366
FXAK68 PAFC 261308
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
506 AM AKDT Fri Jul 26 2024

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

A deep closed upper level low is tracking inland across Southwest
Alaska, with an increasingly negatively tilted trough headed for
Southcentral Alaska. A series of short-waves are ejecting out of
the trough into Southcentral. An atmospheric river extends from
the North Pacific along the leading edge of the trough into
Southcentral. The 12Z Anchorage sounding showed a precipitable water
value of 1.38", which is 163% of normal. The combination of a moist
environment and dynamic forcing is leading to widespread rain
overspreading Southcentral, locally moderate to heavy in intensity
(at least by Alaska standards). At the surface, a weak triple point
low has formed over the northwest Gulf, with an occluded front
tracking northward toward the north Gulf coast.
Ridging out ahead of this front combined with lower pressure as
you head northward across mainland Alaska is leading to locally
gusty gap winds, namely for Turnagain Arm, the Knik River Valley
and the Copper River.

Rainfall will become more widespread, steadier, and in some cases
heavier as we head through the morning hours today - with the
upper low and primary trough axis approaching from the south and
west. Southerly upslope flow will enhance rainfall amounts in the
Mat-Su as well as Prince William Sound and the Gulf coast. Models
vary on the exact track of the upper low, but all bring at least
a portion of the center across interior Southcentral tonight. With
very cold air aloft , this will steepen lapse rates and lead to
showers - and perhaps even an isolated elevated thunderstorms.
Precipitation will taper off tonight through Saturday as the upper
low and trough exit to the north and east. Clouds will decrease,
leading to warmer temperatures and some sunshine on Saturday.

Strong zonal flow aloft will steer a short-wave trough quickly
eastward from the North Pacific to the Gulf and Southcentral on
Sunday. There is model spread with regard to the track of of the
short-wave and whether it catches Southcentral or remains mostly
south over the Gulf. Nonetheless, there is another chance of rain
Sunday as this short-wave moves through. There is higher confidence
in what follows, with another upper level low and trough crossing
the Bering Sea and into southern Alaska Sunday night- Monday.
This will bring another round of soaking rains to Southcentral.
While the rain will lead to rises on area rivers and streams, no
flooding is expected at this point in time. That could change if
we see enough big rain producers all in a row.

-SEB

&&


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

A potent upper level low is moving ashore near the Kuskokwim
Delta this morning, pushing gusty but not particularly strong
onshore winds into Southwest as it heads northeast. Steady rain
along the low`s occluded front close to the Alaska Range is
giving way to showers as seasonably cool air aloft moves into
place with the low and facilitates a more unstable atmospheric
profile. More zonal flow is in place across the Bering and
Aleutians, with the typical low stratus meandering across most of
the region. The next system of interest that will soon bring a bit
more unsettled conditions to the Aleutian Chain is approaching
Shemya from the southwest. This next low and occluded frontal
system will move quickly northeast today into the Aleutians,
spreading rain and gusty southeast to southwest wind to around
small craft intensity into the southern Bering Sea. Very warm low
level temperatures and moisture will also stream north along and
behind the low`s front, likely helping to support a strong
inversion off the sea surface and widespread fog advancing north
and east with time towards the southern AKPen and Pribilofs
through at least Saturday.

Looking more towards the weekend, the pattern will remain
unsettled as multiple weak shortwaves continue to traverse the
mostly westerly flow in place aloft and bring intermittent rounds
of rain to both the greater Bering region and Southwest. Much of
Southwest could actually see a decent break in the rain and even
see a few breaks of sunshine in between systems on Saturday, but
this should prove short-lived. The next stronger shortwave will
move quickly across the Bering Saturday night, helping to maintain
the existing surface low then drifting north of the Pribilofs. The
low and attendant front will arrive in Southwest on Sunday
morning, spreading more steady rain into Bristol Bay and the
Kuskokwim Delta. Meanwhile, a much deeper upper trough will drop
down from Kamchatka into the western Bering between Saturday and
Sunday, then swing east towards the Southwest coast yet again with
more rainfall potential Sunday evening. In short, the fall-like,
cool and wet pattern is here to stay for the foreseeable future.

&&


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

...No change to previous discussion...

An active pattern is expected to persist throughout the period.
A shortwave over the Bering embedded within the main longwave
trough axis will move over Southwest Alaska late Sunday.
Widespread moderate to locally heavy rain is possible over
Southwest Alaska by late Sunday, with the system spreading
eastward into Southcentral Alaska during the day Monday. Unsettled
conditions with periods of showers are expected to persist for
Southern Alaska on both Tuesday and Wednesday as the now upper
level low remains situated over mainland Alaska. Gusty
northwesterly gap winds are possible across the Alaska Peninsula
on Tuesday as the low departs to the east over the Northern Gulf
and a weak transient surface high builds in over the central
Aleutians. Another system emerges from the North Pacific and
impacts the Aleutians with periods of rain on Wednesday. Overall,
temperatures will remain below average for the long term period
with persistent cloud cover and rain.

-ME

&&

.AVIATION...

PANC...A series of short-waves will lead to multiple rounds of
rain today with varying intensity. As a result, there will be
some fluctuations in ceiling and vis. Initially, low level
southeast flow will keep conditions primarily in the VFR category.
However, as we head through the day the depth of southeast flow
will become more shallow and the strongest dynamics (and heaviest
rain) will arrive, with a period of MVFR conditions likely. IFR is
not out of the question in this pattern, though it would be brief
in nature if it did get that low. VFR conditions will then return
as the primary trough axis crosses through between 00Z and 06Z.
Meanwhile, gusty southeast winds will be in and out of the
terminal, but will ultimately prevail for much of the day.

&&

$$