Source:  NOAA Atlas 14 - California

Example: A 6-hour - 100 Year storm (i.e., 6 consecutive hours of rain) with a return period of 100 years would statistically be 3.11 inches.

VERY IMPORTANT NOTE: A 100-year storm means that there is a 1-in-100 probability of a similar event in any given year.  It does NOT mean there is an interval of 100 years between events.