Hydrometeorological and climatological conditions associated with flash flooding in the Catskill Mountains, NY

Abstract:

Flash flooding is a concern in watersheds of the New York City Water Supply System, as the turbidity associated with rapid flooding is unacceptable in an unfiltered water supply. Previous studies suggest that flash flooding will occur more frequently in this region in a changing climate. Therefore, a thorough understanding of the conditions associated with flash flooding are important for effective watershed management. Seven flash floods were identified in the hydrologic record for the Neversink River near Claryville, NY from 1 April 1987 through 15 July 2014. Case studies using Weather Prediction Center forecast maps, National Centers for Environmental Prediction/ National Center for Atmospheric Research (NCEP/NCAR) reanalysis daily composites, and co-operative station data were used to characterize each event. Forecast maps indicate synoptic-scale frontal activity concurrent with all flash flood events. The four winter flash flood peaks are associated with rain on existing snowpack, with anomalously warm 1000 mb temperatures and anomalously high precipitation rates. The three summer flash flood peaks are associated with convective activity, high precipitation rates, anomalously warm 1000 mb temperatures, and southerly winds. NCEP/NCAR climate composites for winter and summer flash flood events are consistent with the case studies presented. The frequency of these broad-scale conditions suggest that localized effects of the basin conditions separate flash flood events from other high discharge events. Recognizing these conditions in the context of climate predictions is useful for effective and proactive water management in the region to maintain an unfiltered water supply for the greater New York City area.