California’s Record Rainfall for a 24-hour period was 25.83 inches, occurring January 22-23, 1943, at Hoegee's Camp in the San Gabriel Mountains, Los Angeles County (about 1.8 miles southeast of Mount Wilson and two miles north of Sierra Madre). Not only has this been a record for California, it is the fourth highest rainfall amount for a 24-hour period for the entire United States.
Hourly precipitation values for Hoegee's Campground indicated that the greatest 24-hour rainfall occurred between 1:00 AM on January 22nd to 1:00 AM on January 23rd. During that period, the storm delivered its record 25.83 inches (corrected from previous reports of 26.12 inches). A detailed post-storm report was produced by the Weather Bureau.
Source: State Climate Extremes Committee, NOAA National Centers for Environmental Information.