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000
FXAK68 PAFC 211303
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
503 AM AKDT Sun Apr 21 2024

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

In the upper levels, a ridge centered over the Alaska Interior
is weakening and allowing an east-west oriented short-wave trough
over the southern Gulf to lift northward toward Southcentral. This
short-wave extends from a closed low centered a few hundred miles
south of Kodiak Island. A short-wave digging down the back side of
the low is begin to elongate the center, with the northern trough
pulling it northward and the southern trough pulling it southward.
This will be a key feature for later in the forecast. In the
meantime, ahead of the trough in the southern Gulf is a dissipating
front draped east-west from the central Gulf to Kodiak Island. The
heavy rain of the past 4 days is now in the rear view mirror, with
just some lingering very light rain over the island this morning.
Steady light rain is spreading slowly northward ahead of the upper
short-wave and toward the Gulf coast and Prince William Sound.
Benign weather continues for the bulk of Southcentral under the
influence of the upper ridge.

The upper trough over the Gulf will lift northward across
Southcentral tonight, with clouds and areas of light precipitation
spreading onshore today and inland tonight. The bulk of precipitation
will fall along the coast, with weak forcing and dry low levels
leading to little or no accumulating precipitation inland.
Marginally cooler air aloft will accompany the trough and lead
to lowering snow levels, though areas at and near sea level will
remain safely all rain. With the dissipation of the front over the
Gulf, a surface high will build from Southeast Alaska to coastal
Southcentral this afternoon and evening, leading to development of
summer-like gap winds, which includes Turnagain Arm, the Knik
River Valley, and the Copper River. They won`t be terribly strong,
but do expect some wind in south to west Anchorage, Palmer, and
Glennallen.

As the upper trough tracks into Southcentral tonight, the upper
low behind it will split into two, with the northern low headed
for interior Southwest Alaska. This low and increasingly negatively
tilted trough trailing to the southeast will then come back east
and cross Southcentral Monday through Tuesday. Model spread in the
track of the low and trough is large. The NAM tracks the low
northeastward to the Alaska Range while the GFS/ECMWF/Canadian all
take further south (but different) tracks across Southcentral.
Thus, forecast confidence in the precipitation forecast for
Monday/Tuesday is low. All solutions would bring precipitation to
the astern Kenai Peninsula. Outside of that, it`s hard to say
where the best chance of precipitation will be. The other wildcard
is precipitation-type, as the airmass continues to cool Monday
night into Tuesday. Wherever steady precipitation develops, there
is potential for snow levels to drop down to sea level, especially
across interior Southcentral. Stay tuned. The upper low will
ultimately exit Southcentral sometime Tuesday night or Wednesday,
with dry conditions settling in behind it.

Going back to tonight with the upper low split, the southern low
will track southeastward into the Northeast Pacific and phase with
a short-wave and surface low crossing the North Pacific. This low
will deepen and lift northward toward the Gulf tonight through
Monday. However, progressive upstream flow will quickly force the
upper low eastward, leaving a weakening surface low just south of
the Gulf. Therefore, expect minimal impacts from this system
across the southern Gulf offshore.

-SEB

&&


.SHORT TERM FORECAST SOUTHWEST ALASKA, THE BERING SEA/ALEUTIANS
(Days 1 through 3: Today through Wednesday morning)...

Scattered showers will continue to linger over Bristol Bay through
Monday as an upper-level trough sits overhead with weak shortwaves
rotating around it. The trough lifts out of the area late Monday
with drier weather expected Monday evening into the first part of
Tuesday as a shortwave ridge builds overhead.

All eyes then look westward to the Bering Sea for the next notable
weather-maker. The system is currently getting its act together
in the western Bering early this morning. This storm will continue
to strengthen through Monday evening as it meanders in the
western Bering. Winds are expected to be mostly gale-force with
gusts to storm-force as the initial front sweeps the Aleutian
Chain today through Monday night as it makes its way east to the
Alaska Peninsula by then. Strongest winds are expected along the
central Aleutians as the front generally weakens through its
eastward progression. However, as the core of the storm moves in
the vicinity of the western Aleutian Chain Monday evening, there
could be a small corridor of sustained storm-force winds mostly
for the marine areas on the North Pacific side near and south of
Kiska. The dominant precipitation-type with this system is
expected to be mostly rain or rain/snow mix. The Pribilof Islands
will start as rain Monday evening with the possibility of
changing over to snow Monday night into early Tuesday morning as
the storm occludes and cold air works in.

Eventually, the front makes it to coastal Southwest by Tuesday and
looks to linger off the coast through most of Wednesday. The front
will arrive in a weakened state with sustained southeasterly
small-craft winds likely along the coasts of Bristol Bay and the
Kuskokwim Delta. The Kuskokwim Delta coast stands the best chance
at staying mostly snow for longer due to the availability of
colder air. Areas to the south, along Bristol Bay, will see more
in the way of rain to rain/snow mix due to less cold air. Any snow
accumulations with this system are expected to be light.

&&

.LONG TERM FORECAST (Days 4 through 7: Wednesday through Saturday)...

A strong low in western Bering Sea will dominate the region while
a strong ridge remains stubbornly in place over the interior of
Alaska and Canada through much of the forecast in the week ahead.
The strength and placement of the ridge will influence the
direction and placement that the embedded shortwave troughs will
flow into the Gulf. Expect unsettled weather to continue through
next week. A front will lift through the northern Gulf midweek,
bringing light precipitation to higher elevations across
southcentral due to weak upslope flow and for Kodiak Island and
Kenai Peninsula.

Subtropical moisture will be drawn into the Bering Sea low that
will occlude and spread heavy precipitation and gusty winds across
the Aleutians and the Pribilofs through midweek. While the low
circulation will remain over the Bering, the triple point of the
occlusion appears to be roughly around the Alaska Peninsula with
the front extending north along the SW Alaska coast. This front is
expected to lift towards Southcentral, but there is uncertainty
in how much it weakens before reaching Southcentral. Southeasterly
flow though could keep breezy conditions through Kamishak gap
Thursday.

Toward the later part of the week, the upper level low will begin
to tilt and nudge into the Gulf. Models begin to diverge in the
details for precipitation into the Gulf. A North Pacific low
could potentially bring the next round of relatively substantial
precipitation to the Gulf and Southcentral next weekend. There is
low confidence though due to strength of the ridge over the
eastern Pacific and the interior.

rux

$$

.AVIATION...

PANC...A weak Turnagain Arm wind is likely to develop today as a
weak front lifts north across Southcentral Alaska. Expect winds
to increase out of the southeast at the terminal this afternoon.
Ceilings may drop to near 5000 ft through the evening hours, with
a slight chance for light rain to develop along with brief MVFR
ceilings.

CQ

&&

$$