2018 Southern California mudflows

San Ysidro Ck. after debris flow

Butterfly Beach debris

Early on the morning of January 9, mudflows struck Montecito, which had been affected by the Thomas Fire, and other areas of Santa Barbara County.[16] 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.[17] 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.[18][19] Over 20,000 people lost power, and a 30-mile (50 km) section of U.S. Route 101 (US 101) from Santa Barbara to Ventura was shut down as sections filled with two feet (60 cm) of mud and debris, some of which also reached beaches 2.25 miles (3.6 km) from the mountains.[20][21][22] Following the closure, access to Santa Barbara from the Los Angeles area was limited to a 260-mile (420 km) detour around the Los Padres National Forest or through the use of private ferries to Ventura.[23] On January 11, Amtrak train service was restored to Santa Barbara and US 101 was partially reopened as far west as Carpinteria.[24] The highway was fully reopened on January 21, after Caltrans crews cleared 12 feet (3.7 m) of mud from the roadway.[25][26]

Approximately 21,000 residents of Santa Barbara and Ventura counties in high elevation zones affected by the Thomas Fire were evacuated,[27] but low-lying areas were outside of the mandatory evacuation area.[28][29] Warning messages sent by the National Weather Service and the county government arrived too late to prompt Montecito residents to seek safe areas.[30] Many residents in the mandatory evacuation zone, and most residents in the voluntary evacuation area, ignored warnings and stayed in their homes, probably a result of "evacuation fatigue" left over from the recent wildfires.[31] The mandatory evacuation zone was expanded to cover a majority of Montecito's estimated 10,000 residents two days later (January 11) due to disruptions in electricity, gas, water, sewage and Internet, and due to emergency road works and ongoing search and rescue operations.[32]

As of January 21, the mudflows caused at least 21 confirmed deaths, mostly in the Montecito area, and an additional two people were reported as missing in Montecito.[4][18] Over 150 people were hospitalized with various injuries, including four in critical condition.[5] The mudflows destroyed over 100 homes and damaged an additional 300.[33] Writer T.C. Boyle, whose home was proximate to both the fire and mudslides, documented the collective trauma in The New Yorker magazine.[34]

Cleanup efforts began a week after the initial mudflows, with debris being sent to the Ventura County Fairgrounds for sorting and the Calabasas Landfill for disposal.[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.[37] Mandatory evacuations were ordered for nearby areas of the San Fernando Valley affected by the Creek Fire before the storm, but no major damage was reported.[38][39]

Heavy rain in Corona, affected by the Canyon Fire, covered streets with mud and flooded into several homes, but no major damage was reported.[40]