During the late summer and early autumn of 2011, the Catskill Mountains in south-central New York State experienced devastating flooding associated with former Hurricane Irene and former Tropical Storm Lee. Even in this particularly flood-prone region, the events of fall 2011 are perceived to be unique in the known history of the region and marked a turning point in the perceptions of residents about climate change. Here, precipitation and stream gage records are analyzed to determine just how unusual this season was. The historical precipitation gage record requires careful analysis due to the changing availability of stations. After a set of stations is developed that is appropriate for this analysis, events of lengths 1 day, 5 days, and 60 days are analyzed to identify regionally significant events (as opposed to events localized to one basin) since the early 20th century and to evaluate Irene and Lee in that historical context. Gage record results corroborate the perceptions of local residents that the period between 1996 and 2011, and in particular the events of fall 2011, were unprecedented in the last 100 years and as inferred from analyses of regional tree rings, probably in the last 500 years.

The Catskill Mountains and surrounding counties in south‐central New York State are home to almost 400,000 residents, and supply drinking water to over 9 million people in New York City and other municipalities. In this study, we identify a set of stations in this region that are appropriate for climatological analysis, and examine variations in precipitation, streamflow, and temperature between 1900 and 2016, extending the time domain of previous studies of the climatology of this region during both the early and recent portions of the record. Temperatures have increased since the mid‐20th century, in particular daily minimum temperatures, at rates that vary with season and elevation. As a result, diurnal temperature ranges have tended to decrease, particularly during the warm season at lower elevations. The most significant hydrological events include the cold drought of the 1960s (a year‐round phenomenon), and the wet period beginning in the late 1990s (primarily a warm season phenomenon). We also find evidence of a particularly wet period at the start of the 20th century. Cyclic behaviour is found in both hydrological and temperature records, with the most prominent cycle in cold season precipitation and streamflow peaking at 28 years.