A synoptic climatology of rain-on-snow flooding in Mid-Atlantic region using NCEP/NCAR Re-Analysis

Abstract

Rain-on-snow (ROS) flooding is common throughout the Appalachians and Mid-Atlantic region primarily during late winter and spring. Six recent (2007–2013) widespread snowmelt-related flood events throughout Pennsylvania and other Mid-Atlantic states are used to gain an understanding of synoptic-scale atmospheric conditions that generate annual maximum flows for the region. NCEP/NCAR Re-analysis output for six individual case studies and an all-event composite suggest similar synoptic-scale atmospheric conditions lead to widespread ROS flooding. Snow ablation and subsequent flooding are heavily influenced by the low-level (850 mb Low Level Jet) advection of warm, moist air from the Gulf of Mexico and Atlantic Ocean around an area of high pressure off the Atlantic Coast towards low pressure in the mid-continent. An anomalously deep 500mb trough in the mid-continent aids in steering heat and moisture into the study area. The sustained advection of warm, moist air into the region allows for the snowpack to first ripen and then ablate, thus producing widespread ROS flooding throughout the study area. Although regional snowpack ablation combined with liquid precipitation drive ROS flooding, the impact of antecedent soil moisture or frozen ground should not be overlooked. New information presented herein provides flood forecasters identifiable synoptic-scale features that may precede ROS flooding.