465
FXAK68 PAFC 231216
AFDAFC

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

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA
(Days 1 through 3: Monday through Wednesday 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 will move from east of the
Barren Islands to an area just southeast of Seward today. The
upper-level wave will move northwest from Prince William Sound
over the Chugach and Talkeetna Mountains. An area of rain is
expected to move inland across the Copper River Basin and Cook
Inlet later this morning and into early afternoon. The strong
winds across the northern Gulf and gusty northerly gap winds from
Seward to Homer and Wasilla will diminish in intensity as the wave
moves north and weakens and the surface front falls apart along
the coastal mountains.

The forecast challenge comes 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. Currently, the forecast leans toward the
NAM/NAMnest and GEM/REM. 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 an extended period of rainfall. This solution also brings two
waves over the Copper River Basin with likely rain this morning
and Tuesday morning and scattered showers in between.

For other inland locations, including Anchorage, Talkeetna, and
Palmer, occasional bouts of light rain are expected through the
next 24 to 36 hours.

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 rain to
Kodiak Island.

-TM

&&


.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.

-KM

&&


.LONG TERM FORECAST (Days 4 through 7: Thursday through
Sunday)...

Broad longwave troughing over the Gulf of Alaska and subsequent
ridging over most of the Bering Sea will persist from the start of
the extended period into Friday morning. Unsettled weather will
be experienced by most of Southcentral while portions of Southwest
near the coastline could remain dry as cold and dry air moves in
from the north. By midday Friday, a new shortwave low enters the
western Bering and tracks southeastward towards the North Pacific
by the middle of the weekend. The shortwave phases with the upper
level trough on Saturday, jetting moderate precipitation towards
Kodiak Island and much of the gulf coast into Sunday.

While a week out, a system that currently has our attention is a
subtropical Pacific low that will move northwards towards the Gulf
of Alaska. Most ensemble and operation model guidance is trending
towards a stronger system moving towards the gulf, but the
mechanisms in place to get it there vary by model. While there is
not major concern for heavy rainfall, unusually strong winds for
the gulf are not out of the question at this point in time.
Continue to follow the forecast for continued updates on this
system.

&&


.AVIATION...

PANC...VFR conditions and northerly winds will persist through
this morning. A strong occluded front will lift northward across
the Gulf today which will lead to increasing north winds for the
terminal this morning along with light rain. The winds are
expected to diminish in intensity through the afternoon and
evening as the front weakens and the pressure gradient relaxes.
Light rain developing this morning is expected to continue through
the afternoon and evening. There may be times when the rain tapers
to sprinkles or shuts off entirely. The diminishing winds both at
the surface and aloft will also allow for more moisture to advect
over the coastal mountains today. This should cause ceilings to
gradually lower throughout the day, with MVFR ceilings likely by
early evening.

-MK/TM

&&


$$