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 does this remain a record for California, it is the fourth highest rainfall amount for a 24-hour period in 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).
Two post-storm reports by the Weather Bureau, Hydrometeorological Report #36 (Table 8-4, p. 111) and Hydrometeorological Report #37 (Section 2-B-4, p. 42, and Section 4-C-3, pp. 125-126), provide more meteorological detail for the event.
Source: State Climate Extremes Committee, NOAA National Centers for Environmental Information.