Flood in Alaska, August 12 to 13, 2025

Name: Flood in Alaska, August 12 to 13, 2025
Start: 2025-08-12T11:30
End: 2025-08-13T12:55
Location: Alaska

August 12 to 13, 2025 · NWS AJK · 1 event in 1 county or zone

Where it happened

Every event with a place to put it. Tap one for its entry below.

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Tropical storms and surge
Floods
Wind
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Winter weather
Heat and drought
Cold
Fire, smoke and dust
Marine and coastal
Other

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The toll

Deaths: 0
Injuries: 0
Property damage: $5.9 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 5 other years (2018, 2019, 2020, 2023 and 2024), was full to the top of the Mendenhall Glacier (top of the ice dam) and started to flow down a side drainage channel on August 10th. In past years it has take from 2 to 4 days to begin to drain sub-glacially, but this year the detection of the sub-glacier release was complicated by the concurrence of heavy rainfall from an atmospheric river. This atmospheric river produced five to nearly eight inches of rain from August 7-12. A series of weather fronts moved across the region which tapped into sub-tropical moisture and transported that moisture poleward out of the North Pacific into the Gulf of Alaska and over Southeast Alaska. Not only was there heavy rainfall but very warm temperatures increased ice melt from the Mendenhall Glacier into the Mendenhall Lake and River system which increased the stream flows above minor flood stage leading up to the release.

The USGS laser sensor within the basin started to indicate a faster water level drop by the morning of Tuesday August 12th. USGS Mendenhall Lake gauge, which was already above the minor flood stage of 9.0ft, started to rise steadily from the additional water from the basin. Visual reports from aerial reconnaissance of the basin confirmed that the basin had started to drain sub-glacially. The Mendenhall Lake gauge steadily rose through Tuesday with the rate of rise increasing 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 eight feet over less than two days and it crested at 16.6 ft on Wednesday morning August 13th. This event broke all previous records, even last years record event by just over one half foot. This flooding event caused significant impacts but it was not as widespread as last years event due to flood mitigation actions. There were moderate to major flooding impacts in the Mendenhall Valley, mainly near the View Drive area.

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 12, 9:03 AM AKST, 2 hours 27 minutes earlier.

Area forecast discussion, Aug 12, 9:03 AM AKST, 2 hours 27 minutes before

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
1003 AM AKDT Tue Aug 12 2025

.UPDATE...Hydrology update as Suicide Basin is now releasing.
Aviation update for 18Z TAF issuance.

&&

.SHORT TERM...Longwave upper level trough is currently dig south,
dragging any remaining precipitation along with it. Embedded
within the trough at the vorticity maximum, strong WSW flow
streaming directly into Mendenhall Valley in Juneau has lead to
enhanced, terrain induced convective rain showers tonight. Dry, NW
flow aloft has begun to move into the Juneau area, marking the
end of the rain this morning. Comparing this to Lynn Canal, which
is currently experiencing breaks in clouds, shows what is in store
for the rest of the panhandle as this trough continues to move
south. Some low clouds and fog down to one quarter mile are
present along the northern panhandle and NE gulf coast, but are
expected to clear by 10 AM this morning with both solar radiation
and increased northerly winds from rising pressure in Canada.
Needless to say, with the sun's rising, most of the cloud cover
north of the trough is expected to dissipate, leaving mostly clear
skies and temperatures reaching the upper 60s to low 70s across
the panhandle.

.LONG TERM...For the start of the long term period, high pressure
continues to remain over the eastern Gulf Wednesday morning. This
high pressure will allow for a continued break from the rain
across the panhandle. With partly to mostly cloudy skies,
temperatures will be able to warm up somewhat during the day on
Wednesday.

This break in the clouds will be short though as the clouds move
back in for Thursday ahead of our next system that is expected to
impact the area late Thursday. An upper level trough with
associated low pressure will move SW of Prince of Wales and become
vertically stacked, bringing multiple waves to the southern
panhandle. One change is keeping the northern half of the
panhandle drier with NE Canadian air. That is until enough warm
air advection moves into British Columbia, forming a deformation
band. Then, moisture looks to get wrapped up in the easterly flow
and move over the northern half of the panhandle.

Accumulation amounts continue to look to be on the lighter side
until Friday when heavier precip moves in. This heavier precip
looks to be more focused on the southern panhandle approximately
from Sitka to Kake southward at the moment while places north of
there are expected to stay on the lighter side. Winds are expected
to increase Thursday night into Friday morning as well with most
places along the outer coast seeing winds range from 10 - 20 kt at
the time of writing. Through the Inner Channels, winds could be
up to 10 - 15 kt during this time. This will need to be monitored
as we head into the middle of the week for any potential changes.

&&

.AVIATION.../Update for 18Z TAF issuance/...Dense fog early
Tuesday morning has begun to rise for a majority of locations.
Northern locations are already seeing clearing skies, while
central and southern locations are still clinging on to this low
stratus deck for a little longer. These IFR to MVFR regions will
continue to increase to VFR for the rest of Tuesday, expecting
skies to fully clear out by this afternoon. Winds will remain
lighter through the day, with clear areas increasing up to 10 kts
and dropping down to calm by early Wednesday morning.

