2024

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

Unsettled weather will remain over much of Southcentral through
mid-week as a quasi-stationary upper-level trough and surface low
sit anchored over the northern Gulf. The extent of cloud cover
and precipitation will be largely driven by a series of upper-
level waves rotating around the base of the trough combined with
their respective surface troughs driving north into the coastal
mountains.

The first of these features has moved overhead and produced a
swath of rain from the Copper River Basin to Cook Inlet much of
today. Meanwhile, the strong winds across the northern Gulf and
gusty northerly gap winds from Seward to Homer and Wasilla
continue to diminish in intensity as the wave moves north and
weakens and the surface front falls apart along the coastal
mountains which helps to further weaken pressure gradients.
However, somewhat gusty conditions look to remain through Valdez
Arm and Valdez Narrows through tonight.

The forecast challenge continues to be later this evening through
Tuesday as the subsequent waves and troughs approach the
Southcentral coast. Models continue to struggle with which
features will be the dominant players, due not only with the
position of various vorticity maxima, but with the extent of cold
air advection in the wake of the main trough. The advection of
colder air across Kodiak Island and the Barren Islands will likely
help to spin up mesoscale surface features driven by the
aforementioned upper-level vorticity advection. How each moves
north and affects the mid-level wind field will dictate the extent
of precipitation beyond today. With that, this forecast continues
to lean toward the NAM/NAMnest and GEM/REM as the previous one did.
This solution lingers a mid-level trough axis over the central
and southern Kenai Peninsula, extending into the western half of
the Susitna Valley through Tuesday, resulting in extended
unsettled weather. This solution also brings two waves over the
Copper River Basin with likely rain that affected the area this
morning and another one Tuesday morning and scattered showers in
between.

For locations including Anchorage and the Western Kenai Peninsula,
expect periods of rain to continue through Tuesday. Places such as
Talkeetna and Palmer should expect occasional bouts of light rain
are expected through the same time period.

By Wednesday, most of the area continues to remain within
cyclonic flow and thus will continue to see cloudy conditions with
most of the showery activity confined to the coast and higher
elevations. The aforementioned trough over the Kenai Peninsula may
also drift south by Wednesday, bringing another round of light
rain to Kodiak Island. The case remains the same on Thursday,
although shower coverage looks to be less than Wednesday. With
cyclonic flow still present, expect very little to no sunshine on
Thursday with mostly cloudy conditions.

&&


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

Broad longwave troughing stretches from Interior Alaska across
Southwest Alaska and over the Eastern Aleutians, with higher
pressure moving into the western Bering. Cold northerly flow
advects across the eastern Bering Sea and and Southwest, elevating
winds through the gaps and passes south of the Eastern Aleutians
and southern Alaska Peninsula. Winds will gradually diminish from
west to east through Tuesday evening. Morning low temperatures
across Southwest Alaska will continue in the low to mid 30`s, and
low 40`s along the Alaska Peninsula and Eastern Aleutians. Monday
into Tuesday mostly drier weather is anticipated for Southwest
Alaska. While a few showers will be possible during the day
on Monday, within weak northerly flow, the bulk of any
precipitation should largely be confined to the higher elevations.
There is potential for some light snow from Sleetmute to Iliamna
and areas north and east Tuesday night into Wednesday morning with
the development of a weak deformation band.

The next upper low will emerge from Asia into the western Bering
late Tuesday into Wednesday. An attendant front may reach the
Western Aleutians by Wednesday afternoon with renewed shower
activity and perhaps some gustier winds with any cold air
advection behind the arriving low.

&&


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

Broad longwave troughing will persist over mainland Alaska through
the extended forecast period, resulting in unsettled weather
leading into the start of next week. The front of a shortwave low
in the Bering will move eastward onto Southwest Friday afternoon,
providing them with their next round of widespread steady
rain.The shortwave low will continue to slowly trek southward
before situating itself near Kodiak Island Sunday morning. Strong
southerly flow from the system will shoot ample moisture
northwards towards the Kenai Peninsula and western Alaska Range,
with enhanced amounts of both rain and snow possible in those
locations. This will be short-lived however as the shortwave
dissipates and makes way for a potentially strong North Pacific
low to enter the Gulf of Alaska on Monday.

The aforementioned low entering the gulf on Monday has not been
picked up well by deterministic models and ensemble guidance,
resulting in an uncertain forecast for this part of the forecast
area. The intensity, track, and timing of this low has been
considerably variable over the last couple of days, so confidence
on the impacts of this storm are very low. Continue to monitor for
future updates as we get more guidance on this system for early
next week.

&&


.AVIATION...

PANC...As our occluded front weakens and pressure gradient
relaxes through Monday afternoon/evening, there is confidence
winds will continue to lighten both at the surface and aloft,
causing ceilings to drop to MVFR by Monday evening. Moisture will
continue to advect into the area and there is potential for IFR
ceilings starting early Tuesday morning and persisting through
much of the day. Periods of light rain should accompany these
diminishing conditions Monday afternoon through Tuesday.


&&


$$