Streamflow Associated with Northeast Tropical Cyclones and Dry Antecedent Conditions in the Connecticut River Watershed

Extreme precipitation events within the northeastern United States continue to pose a challenge for stakeholders such as forecasters and emergency management personnel. Tropical cyclones (TCs) are a well-known source of these high-impact events. To determine the importance of antecedent conditions on streamflow, TCs were examined from 1938 to 2012. Sixteen storms were ultimately chosen based on meeting sufficient rainfall requirements in the Connecticut River Watershed. These storms were categorized by antecedent condition as wet, dry, or average using the Palmer Drought Severity Index (PDSI) and soil moisture percentiles. This study focused primarily on TCs with dry preexisting conditions and contains an in-depth examination of discharge associated with Hurricane Floyd (1999), a representative storm with a prior dry environment. A parallel study, Fernández et. al 2018, focused on the TCs possessing wet antecedent conditions. The two studies were compared to determine whether antecedent soil moisture and seasonal precipitation played an integral role in the streamflow patterns and resultant flooding.

Daily discharge measurements from the United States Geological Survey were used to build composites of dry vs. wet environments prior to the TCs. These preexisting conditions were found to have a significant impact on discharge levels within the watershed, particularly downstream. Peak discharge values at downstream stations for the dry composite were less than that of storms possessing wet antecedent conditions by a factor of three. This is likely due to the fact that drier soils allow for more absorption, while rainfall on saturated soils is converted much more quickly to streamflow. Relationships between precipitation and discharge peaks down the length of the watershed were also examined, as were the impact of these storms on PDSI values.