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239
FXAK68 PAFC 261324
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
524 AM AKDT Sun May 26 2024

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Today
through Tuesday Night)...

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We did it! Anchorage finally cracked 60 degrees yesterday,
marking the first 60 degree since September 5th. This is
approximately two weeks later than the average first 60 degree day
and the latest occurrence of the first 60 degree day since 2000.
Similar weather with highs around 60 are on deck for Southcentral
Alaska for the next several days.

GOES water vapor imagery shows a couple distinct features that`ll
play a role in our weather to start the week. The first and
foremost feature impacting weather across southcentral is a very
broad upper-level low positioned across the Gulf of Alaska / far
Northeast Pacific. Satellite also shows an upper-level low moving
from the North Slope towards west-central Alaska, as well as a
series of low-amplitude shortwaves from eastern AK back through
Canada. This places much of southcentral Alaska in a col, or
essentially a saddle between troughs and ridges. Precipitation for
the next 2-3 days will largely be driven by 1) westward-
propagating shortwave troughs (easterly waves) on the northern
periphery of the Gulf of Alaska low into the Copper River Basin
and 2) diurnally-driven instability.

As of 4am, satellite imagery shows one of these subtle easterly
waves moving across the Copper River Basin. Radar imagery and
ground observations note rainfall primarily from Glennallen south
into the central and eastern Chugach and Prince William Sound.
This area of rainfall (and high-elevation snow) will continue
shifting east through the morning but essentially taper off as the
wave shears apart. High-resolution ensemble forecast (HREF),
particularly the HRRR, suggests convection across the Matanuska
Valley and into Anchorage later this evening. The probability
remains low and will be dependent on instability and what`s left
of the aforementioned wave- but did increase PoPs from the
previous forecast. Otherwise, today will be a rather pleasant day,
particularly for areas west and south of the Talkeetnas, where
breaks in the cloud cover are likely observed.

Monday and Tuesday will be similar to Sunday in that the highest
PoPs will be focused across the Copper River Basin southwards
towards the eastern and central Chugach. In fact, confidence is
increasing that much of the Copper River Basin will receive
rainfall throughout Monday. The Mat-Su Valley, Anchorage, and
Kenai Peninsula may see isolated showers with daytime heating-
driven instability, but synoptic-scale features are lacking for
more organized precipitation.

-Brown
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&&


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

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Nighttime microphysics satellite imagery shows clearing
conditions spreading southward into Southwest Alaska this morning.
Weak northerly flow and "drier" conditions will filter their way
down into Bristol Bay. As a result, afternoon highs are expected
to warm into the upper 50s with low 60s possible for parts of the
Lower Kuskokwim Valley and Greater Bristol Bay areas. Saturday
saw some showers and thunderstorms develop along the Kuskokwim
Mountains in the afternoon and evening hours. On Sunday
instability will be more limited, but roughly 200 J/kg of MUCAPE
may be realized over eastern parts of Bristol Bay, nudged up
against the Western Alaska Range, where renewed showers and wet
thunderstorms will be possible.

Over the Bering and much of the Aleutians, widespread low stratus
and areas of fog have developed beneath subsidence created by the
transient ridge of high pressure. A Kamchatka low is bringing a
front into the Central Aleutians with increasing southeasterly
winds gusting to gale force. However, this front will weaken
throughout the day on Sunday on the thermal gradient along the
front becomes more diffuse. An elongated area of low pressure is
expected to form along the front on Monday as the ridge begins to
break down over the Bering. Unsettled conditions are forecast over
the Eastern Aleutians and Lower Alaska Peninsula on Monday, with
low pressure becoming more amplified over the Bering on Tuesday.

-BL
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&&


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

Ridging over mainland Alaska on Wednesday will help clear skies up
over Southcentral during the day. The calmer weather will be short
lived however as energy from an upper-level trough in the eastern
Bering surges eastward and pushes the ridge to the north. This
pattern change will re-introduce widespread cloud cover and
chances for precipitation across the region as the system
progresses. The trough will dive further south and east into the
North Pacific by Thursday afternoon before the upper-low migrates
northward into the Gulf and towards Prince William Sound by
Saturday afternoon.

Another upper-low will start to form near the western Aleutians
late Thursday and move eastward through Saturday, providing a
chance for precipitation across the Aleutians and Bering. The
positioning of this low and the one in the central Gulf by
Saturday is still undetermined. Some model guidance is suggesting
that they will interact and even start to merge near the Alaska
Peninsula, while other models keep both systems separate from each
other. This would result in the Gulf low remaining closer to
Prince William Sound and the western low positioning itself near
the central Aleutians. In either scenario, broader troughing will
persist over most of the forecast area, producing cooler and
unsettled weather through at least the start of next weekend.

-BS

&&

.AVIATION...

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PANC...VFR conditions and light winds will generally persist. A
week upper disturbance moving in from the east could help set off
a few rain showers near the terminal later this afternoon.
Increasing cloud cover ahead of this approaching upper wave is
likely through this afternoon as well, and ceilings with this deck
could hover near the 5000 ft threshold at times.
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&&


$$