A Seasonal Shift in the Frequency of Extreme Hydrological Events in Southern New York State

The recent sequence of extreme hydrological events across the eastern United States (e.g., Hurricane Irene in August 2011, Tropical Storm Lee in September 2011, and Hurricane Sandy in October 2012), which led to unprecedented flooding including in various parts in the study region, the Catskill Mountains, and Hudson River Valley in southern New York State, have raised the question of whether the frequency of extreme events across the region is changing. In this study variations in the frequency of extreme precipitation and streamflow events available from historical records are analyzed. This study finds that there has been a marked increase in the frequency of warm season (June-October) extreme hydrologic events during the last two decades, with an accelerated rate of increase since the mid-1990s. The most recent decade has the highest frequency of extreme warm season events in the last 100 years across the study region. No such trend is observed between November and May; in fact the frequency of 4-day extreme precipitation events during the cold period has declined during the last two decades.