Debris Flow — San Bernardino, California

2022-09-12 · near Forest Falls, San Bernardino, California

Event narrative

Heavy thunderstorms associated with remnant moisture from post-tropical cyclone Kay produced torrential rain over the Apple and El Dorado burn scars on the afternoon of September 12. Multiple debris flows occurred following torrential one-hour rainfall rates exceeding debris flow guidance from the USGS and CAGS. Two of the debris flows traveled several miles including the Banning Canyon wash showing debris logs and ash water. The most impactful debris flow occurred in Forest Falls and Oak Glen, where the flow traveled at least 3 miles down Oakland Road with 4 foot boulders, logs over-topping the road with several feet of mud, with several power poles destroyed. Another highly impactful debris flow formed in Forest Falls on Prospect Road, where large boulders, black water and logs took out trees and damaged several homes. Another debris flow occurred 0.5 mile away on Valley of the Falls Road, producing a 12-foot wall of dirt and rocks. An additional debris flow occurred on State Highway Route 38 just before Angeles Oaks. These flows are the most significant that have occurred since the fires in 2020. An estimated 30 homes were damaged and three vehicles were moved and damaged.

The following are recorded rain amounts from the same region:

Raywood Flats, 6 NNW Forest Falls: 2.44 inch in 1 hour and 2.75 inch for the day, resulting in the full closure of Highway 38.

Mesonet Station Yucc1 Little San Gorgonio Peak Yucaipa Ridge, 2 SE Forest Falls: 1.06 inch in 60 minutes.

Mesonet Station FFPC1, 1.5 W Forest Falls: 1.93 inch.

Wider weather episode

Tropical Cyclone Kay formed on September 4th, 2022 and made its closest approach to Southern California on the 9th and 10th, with the center of Kay coming within 120 miles of San Diego. Tropical moisture associated with Kay managed to produce heavy rainfall leading to flooding across mountainous terrain in Southern California on September 9th and 10th. This led to a number of wind damage and flooding events in the mountains and desert foothill locations in Southern California. Kay transitioned to a Post-Tropical Cyclone that weekend, but left enough lingering moisture to support additional rounds of heavy thunderstorms through the 11th and 12th of September. These later rounds of heavy rain would lead to multiple significant debris flow events in the San Bernardino County Mountains, namely in and around the Apple and El Dorado burn scars.

View location on OpenStreetMap → (34.0391, -116.9694)

Source: NOAA Storm Events Database, event_id 1056857. Narrative written by the NWS forecast office that issued the report.