Debris Flow — Flathead, Montana

2021-09-10 to 2021-09-11 · near Lake Mcdonald, Flathead, Montana

Event narrative

Backpackers camping at the edge of Lake Ellen Wilson in Glacier National Park were awakened by torrential rainfall and the clapping of thunder around midnight. Bias-corrected radar data estimated over half an inch of rain fell in one hour through 1 am on the 11th. The rain continued at moderate intensities for a few more hours and ended by late morning with an radar estimated total rainfall amount of over 2 inches. The campers reported boulders rolling down Mount Jackson into the lake throughout the night with 6 new waterfalls cutting off their exit trail towards Sperry Chalet. Three backpackers had to bed down at Gunsight Pass for an extra night due to all the rock slides and fast-moving water through the trails.

Wider weather episode

An upper level trough with near record atmospheric moisture levels moved around the western U.S. ridge from Oregon to northwest Montana. This system initiated several waves of heavy rainfall over Glacier National Park which resulted in high elevation debris flows and impacted backpackers.

View location on OpenStreetMap → (48.6204, -113.8389)

Source: NOAA Storm Events Database, event_id 975149. Narrative written by the NWS forecast office that issued the report.