Flood — Juneau Borough, Alaska

2020-08-10 to 2020-08-11 · near Douglas, Juneau Borough, Alaska

Event narrative

July and the beginning of August was very wet and the soils were primed. As moderate to heavy rains moved over the area on the morning of August 9th Mendenhall Lake continued to rise from the previous rains but at a steeper rate. There was a short break in the rain amounts and the rate of rise on the lake was not as steep. As the stronger portion of the weather front moved over the northern half of the panhandle high rain rates increased again and Mendenhall Lake began to rise at a steep rate during the evening hours on the 9th. Rainfall totals from this time frame ranged from one to three inches near sea level with higher amounts in the headwaters at higher elevations. The very warm temperatures created very high freezing levels that increase runoff from glacier ice melt. Mendenhall Lake rose to minor flood stage of nine feet in the early morning hours of August 10th and continued to rise at steep rate into the early afternoon hours and crested at ten point two feet around 245pm AKDT. This was above moderate flood stage of ten feet. The lake did not go back below flood stage until the early morning hours on August 11th.

There was minor to moderate flooding around Mendenhall Lake and the upper portions of the river above Back Loop Road. The Mendenhall campground was evacuated due the potential of moderate flooding the night before impacts took place. There was up to two feet of water at the entrance and within the campground. The West Glacier Spur Road and the area around Skaters Cabin was also flooded. View Drive was flooded with less than one foot of water. There was also flooding along the dredge lakes trail area and few other trails around the Mendenhall Glacier Visitor Center run by the USDA Forest Service.

Wider weather episode

A very strong and wet front for this time of year moved across the Gulf of Alaska and over the northern panhandle. There were 2 bursts of heavy rain one early Sunday morning with moderate to heavy rain of around one inch. Then there was a break in the rain before a second round of deep moisture, significant orographic lifting and dynamics moved over the northern areas Sunday night and continued into Monday morning. There was very high IVT values ranging from 300 to 700. Storm total rain amounts were from two to four with higher amounts in the mountains and headwaters. The ground was already very staturdated from the wet conditions leading up to this event. All of the rain produce moderate flooding around Mendenhall Lake and flooded areas around the lake and along the upper portions of Mendenhall River above the Mendenhall River bridge on Back Loop Road. The river crested around ten point two feet which is above moderate flood stage of ten feet in the afternoon hours of August 10th.

View location on OpenStreetMap → (58.4304, -134.5798)

Source: NOAA Storm Events Database, event_id 915712. Narrative written by the NWS forecast office that issued the report.