Alaska, 2023

Flood in Alaska, August 5 to 6, 2023

August 5 to 6, 2023 · NWS AJK · 1 event in 1 county or zone

Since 2011, Suicide Basin, a side valley that is dammed by the Mendenhall Glacier, fills with water from snowmelt and rainfall then drains during the summer as a glacial outburst flood. This year, the glacier dammed lake in Suicide Basin, like in 3 other years (2018, 2019 and 2020), was full to the top of the Mendenhall Glacier (top of the ice dam) and started to flow down a side drainage channel.

Where it happened

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The toll

Deaths
0
Injuries
0
Property damage
$7.0 million
Crop damage
None recorded
Events
1

Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.

What happened

The episode as NWS AJK wrote it up.

Since 2011, Suicide Basin, a side valley that is dammed by the Mendenhall Glacier, fills with water from snowmelt and rainfall then drains during the summer as a glacial outburst flood. This year, the glacier dammed lake in Suicide Basin, like in 3 other years (2018, 2019 and 2020), was full to the top of the Mendenhall Glacier (top of the ice dam) and started to flow down a side drainage channel. After about 7 days, Suicide Basin began to drain sub-glacially. Mendenhall Lake started to rise from the additional water from the basin Thursday evening into the early morning of Friday, August 4th. In the morning hours of Friday, August 4th visual reports from the USGS Mendenhall Lake gage and webcam confirmed that the basin had started to drain. The Mendenhall Lake gage steadily rose through Friday, then the rate of rise increased exponentially as the inflow to the lake from Suicide Basin was getting larger. Over the course of this glacier dam outburst flooding event, Mendenhall Lake and River rose an impressive nine and a half feet over less than two days, breaking all previous records, and causing significant impacts. This flooding event was historic due to areas seeing flooding that usually do not flood along with the severity of the flooding and the rate of riverbank erosion. There were moderate to major flooding impacts in the Mendenhall Valley, where a majority of Juneau’s population resides, and historically does not flood.

What forecasters said that day

The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.

NWS Juneau, AK wrote its last area forecast discussion before the first event at Aug 5, 4:15 AM AKST, 1 hour 45 minutes earlier.

Area forecast discussion, Aug 5, 4:15 AM AKST, 1 hour 45 minutes before
Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
515 AM AKDT Sat Aug 5 2023

.SHORT TERM.../Through Saturday night/...The high pressure over
the panhandle continues to decay, and a higher cloud deck which
began to roll in during the evening hours will continue to spread
across the area through the day on Saturday. While numerous breaks
and abundant sunshine are anticipated - especially N of the Icy
Strait corridor - this cloud deck will likely put at least some
damper on temperatures. Think that 70s for much of the area remain
likely, but 80s may prove harder to reach barring some well-timed
breaks in some locations. Otherwise, anticipate that the clouds
will be the most substantial along the outer coastal areas. The
source of this cloud deck - and of some chances of showers for the
outer coastal areas and perhaps even for more interior parts of
the panhandle - is a decaying frontal band over the Gulf. The
frontal band will largely stay offshore of the panhandle as its
associated low falls apart through the next few days, but some
showers and clouds associated with it do remain possible. Winds in
the outer coastal waters will reach between 20 and 30 kt as this
band lifts N through the weekend. Otherwise, the inner channels
should largely remain around 15 kt or lower - with many having
variable winds - through the daytime and evening hours for
Saturday.

.LONG TERM.../Sunday through Friday/...Weather pattern trend in
the upper levels remains on track, with the ridge in western
Canada expected to gradually break down through the middle of next
week. During this time, a broad upper level trough will extend
into the central Gulf of Alaska. This will lead to a transition to
overall onshore flow over the panhandle for early to mid next
week.

As a result of this shift, the seasonably warm and dry weather will
be limited to this weekend. Light showers and cooler temperatures
return to the panhandle starting Sunday afternoon/evening as a
diminishing short wave feature moves into the south central gulf.
While precipitation is expected to be primarily light and showery
from this feature, model guidance is continuing to suggest a more
organized system with a stronger front approaching the panhandle
sometime late Monday into Tuesday. Inherited wind forecast in the
gulf was kept, with winds trending towards 25kt or greater
approaching the outer coast for that timeframe. Overall for next
week the pattern will remain cooler and wet as systems are
expected to transit the gulf bringing showers and increased winds
with associated frontal passages.

&&

.AVIATION...Mid to high clouds area passing over the area from
south to north from surface low to the SSW. Some light showers can
be seen with these clouds over the gulf, and some may reach
portions of the southern outer coast through the day. Those
showers spread northward along the coast through evening with
short-lived MVFR CIGs as reported by PAHY so far this morning.

