Powerful, but slow moving, thunderstorms brought heavy rain to the mountainous region of northern Los Angeles County, California on Thursday afternoon. With the soil hardened from years of drought, the water ran off downhill and unleashed flashfloods and mudslides across the area.

Local officials say the mud – up to 5 feet deep in some spots – trapped hundreds of motorists in their vehicles on Interstate-5 and Route 58. Homes in the Elizabeth Lake area were also surrounded by mud and debris flows. Luckily, no fatalities have been reported.

According to the NWS, Antelope Valley, which sits between I-5 and Rt. 58, received 1.81 inches of rain in 30 minutes. They have described that as a “1,000 year rain event”, which means there is a 1-in-1,000 (0.1%) chance of this type of event happening in any given year. It is interesting to note that this is the second “1,000 year rain event” to happen in the US this month. The other was the historic flooding in South Carolina.

The intensity of this California deluge had two main drivers. The first was a cut-off low-pressure system over the area that provided lift. The second involved the warmer than normal sea surface temperatures in the Pacific Ocean, which allow for increased amounts of evaporation and higher levels of humidity.  Combined, these two factors were able to generate enough instability in the atmosphere to produce heavy thunderstorms.

While experts say this storm was not related to El Niño, it does offer a glimpse of what may be in store for the Southwest over the next few months. The impacts of El Niño are typically strongest during the winter season.

Vehicles stuck in the mud along California’s Rt. 58. Credit: Caltrans/EPA