As clearing occurs, some locations are expected to develop
widespread fog or low stratus decks Tuesday night through
Wednesday morning. Areas where fog does develop should expect
significantly reduced visibilities and ceilings, both of which
could plummet back to IFR/LIFR for these areas until after sunrise
Wednesday morning, at which point daytime heating will see the
fog lift and dissipate, with dry weather expected through the day
on Wednesday for most areas, though a marine layer may linger for
some time in the morning along the outer coast.

&&

.MARINE...Outside: With rising pressures in the northern and
central gulf, and a trough digging southeast through today, look
for an increase in NW winds, particularly this afternoon and
overnight to a moderate to fresh breeze. With very light SW swell,
expecting wave heights to be maximized this morning and slightly
diminish by this afternoon. Tonight will see a marine layer begin
to develop, bringing low clouds and dropping visibilities down to
one mile, particularly near Cape Spencer.

Inside: With the trough digging south to southeast and a ridge
building north and west of the panhandle, this has lead to a
mostly light to northerly wind pattern for the inside waters for
Tuesday. Several locations look to be impacted with increased
winds as the trough digs, mainly east west channels. Icy Strait
and Cross Sound, in particular, will have increasing winds from a
tightening gradient to 15 to possibly 20 knots later this morning
into this evening. Similarly, eastern Frederick Sound zone just
north of Petersburg and Sumner Strait look to see a brief increase
in westerly winds to a moderate breeze.

&&

.HYDROLOGY...

SUICIDE BASIN HAS BEGUN RELEASING AT THIS TIME.

A flood warning for the release of Suicide Basin has been issued.
USGS laser gauge data from Suicide Basin and Mendenhall Lake
levels indicate a release from the basin has started. A crest is
expected around Wednesday afternoon near record levels of
somewhere between 16.3 and 16.8ft. Review published Flood Warning
for more information.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...None.

&&

SHORT TERM...NC
LONG TERM...NC/SF
AVIATION...GFS/ZTK
MARINE...NC
HYDROLOGY...ZTK

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

In the hours before

Special weather statement, Aug 12, 8:42 AM AKST, 2 hours 48 minutes before

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE JUNEAU AK
942 AM AKDT TUE AUG 12 2025

AKZ325-121752-
CITY AND BOROUGH OF JUNEAU-
942 AM AKDT TUE AUG 12 2025

...SUICIDE BASIN IS RELEASING...

THE SPECIAL WEATHER STATEMENT HAS BEEN REPLACED WITH A FLOOD WARNING
FOR THE MENDENHALL RIVER. PLEASE REFER TO THE FLOOD WARNING FOR
INFORMATION REGARDING THE MENDENHALL RIVER.

SPENCER.FIELDING

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 12, 11:30 AM AKST to August 13, 12:55 PM AKST
Where: Juneau Borough County, 0.6 mi SSW of Mendenhall Valley to 3.44 mi SW of Mendenhall Valley
Cause: Dam / Levee Break
Property damage: $5.9 million
Reported by: River/Stream Gage
Coordinates: 58.362, -134.594

The level of Mendenhall Lake was already above the minor flood stage of 9 feet due to an atmospheric river and heavy rain in the days leading up to the event when the outburst flood started on the morning of Tuesday August 12th. By 1200 AKST the water levels had risen 1 foot to be over the moderate flood stage of 10 feet. By this time, areas near Skaters Cabin Rd and the Mendenhall Glacier Visitor Center were experiencing moderate flooding impacts with the USFS Mendenhall Lake Campground being evacuated. As water levels continued to rise, homes along View Drive were cut off with flood waters from 1 to 3 feet deep. The rate of rise continued to increase through the evening hours with Mendenhall Lake going over the major flood stage of 14 feet at 2325 AKST. The rate of rise was higher than last year and the highest observed with an over 3000 CFS increase per hour. For context, the average flow for this time of year for the Mendenhall Lake is around 3500 CFS. The river continued to rise and went over the previous flood stage record of 16 feet around 0420 AKST on August 13th and crested at a new record of 16.65 feet (preliminary crest from the USGS) at 0615 AKST (1515 UTC). The Mendenhall Lake gauge was back below minor flood stage of 9.0 feet at 1440 AKST on Wednesday August 13th. The river system was above flood stage for approximately 38 hours.

According to the City and Borough of Juneau (CBJ), this major flooding event produced significant impacts upstream of the Back Loop Road Bridge, along with minor impacts downstream along the Mendenhall River. These impacts below back loop bridge are different from last years impacts and not as severe due to flood protection. Based on FEMA damage definitions, CBJ identified 35 homes as affected, including 11 with minor damage and 6 with major damage. The most severe flooding occurred in the View Drive area, where CBJ reported 5 homes sustained major damage after 4 5 FT of water entered living spaces. Additional homes along Meander Way (15 total) were also impacted, including 2 with minor damage and 1 with major damage caused by water seeping under or through HESCO barriers (flood protection measures). Homes near River Road were also affected, and storm drains near the Safeway parking lot at Brotherhood Bridge backed up, contributing to localized flooding. CBJ also documented severe erosion along the left bank (river left) near the Back Loop Road Bridge at the event peak. This erosion damaged the bridge approach, which was already closed, causing part of the sidewalk to fall into the river and severed telecommunication and power lines. The loss of a utility pole into the river cut power and internet connectivity to nearby homes and businesses.

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

Every storm in Alaska in 2025 Every state in 2025 This day in weather history Warnings in Alaska, August 2025

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: 1292221.