Debris Flow — Ketchikan Gateway Borough, Alaska

2024-08-25 · near Ketchikan, Ketchikan Gateway Borough, Alaska

Event narrative

The second landslide, on the opposite slope of the first, went down Copper Ridge and affected a private construction yard. Construction materials stored at the yard were destroyed in the slide, which was significant loss, along with the damage/destruction of the upper part of the access road.

Wider weather episode

A low-pressure system moved out of the North Pacific Ocean into the Gulf of Alaska and quickly intensified into a 995 MB gale-force storm on the Sunday morning of August 25th, 2024. By the late afternoon, this system had nearly stalled along the northwest Gulf Coast, stretching an occluded front across the central and southern Panhandle. This setup was accompanied by a moderate atmospheric river (AR) which transported subtropical moisture over Southeast Alaska. The AR produced hourly rainfall rates at elevation that exceeded a half inch per hour at times with storm total amounts ranging from 2.55 inches at the airport to over 10 inches at higher elevation sites. There were other stations at sea level that did report over 5 inches for this event. This AR was also associated with strong surface sustained winds of 15 to 25 MPH with gusts ranging from 30 to 50 MPH.

The most intense rainfall came in a 12 hour period ending at 20:00AKST on August 25th with a precipitation station reporting a 10 year annual reoccurrence interval and others reporting a 6 hour period of a 5 year reoccurrence interval for rain intensity. This did produce large river rises on small streams across the area like Ketchikan Creek but no flooding was reported from rivers. Two landslides were reported that destroyed homes, caused damage to roads, businesses, and infrastructure, and resulted in one fatality.

View location on OpenStreetMap → (55.3543, -131.6604)

Source: NOAA Storm Events Database, event_id 1199212. Narrative written by the NWS forecast office that issued the report.