A lake atop one of Alaska's most well-known glaciers is draining quickly and triggering flooding in parts of Juneau.

The National Weather Service (NWS) issued a flood warning for Mendenhall Lake and areas immediately downstream, including homes in Juneau's Mendenhall Valley.

A lake crest of just under 4 feet above flood stage was forecast by the NWS late Friday night, rivaling the record crest set on July 11, 2014 (3.85 feet above flood stage). 

(MORE: Check the Latest Mendenhall Lake Stage, Flood Forecast)

Friday morning, the Mendenhall River was rising at the rate of 2 inches an hour. Based on the NWS forecast, Mendenhall Campground and homes along View Drive were expected to see at least 3 feet of flooding. 

The flood is being triggered by a sudden release of accumulated water from an area known as Suicide Basin, dammed about 2.5 miles above the edge of the Mendenhall Glacier about 5 miles north-northeast of Juneau International Airport.

In just a 33-hour period from late Wednesday night to early Friday morning, the water level in Suicide Basin plunged by just over 57 feet, indicating the outburst flood event had begun. 

Known as a glacial lake outburst flood, or by the Icelandic term jokulhlaup, this event typically occurs in Alaska in the summer months, as the the combination of water from snowmelt, glacial melt and rain builds up behind an ice dam in the glacier.

"Water has been building up in the basin for several months," said Dr. Eran Hood, Professor of Environmental Science at the University of Alaska Southeast. 

Interestingly enough, a rare thunderstorm in Juneau likely added additional water to the dammed-up lake.

Hood says hydrostatic pressure from the water behind the ice dam first lifts up the glacial ice in the ice dam.

The outburst flood begins with relative slow draining, followed by an accelerated draining once the friction of the water melts additional ice. This is similar to pulling the plug in a bathtub drain, except if somehow the drain hole increased in size over time, according to Hood.

Video above courtesy of Alaska EPSCoR and Dr. Eran Hood

In this particular case, the water then flows into Mendenhall Lake, then the rather short stretch of the Mendenhall River downstream cutting through the most populated valley in the city of Juneau.

"The main impacts are around the lake itself and the (Mendenhall Glacier) visitor center," said Hood.

There has also been some erosion on the grounds of Juneau International Airport not only from these glacial lake outburst floods, but from other rainfall flood events on the river, requiring some reinforcement, but nothing of major consequence. 

The most notable events on Mendenhall Lake occurred in July and August 2014 and 2011, but at least one event has occurred virtually each summer in recent years. 

"There are hundreds of these events in Alaska, usually occurring in wilderness areas where nobody sees them," said Hood. 

The largest-known glacial lake outburst flood witnessed in Alaska was a 1986 event near Yakutat, when a peak discharge of 3.97 million cubic feet per second poured out of a dammed-up Russell Lake, according to Alaska-based climatologist, Dr. Brian Brettschneider.

This discharge was roughly double the peak discharge of the Mississippi River at Memphis, Tennessee, during the great flood of 1937, according to the U.S. Geological Survey.

It remains to be seen what effect climate change will have on these glacial outburst floods.

"Mendenhall Glacier is rapidly thinning and retreating," said Hood. "We expect the frequency and magnitude of these floods to change over time."

Jonathan Erdman is a senior meteorologist at weather.com and has been an incurable weather geek since a tornado narrowly missed his childhood home in Wisconsin at age 7.

unknown node

unknown node

MORE ON WEATHER.COM: Glaciers In Retreat (PHOTOS)