A glacier outburst is underway in Alaska. It’s sending a surge of water downstream toward Juneau

By CNN Meteorologist Mary Gilbert, Amanda Musa, CNN

(CNN) — A wave of water gushing out of an Alaskan glacier is causing a river to rise rapidly and is threatening significant, potentially record-breaking flooding in the state capital of Juneau for the third consecutive August.

“A glacial outburst has occurred at Suicide Basin. The basin is releasing and flooding is expected along Mendenhall Lake and River,” Juneau officials said in a Tuesday news release. “Officials recommend residents in the 17ft lake level inundation zone evacuate the area until the flood waters recede.”

Glacial lake outbursts happen when a lake of melting snow and ice and rain “drains rapidly — like pulling out the plug in a full bathtub” after pooling up so high it overtops the glacier that holds it back, according to the city of Juneau. In this case, it’s due to the Mendenhall Glacier and its counterpart, dubbed Suicide Basin, which run up against the western portion of Juneau, about 10 miles from the city’s center.

The Mendenhall River, which runs along the west side of the city, has risen about 6 feet since the National Weather Service issued the first flood warning for the area Tuesday morning.

The river entered major flood stage — Level 4 of 4 — early Wednesday, and was nearing 15 feet not long after. Major flooding is expected for some homes along the river once it tops 15 feet, according to the weather service. It’s expected to crest at around 16.6 feet late Wednesday morning, which would eclipse the record crest set just last August.

The river pulled in large trees as it rose overnight, making conditions even more hazardous. “It’s very dangerous right now. Stay away if you can,” said Andrew Park, a meteorologist with the National Weather Service in Juneau, warning any mariners in the area to “keep your eyes peeled for big trees.”

Waters rose quickly after the glacial outburst began late Tuesday morning. Significant flooding was reported in the Mendenhall Lake that evening, with more than three feet of water gathering in some areas, according to the weather service.

Last year’s glacier outburst caused the river to surge to 15.99 feet, unleashing destructive flooding in Juneau that impacted more than 100 homes. City officials characterized the flood severity as “unprecedented.” The river level in 2024 topped the one reached during 2023’s early August glacial flood by a full foot.

Officials urge residents to prepare

Juneau installed a flood barrier along the area that was inundated in 2024, a city spokesperson told CNN. There are roughly 1,000 residents and businesses in that part of the city, the spokesperson said Tuesday.

Officials urged everyone in the inundation area to evacuate Tuesday night.

“Don’t wait, Evacuate TONIGHT,” the city said in a Facebook post.

“This is likely to become a life-threatening situation. If you are told to evacuate, stop what you are doing and immediately go to an emergency shelter or another safe location,” said Republican Sen. Lisa Murkowski in an X post Tuesday.

On Sunday, Alaska Gov. Mike Dunleavy issued a preemptive disaster declaration to better streamline emergency response efforts to the outburst.

“By issuing this declaration before the flood occurs, we can position state resources and personnel in advance to support local and tribal governments in their efforts to protect lives, homes, and essential services,” Dunleavy said in a statement. “Our goal is to act early to reduce impacts and preserve community safety.”

The capital of Alaska, Juneau has a population of about 33,000, according to the the US Census Bureau. The city is surrounded by coastal waters to the west and south.

Why does this keep happening?

The glacial lake outbursts have become a regular occurrence since 2011 and have worsened considerably each year since 2023.

They are yet another consequence of climate change due to fossil fuel pollution. The Arctic, including Alaska, is warming twice as fast as the rest of the planet as global temperatures rise.

This is causing glaciers, like these in Alaska, to thin or melt altogether. Part of the once icy expanse of the Suicide Basin has been replaced by an earthen divot that gets filled up like a bathtub each summer, setting the sequence in motion.

Rising temperatures have severely reduced the expanse of the Mendenhall Glacier and its Suicide Basin, creating the annual glacial lake outbreak hazard as ice gets replaced by liquid water and rises closer and closer to that “bathtub” edge throughout the summer.

Studies in recent years have shown that between 10 and 15 million people globally are exposed to impacts of glacial lake outburst flooding like what’s unfolding in Juneau.

Glaciers are melting and losing mass globally at an increasingly rapid rate as the world warms, producing more and larger glacial lakes, a 2024 study found. The uptick in number and size of glacial lakes is expected to increase the frequency of outburst flooding events in the future.

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CNN’s Eric Zerkel, Zenebou Sylla and meteorologist Briana Waxman contributed to this report.