&&

.HYDROLOGY...A flood warning continues for a glacier-dammed lake
outburst. Flooding caused by a glacier-dammed lake outburst is
expected or already occurring as of 5am for the Mendenhall Lake
and River in Juneau, AK. This warning is in effect until 10AM AKDT
Sunday.

The Mendenhall Lake gauge began rising Thursday evening, and
through Friday night has been rising faster than previously
anticipated. If this event follows the pattern of previous
releases, Mendenhall Lake will rise for the next 1-2 days with a
peak approximately 2 days after the rise began. This would put
peak levels for the lake on Saturday.

There is uncertainty in the exact timing of this event. Based on
recent events, the lake level should rise 4-6 feet. How quickly
Mendenhall Lake rises depends on several factors including how
much water is stored in Suicide Basin and how fast the water
releases. Please continue to monitor weather.gov/ajk/suicidebasin
as well as social media for any additional updates.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ652-661>664-671-672.

&&

SHORT TERM...GFS
LONG TERM....STJ
AVIATION...Ferrin
HYDROLOGY...SF/GJS/GFS

Visit us at http://www.weather.gov/Juneau

In the hours before

Local storm report, Aug 5, 3:08 AM AKST, 2 hours 52 minutes before
Preliminary Local Storm Report
National Weather Service Juneau AK
408 AM AKDT Sat Aug 5 2023

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0230 AM     Flood            4 N Mendenhall Valley   58.43N 134.58W
08/05/2023                   Juneau             AK   Other Federal

            Mendenhall Lake gauge (MNDA2) rose above
            minor flood stage of 9 feet.

&&

NAF

The radar

The national radar mosaic for the moment the first event began.

Open the radar archive at this moment

Event by event

1 flood. Local standard time, as filed.

Flood, Juneau Borough

When
August 5, 6:00 AM AKST to August 6, 7:45 AM AKST
Where
Juneau Borough County, 13.82 mi NNW of Douglas to 12.32 mi NW of Douglas
Cause
Dam / Levee Break
Property damage
$7.0 million
Reported by
NWS Storm Survey
Coordinates
58.445, -134.564

The level of Mendenhall Lake was near normal levels of 5.5 feet before this event started, late Thursday August 3rd, and by Saturday morning water levels were over moderate flood stage of 10 feet. By this time areas near Skaters Cabin Rd, View Dr, and the Mendenhall Glacier Visitor Center were experiencing moderate flooding impacts with the Mendenhall Lake campground being evacuated. By midday on Saturday, the water levels exceeded the previous record from 2016 of 11.99 feet and impacts were increasing along the Mendenhall River. Homes along View Dr were cut off due to the significant amount of flooding over the road along with flood waters from 1 to 3 feet deep. Areas below the Mendenhall Back Loop Road bridge started to see flooding of yards and increasing bank erosion in the afternoon of August 5th.| |The river continued to rise at a fast pace through the day into the evening hours and the City of Juneau then set up their Emergency Operations Center. In the evening hours, many areas (roads and homes) below Mendenhall Back Loop Road bridge along the Mendenhall River were experiencing moderate flooding along with significant bank erosion of 50 to 200 feet. From that bank erosion one home fell into the river with another large 3 story condominium building was evacuated as it became compromised and could also fall into the river. Going into the late evening hours a portion of another house fell into the river with many other homes along the river being undermined and at risk of falling into the river. Homes and roads around Meander Way, Marion Dr, River Court and Killewich Rd flooded with moderate impacts in the late evening and the local electric company cut power to those residences due to flood waters being 2 to 4 feet deep.

The river crested near 14.97 feet (preliminary crest from the USGS) around 11:30 PM AKDT on August 5th. At the peak of the event there was 3 to 7 feet of water in and around homes along View Dr. The Mendenhall Lake and River were back below minor flood stage of 9.0 feet by 9:45 AM AKDT on Sunday, August 6th.

While other locations in the Mendenhall Valley, above Mendenhall Back Loop Road bridge, have experienced flooding impacts from these types of events in the past, they saw major damage this time around. This event was 3 feet higher than the previous record from 2016 and the Mendenhall Lake and River rose approximately 9.42 feet.

In the end there were about 48 homes affected with 13 destroyed, 8 with major damage and 7 with minor. This historic event saw the largest amount of water released from Suicide Basin in a single event and approximately 14 billion gallons of water came out of the basin and into the Mendenhall Lake and River system.

What these are

Flood
Flooding that develops more slowly than a flash flood: rivers leaving their banks, or water standing over land for an extended time.

After NWS Directive 10-1605, the rules offices file Storm Data by.

The same day elsewhere

More

Every storm in Alaska in 2023Every state in 2023This day in weather historyWarnings in Alaska, August 2023

Sources

NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 1127010.