Flooding in Alaska, September 8 to 17, 2023

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
507 AM AKDT Fri Sep 8 2023

.ANALYSIS AND UPPER LEVELS...
An anomalous deep upper level low sits over the southern half of
the state. 500mb temperatures are around -30C. This cold pool
within the trough is responsible for steep lapse rates in the
lower and middle atmosphere. Several smaller scale disturbances
are orbiting the trough. The main cold front associated with the
system has dropped well into the northern Pacific and western
Gulf, but a secondary center of low pressure will establish itself
over the western half of the state by Sunday.

The freezing level on the Thursday afternoon sounding was just
below 4100 feet. This was verified by a spotter in the Chugach
who photographed snow on the ground at 4100 feet, and falling
snow down to the 3500 foot level. Instability and cold air aloft
will encourage continued showers and thunderstorms, meaning a
continuation of this cooler, cloudier, and showery pattern into
next week.

&&

.MODEL DISCUSSION...
Guidance is in good agreement about the large scale evolution of
the continued instability, though with this unsettled showery
pattern, precipitation will be sporadic. The small scale
disturbances which develop and feed on instability will determine
the placement and strength of shower and thunderstorm activity.
The next synoptic system is fairly well agreed upon, wrapping into
the northern Gulf from the eastern Gulf on Saturday morning,
moving inland over northern Cook Inlet on Sunday.

&&

.AVIATION...
PANC...Lower ceilings and shower activity will be the main concern
for the terminal. Generally weak wind flow in the low levels and
shower activity keeping moisture at the surface. Periodic MVFR
ceilings over the terminal are expected in the short term.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Today through Sunday evening)...

A broad upper-level trough sits over the western half of Alaska,
anchored by an upper-level low spinning south of Kodiak Island
this morning. Upper shortwaves embedded within the cyclonic flow
continue to pinwheel around the low. One wave is associated with
an inverted surface trough and is pushing showers across Prince
William Sound and over the coastal mountains this morning. This
trough is also associated with a colder airmass, and, as such,
snow levels remain around 3,000 ft. Overnight low temperatures
have also fallen into the lower to mid 30s for locations with
less cloud cover. Areas of fog have also developed overnight,
especially across the Western Kenai Peninsula, where skies
remained mostly clear. A Dense Fog Advisory is out for this area
until 8 AM, with some locations reporting visibility as low as
one quarter mile.

The upper trough will continue to drop slowly southward into the
Gulf through Saturday and become more negatively tilted. A surface
low will then merge with another low pressure system that tracks
out of the North Pacific and into the Central Gulf on Saturday.
This newly formed low will bring steady precipitation to the coast
by late Friday night or early Saturday morning. Strong easterly
winds are also likely across the north Gulf coast, with gales and
a small area of storm-force winds from Kayak Island to the
entrance of Prince William Sound. The surface low is expected to
track from near Middleton Island to near Whittier by Saturday
morning before moving inland and weakening rapidly through
Saturday and Sunday. Coastal locations along Prince William Sound
and the eastern Kenai Peninsula will see an increase in offshore
winds early Saturday morning before the winds switch direction as
the low moves onshore. Precipitation will be heaviest along the
coastal mountains; however, a period of steady rain is expected
for areas along the lee of the mountains Saturday morning as the
low, and its attendant upper-level wave, move from Prince William
Sound to the Susitna Valley. The precipitation becomes more
showery in the wake of these features, with the bulk of the shower
activity along upslope areas of the higher elevations due to the
persistent southeasterly flow.

&&

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

Eastward progression of the front across the Bering Sea and
Aleutians continues through early Saturday morning. The core of
strongest winds will be gale force gusts along the leading frontal
boundary, winds will be initially southwesterly becoming westerly
as the front continues to track eastward. A core of gusty
westerly winds may also develop along the the southern side of the
Central Aleutians by late tonight into Saturday morning as the
parent low continues to dive southeastward. The low center cross
over the Aleutians and tracks south of the Alaska Peninsula
by Saturday afternoon, bringing an end to most of the heavy rain
and gusty winds for most areas. Rain is likely to continue across
the southern AKPen and along the Bristol Bay coast through
Saturday night before tracking further inland in Greater Bristol
Bay and the Lower Kuskokwim Valley Sunday morning.

&&

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

An elongated trough stretches from Siberia into the Gulf of
Alaska through the forecast period. Small variations in the
models, but the overall pattern remains intact. Widespread low
cloudiness, locally heavy rain and gusty winds approaching gale
force sweep across the Southern Bering, Aleutians and Alaska
Peninsula through Thursday.

The existing low in the Gulf of Alaska get reinforcements as
short waves exit the Bering into the Western Gulf. Gale force gap
winds start around Kodiak Island early Monday, then as the low
crosses the Gulf, will bring an Easterly barrier jet along the
North Gulf Coast late Monday into Tuesday, subsiding Tuesday.
Frequent heavy rains are expected across the Southcentral coasts,
with lesser amounts inland, diminishing late Tuesday. the low will
weaken as it moves into Prince William Sound through Thursday.

&&

SYNOPSIS AND MODEL DISCUSSION...ER
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KM
LONG TERM...BL

SPECIAL WEATHER STATEMENT...CORRECTED
National Weather Service ANCHORAGE AK
624 AM AKDT Fri Sep 8 2023

AKZ121-125-101400-
Western Kenai Peninsula-Western Prince William Sound-
Including the cities of Kenai, Soldotna, Homer, Cooper Landing,
Whittier, Seward, Girdwood, and Moose Pass
624 AM AKDT Fri Sep 8 2023

...MOOSE GLACIER DAMMED LAKE IS RELEASING AND COULD CAUSE HIGH
WATER IN KENAI LAKE AND COOPER LANDING AREA...

WHAT: There is reason to believe the Moose Glacier Dammed Lake
(GDL) has begun releasing.

WHERE: Biggest impacts at Snow River leading into Kenai Lake and
downstream into the Kenai River and the Cooper Landing area.

WHEN: Using a fall 2015 release as an analog, estimates on the
Snow River are expected to peak on Sunday, September 10th around
10:00 AM AKDT at 13 ft. We expect Cooper landing to crest Sunday
night, September 10th around 10 PM AKDT at 13 ft.

POTENTIAL IMPACTS:

- Cooper Landing: Quartz Creek Campground boat ramp submerged and
  water encroaches campsites that are located next to the lake.
  North Cooper Creek Campgrounds begins to flood.

- Snow River leading into Kenai Lake: Minor flooding at Primrose
  Campground.

PREPAREDNESS: There is a lot of uncertainty with this event as
this particular GDL does not release regularly. Please monitor
local river levels and forecasts for the latest updates.