Flood — Juneau Borough, Alaska

2021-08-13 to 2021-08-14 · near Douglas, Juneau Borough, Alaska

Event narrative

The Mendenhall Lake rose sharply through early morning on August 13th and went above minor flood stage of 9.0 feet by 545AKDT. The precipitation intensity continued to increased through the late morning of August 13th and combined with the high magnitude of warm air associated with the atmospheric river the amount of runoff from glacier/snow melt elevated the rate of rise on Mendenhall Lake significantly. The lake rose rapidly through the morning to go over moderate flood stage of 10.0 feet by 945AKDT from. As the heavy rain taper off in the afternoon of the 13th the lake continue to rise but not at a steep rate from all the runoff in the mountains and water funneling through the Mendenhall Glacier and surrounding Juneau Icefield. The lake did not peak until the early morning hours around 545AKDT on August 14th at 11.55 feet and was not was not below flood stage until 1815AKDT.

The Mendenhall Lake campground was evacuated from significant flooding along with road closures of View drive and Skaters Cabin road. It should also be known that some locations along the Mendenhall River below the back loop bridge had significant amount of water in their yards.

Wider weather episode

A strong atmospheric river with sub-tropical moisture moved out of the north Pacific and over Southeast Alaska. This impacted the northern inner channels with 24 to 36 hours of heavy precipitation from a deep upper level trough with a long fetch transporting a significant amount of precipitable water over the complex terrain of the Alaska panhandle.

This atmospheric river produced very high freezing level with very warm air going all the way up to 14,000 feet. The warm rain melted snow that was left from the previous winter in the mountains along with ice melt from area glaciers to increase runoff into the Taiya and Mendenhall River drainage basins. Rainfall amounts from August 13th to the 14th in the Taiya River basin ranged from 0.50 inches at sea level to over 1.30 inches in the headwaters with more than 4 inches over the a 5 day period. The Mendenhall River basin say precipitation values from 3.34 inches near sea level to as much as 5.0 inches in the headwaters. Satellite rainfall estimates indicated there was a lot more rainfall over the Juneau icefield to increase the amount of runoff into the Mendenhall Lake and River system.

There was moderate flooding around the Mendenhall Lake area from the morning hours on August 13th through the afternoon hours of the 14th which fooded multiple roads near the lake and the Mendenhall Lake campground. This was the 3rd highest peak in its period of record going back to 1966 and levels not seen since the glacier dam release from 2016.

There was moderate flooding around the Taiya River from the early evening on August 12th to early afternoon on August 14th which included the the Chilkoot trail and the campground near Deya. This is the second highest crest on record behind the record crest of 19.89 feet from 2002 when a moraine dam broke free along West Creek a tributary of the Taiya River and this was the highest crest seen since 2013.

View location on OpenStreetMap → (58.4021, -134.5808)

Source: NOAA Storm Events Database, event_id 979439. Narrative written by the NWS forecast office that issued the report.