Storm Warning 108, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
431 PM AKDT Sat Sep 16 2023

.ANALYSIS AND UPPER LEVELS...
A closed upper low pressure (521 dam)anchors the pattern directly
over the Bering Strait. A very energetic shortwave trough is
at the base of the longwave in the western Gulf of Alaska. The
previous system in the northern Gulf of Alaska has moved ashore
and is being overtaken by the stronger system. An already wetter
than normal airmass is overhead as the antecedent low pulled sub-
tropical moisture with it to the tune of 130% of normal, but the
incoming system has values approaching 250% of normal although it
is in the process of occluding so those values will not become
widespread across Southcentral Alaska. Further out west a pair of
features look to be loaded up near Kamchatka and the northwest
Pacific, a stronger shortwave trough moving through the flow and a
baroclinic zone. These look to help shape the pattern moving
forward.

From a sensible weather perspective, widespread rain continues
along the northern Gulf coast as the remnants of the last
front/low are pushed ashore by the incoming system. the Cook Inlet
region is drier from cross-barrier flow, but the far western Kenai
through Kachemak Bay is seeing steadier rainfall. Gale-force winds
are coming into Prince William Sound but otherwise fairly quiet on
the winds inland. The incoming low has been sporting hurricane-
force winds, mainly south of our area of responsibility.

&&

.MODEL DISCUSSION...
Guidance is in good agreement with the incoming system as it is
peaking in intensity and entering a weakening state. The wind and
precipitation fields are pretty consistent between models as the
system slides in to the eastern Gulf of Alaska. There are bigger
differences with the next shortwave trough from the northern
Pacific, which looks to be similar to the system today, a compact
but strong system moving just south of the AKPen.

&&

.AVIATION...
PANC...VFR rain looks to continue through the evening with
ceilings below 5000ft. Early Sunday morning, the general low level
wind flow switches from the northwest which will bring a period of
rain and lower ceilings possibly to MVFR, however, continued
northwesterly flow should bring ceilings back up toward 5000 ft
quickly throughout the day.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...

On satellite imagery, there are 2 distinct fetches of modified
tropical moisture from the Central Pacific. The first fetch is in
Prince William Sound, while the second plume of deep moisture is
south of Kodiak. A broad longwave trough extending across the
state of Alaska and Bering, keeping an active weather pattern in
place. There is a strong low at the surface with an estimated
magnitude of 969 mb and a tight pressure gradient impacting the
Gulf Of Alaska and localized winds patterns over Southcentral AK.
At 500 mb there is a vigorous major shortwave trough south of
Middleton Island and a secondary lobe of vorticity over Cook Inlet
with multiple smaller shortwaves embedded in the cyclonic
circulation aloft. Expect widespread unsettled weather with gale
force winds at times which will impact the Gulf. While places like
Prince William Sound will get hammered with precipitation.
Precipitation in association with the system is occurring along
the coast, with a downslope keeping much of the western Kenai,
Anchorage, and Mat-Su Valleys relatively dry with only scattered
areas of very light rain as of this afternoon.

&&

.Hydrology...

A Flood Warning for a glacier-dammed lake outburst remains in
effect until noon AKDT Sunday along the Kenai River. This warning
is for the Kenai River below Skilak Lake, including the Kenai
Keys. Flooding of rivers, creeks, streams, and other low-lying and
flood-prone locations is imminent or occurring. Although water
levels have dropped below Wednesday's crest of 15.2 ft, river
levels remain elevated (12.5 ft) as of Saturday afternoon. River
levels will continue to slowly recede and take until Sunday to
drop below flood levels.

&&

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

An upper-level low remaining nearly stationary near the Bering
Strait will help keep Southwest Alaska unsettled and showery
through tonight before a shortwave ridge builds in for Sunday
morning. A break in precipitation will last through much of the
day Sunday before another round of rainfall comes late Sunday
evening. Meanwhile, tonight, winds from the influence of a storm
force low that tracked south of the Alaska Peninsula today will
continue to to keep winds gusty in the eastern Bering and south of
the Alaska peninsula, especially through bays and passes. As the
aforementioned ridge builds in for Sunday, winds will relax across
Southwest and the eastern/central Bering and chances for
precipitation will also decrease.

Behind the ridge, two systems will be the next weather makers for
the domain. Another North Pacific low will track in close
vicinity to the eastern Aleutians and Alaska Peninsula Sunday into
Sunday night before tracking into the western Gulf for Monday.
This low is expected to deepen as it moves eastward in the
vicinity of the southern Alaska Peninsula. However, there are
still questions as to the strength and exact track of this system.
There are extremely high chances (70-90%) high-end gale force
winds will accompany this low, but there's a moderate chance
(25-50%) that a swath of storm force winds could also develop from
this system. It is important to note that this system is being
resolved fairly similar to the evolution of the previous low that
tracked close to the Alaska Peninsula Friday night (last night).
However, there is a low chance (10-25%) this system will be as
strong as the previous. Further west, a low dropping out of
Kamchatka will begin to impact the western Aleutians Saturday
afternoon as it elongates and stretches into the Bering by Sunday
afternoon. Small craft winds with pockets of gale force winds will
accompany light to moderate rain with this feature as it tracks
eastward behind the ridge through Tuesday. Overall, expect
generally unsettled conditions and a downward trend in
temperatures as an upper- level trough and its associated upper-
level low continue to dominate the sensible weather.

&&

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

Few changes since yesterday's long-term discussion. Active weather
remains in the forecast due to a progressive and amplified
pattern, though there will be brief breaks between systems. The
hazards we'll be keeping an eye on for this period are: (1) an
atmospheric river moving into the Gulf on Wednesday and (2) the
potential for higher water along Southwest Alaska coastlines for
Friday and Saturday.

The long-term forecast begins on Wednesday with an occluding low
near the Alaska Peninsula and an atmospheric river moving into
the Gulf of Alaska. The influx of moisture and vorticity advection
with these two systems will likely bring widespread rain to all
but the Central and Western Aleutians/Bering Sea. The highest rain
rates and amounts are expected around Prince William Sound through
the day on Wednesday. Today's model runs seem to have trended
towards higher rain amounts than yesterday's. While it's too early
to accurately forecast rain amounts, guidance is currently
suggesting a 15-30% chance that communities in Prince William
Sound will see 2 inches of rain in 24 hours. We'll get a clearer
picture of rain amounts as we get closer to the event. Either way,
a rainy day is in store for much of Southern Alaska.

The Western and Central Aleutians/Bering Sea will get their own
rain day(s) from Wednesday evening into Thursday as a potent
gale-force front moves through. The GFS and ECMWF operational
models are in good alignment with the strength of the low,
suggesting around 970-980 mb at its deepest. These two models also
agree with the general track of the low and its front, but
overall model agreement deteriorates past Thursday night, leading
to uncertainty for the latter half of the extended period. While
the low's track and strength bring the potential for persistent,
stronger onshore winds and higher surf, confidence is too low at
this time to say whether higher surf and elevated coastal water
levels will occur.

&&

SYNOPSIS AND MODEL DISCUSSION...ML
SOUTHCENTRAL ALASKA...PJS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...NS
LONG TERM...KC