These maps show, for emergency service managers in the San FranciscoBay region, the threshold rainfall that may be capable oftriggering a level of debris-flow activity likely to threatenpublic safety. The maps are products of a continuing series ofstudies that began after a catastrophic storm on January 3-5, 1982triggered 18,000 debris flows in the San Francisco Bay region,causing 25 deaths and $66 million in property damage. Thethreshold rainfall values were estimated by re-evaluating aprevious empirical analysis of data from the 1982 storm, and otherhistorical rainfall records, that normalized the rainfall intensitydata by dividing by the mean annual precipitation (MAP) of thecorresponding rain gage. The present analysis also takes intoaccount the rainfall frequency, the mean annual number of days withnon-zero rainfall (#RDs), thereby adjusting for the difference inrainfall frequency between windward-facing slopes where rainfall isorographically enhanced and leeward-facing slopes and valleys thatlie within rain shadows where precipitation is reduced. Thedebris-flow threshold maps were created by digitally combining anexisting regional map of mean annual precipitation, a newly compileddata set of #RDs from an analysis of long-term (20-40 years)records of daily rainfall for 33 rain gages in the region, and there-normalized thresholds from the empirical analysis of historicalstorm data.