DEVELOPMENT OF A FLOOD CLIMATOLOGY FOR THE UPPER ALLEGHENY RIVER BASIN IN NORTHWESTERN PENNSYLVANIA

Meteorological floods can occur during any month of the year and are generated by a mixture of causal mechanisms. Relatively unimpaired, and climate-sensitive, U.S. Geological Survey gaging stations (USGS-HCDN) at locations with upstream drainage areas > 30 mi²are used to develop a better understanding of flood hydroclimatology within the upper Allegheny River Basin (uARB) in eastern North America. Dates of all annual peak flows (floods) from USGS-HCDN gaging stations were used to develop a regional flood climatology. The flood climatology included an assessment of the timing of peak annual flows over each period of record and an assessment of flood generating mechanisms for the 1995-96 through 2014-15 water years using a manual environment-to-circulation classification scheme.

Composites of uARB USGS-HCDN station records show that 59% of flood events occur from December to March, with March flooding contributing the highest portion of the total (~22%). Conversely, flooding is least common during August, which experiences only 2% of peak annual flows. Annual peak flows from 1995-96 through 2014-15 are dominated by snowmelt or rain-on-snow events (~65%), primarily from December through March. Flooding from synoptic scale frontal passages, including convective precipitation and orographically enhanced events that produce extreme rainfall rates in some cases, are responsible for generating (~26%) of flooding throughout the uARB. Although rare (~ 4%), tropical systems have produced very large annual maximum floods. Understanding seasonal variations in flooding and the development of a flood climatology for the uARB are important pieces of information for flood forecasting, flood hazard assessment, and water resource management.