[7] At the beginning of December, the Santa Ana winds fanned wildfires across southern California, which were fueled by a lack of rainfall during what is normally the region's rainy season.

[4][5] The Thomas Fire, which devastated parts of Santa Barbara and Ventura counties, burned large amounts of vegetation whose roots had helped stabilize topsoil in hillsides and other vulnerable areas.

[11] The system moved onto the mainland on January 8, bringing heavy rain to Southern California and prompting mandatory evacuations in parts of Los Angeles, Santa Barbara, and Ventura counties, over potential mudslides in areas affected by wildfires.

[15] An estimated 0.5 inches (13 mm) of rain fell within a five-minute period at approximately 3:30 a.m., causing mud and boulders from the Santa Ynez Mountains to flow down creeks and valleys into Montecito.

[16] The debris flows were up to 15 feet (5 m) in height of mud, boulders and tree branches, moving at estimated speeds of up to 20 miles per hour (30 km/h) into the lower areas of Montecito.

[27][28] Warning messages sent by the National Weather Service and the county government arrived too late to prompt Montecito residents to seek safe areas.

[35][36] Parts of Burbank and Sun Valley, previously affected by the La Tuna Fire in 2017, received four inches (100 mm) of rain and were evacuated ahead of potential mudslides.

A debris flow into a residential area of Sun Valley damaged 40 to 45 homes and carried a vehicle that struck a natural gas pipeline, which began to leak.