980
FXAK68 PAFC 131318
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
518 AM AKDT Mon May 13 2024

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Wednesday night)...

A low tracking along the upper jet just south of Kodiak island is
brining some light rain to the island this morning. Meanwhile, to
the north, an E-W oriented trough axis over coastal Southcentral
is rotating northwestward into Southcentral as the upper low and
short-wave south of Kodiak move into the Gulf. This is producing
showers across the Prince William Sound region. Isolated showers
are noted on radar trying to move inland, but some (or all) of
this could be virga (precipitation not reaching the ground).
Most inland areas are dry with some clear skies over the valleys,
particularly the Copper River Valley and Anchorage into the
southern Mat-Su. Temperatures have cooled to near or below
freezing where skies have cleared, with mid to upper 30s for
much of the remainder of Southcentral.

The evolution of the upper levels has changed a bit, resulting
in some tweaks to the precipitation forecast for today. The low
near Kodiak will track quickly off the northeast across the Gulf,
with wind and steady rain remaining offshore of the north Gulf
coast. However, it will continue to push the trough axis inland,
while the western end of the trough from Cook Inlet to Bristol Bay
is pushed southward by an upstream ridge. This will lead to
development of a closed upper low centered over the Kenai
Peninsula, Anchorage, and much of the Mat-Su. Surface heating,
with temperatures generally a few degrees warmer than yesterday
combined with colder air aloft with the upper low overhead will
lead to greater instability. Vorticity-maxima rotating around
the low center will help provide some upward vertical motion.
Thus, it looks more favorable for showers to develop as we head
through the day, especially from the Mat Valley south across
Anchorage to inland portions of the western Kenai Peninsula. There
could even be a few lightning strikes, with the highest potential
in the warmest areas of the southern Susitna Valley and interior
Kenai Peninsula. Steering flow will start out east to northeasterly
this morning and shift to northwesterly this evening, which will
push showers up against the Chugach and Kenai Mountains. While
showers will not be widespread, coverage will be greatest up
against the mountains, where locally heavy downpours are likely
late this afternoon into the evening hours. Otherwise, expect a
mix of sun and clouds today across the remainder of interior
Southcentral with isolated to scattered showers mainly during
the afternoon and evening hours. Showers along the coast will
diminish through the day as the trough lifts inland.

The upper low will drop onto the Gulf on Tuesday. Continued
cyclonic flow aloft and airmass instability will initiate some
afternoon/evening showers, particularly in the Copper River Basin.
Warmer and drier conditions will prevail elsewhere. The trend
toward warmer temperatures and sunnier skies will continue
Wednesday as the aforementioned upper level ridge builds into
Southcentral. Expect a light wind regime for most of the region
the next few days due to lack of strong surface features.
However, a surface high will strengthen over the Gulf Wednesday,
which will lead to strengthening sea breezes along the coast.

-SEB

&&


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

A weakening low south of the Alaska Peninsula continues to move
eastward towards the Gulf, producing areas of light rain and
small craft winds. As the low continues to exit the region, a
building ridge will move over Southwest and the eastern Bering,
providing calmer conditions over the next couple of days. Showers
will also be possible this afternoon with a passing shortwave west
of the Alaska Range developing near the building ridge. While
there will be sufficient instability for showery conditions, a
stable air mass aloft will limit the potential for any thunderstorm
activity to occur.

Our next area of concern will be a Kamchatka low forming later
today that will extend a warm front into the western Aleutians.
Widespread southerly gale force winds are expected as the system
moves eastward. Warm air behind the front will quickly switch any
wintry precipitation to rain as it passes into the central Bering
Tuesday night. A new cold front will begin to develop behind the
original warm front starting tonight, before occluding with the
warm front Tuesday. Winds will remain strong across much of the
Bering, with gale force winds persisting through Wednesday
morning.

The low will start to weaken Tuesday night as it moves closer to
St Lawrence Island and as the front moves towards the western
Alaska coast. While winds will begin to taper by Wednesday
morning, strong southwesterly flow near the Kuskokwim Delta will
persist through the morning hours, with some locations receiving
gale force winds and/or gusts. Widespread precipitation is also
expected as the front enters the western Alaska coastline Tuesday
night through at least Wednesday evening. While the exact timing
and track of the storm are still uncertain in recent model
guidance, there is certainty that this system will be impactful
for much of the region throughout the forecast period.

-BS

&&

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

For Southcentral...

The long term for southcentral Alaska will begin with ridging in
place across much of the eastern mainland, resulting in fairly dry
conditions and temperatures at or just above climatological average.
Afternoon showers will be possible during the afternoon in close
proximity to the mountains, particularly the Wrangells. Deep upper-
level troughing then shifts east across southcentral Alaska by
Friday, bringing increased cloudiness, coastal rain, and high-
elevation snow to the area. Present forecast guidance keeps this
system only precipitating on Gulf communities and in the mountains,
though there is a chance the system will shift north, bringing wider
spreading rains to the mainland. Overall, temperatures will continue
to rise, with daytime highs warming with the exception of Friday,
which is likely to be the coolest day of the forecast period.

For Southwest and the Bering/Aleutians...

By Thursday, confidence is high troughing will exist across the
eastern Bering, Aleutians, and into Southwest AK. However, models
diverge significantly with regards to potent shortwave trough
rounding the base of the longwave trough. The 12z operational GFS
run brings a low into southwest that would result in the potential
for low-elevation snow and windy conditions. Meanwhile, the ECMWF
and Canadian models move that energy a bit farther south. Upper-
level ridging is likely to build in behind the trough as it shifts
east. The exception to this is across the western Aleutians,
where continued storminess is likely.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist.

&&


$$