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FXAK68 PAFC 241250 CCA
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
450 AM AKDT Tue Sep 24 2024

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

Unsettled weather will continue over much of Southcentral through
Thursday as a quasi-stationary upper-level trough weakens and
becomes negatively tilted over the northwestern Gulf. The extent
of cloud cover and precipitation will be largely driven by a
series of upper-level waves and mesoscale surface lows / inverted
troughs retrograding east to west across the northern Gulf and
Prince William Sound.

The focus today will be on a weak upper-level wave moving inland
over Prince William Sound this morning, bringing with it another
round of showers to the coast with lighter periods of rain inland
and along the lee side of the Chugach and Kenai Mountains. A
secondary wave and attendant mesoscale low will then move from
east of Middleton Island northwest into Prince William Sound
later this afternoon. There is some uncertainty in the timing and
strength of the surface feature, whether it remains a closed low
or strong inverted trough, as it pivots westward. Whatever the
timing, though, it does appear that coastal locations from
Cordova to Whittier will see a second round of showers along with
a period of gusty easterly winds.

This band of showers will also move inland across the western half
of Southcentral for Wednesday morning with yet another round of
rainfall from Talkeetna south to the southern Cook Inlet. Given
the showery nature of the precipitation, rainfall amounts will be
difficult to pin down for many inland locations. However, anywhere
from a tenth to a third of an inch is possible from location
along Cook Inlet north and east into the Susitna Valley and Copper
River Basin through Wednesday evening.

By late Wednesday, the various upper-level waves weaken as they
continue to track inland and the aforementioned upper-level trough
slides south into the southern Gulf. However, 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.

-TM/DN

&&


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

Minor changes to the forecast overnight, but otherwise the bigger
picture remains largely on track. A broad upper-level longwave
trough stretches across much of Mainland Alaska, with weak higher
pressure building into the western Bering. Northerly flow
upstream of the trough advects cold air across the eastern Bering
Sea and Southwest Alaska, 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
tonight. 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.

Through today, upper-level shortwaves rotating around the base of
the longwave trough will curve across Southcentral Alaska and back
towards Southwest Alaska, producing a deformation band along the
Western Alaska Range and Middle Kuskokwim Valley. Compared to the
previous forecast package, the development of the deformation
band has been delayed from this morning to this afternoon. Still,
this band is expected to bring prolonged light precipitation
persisting through at least Wednesday. As temperatures drop
overnight, precipitation type will likely similarly change each
overnight period to snow, returning to a mix of precipitation in
the afternoons.

Across the Alaska Peninsula, gusty northerly winds are expected to
return by Wednesday afternoon as another surge of cooler air drops
through the region. Given shifts in the upper level forcing, have
reduced forecast wind gusts along the southern side of the
Peninsula. For now, expecting gusts tomorrow to be a little weaker
than what was seen yesterday afternoon and evening.

The next system to enter the fray will be a vertically stacked
upper-low entering the northwestern Bering Sea by Wednesday night
into Thursday morning. A front along the eastern leading edge of
the front will spread across the Bering ahead of the main low,
reaching the Pribilofs by Thursday morning. While the front
appears fairly transient, the upper low appears to stick into the
central Bering continuing the active pattern through the weekend
for the Bering, and Southwest coastline.

-CL/KC

&&


.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...Light northerly winds will persist through the TAF period.
VFR conditions are now expected to prevail, likely dropping below
5000 ft at times. Periods of ceilings lowering to MVFR or IFR
are possible, but low decks are mostly expected to remain
scattered. Intermittent light rain showers are also likely to
continue through at least Wednesday morning, after which point a
drying trend is possible with lifting ceilings.

&&


$$