Streamflow Associated with Northeast Tropical Cyclones in the Connecticut River Watershed: Wet Antecedent Conditions and Hurricane Irene

Tropical cyclones (TCs) can produce various impacts on the Northeast, no matter their category at landfall. This study focuses on how streamflow within the Connecticut River watershed is influenced by these storms. It was hypothesized that soil moisture and seasonal precipitation played integral roles prior to each storm, in turn affecting how a watershed responds to these events. Conditions prior to Northeast TCs were classified on the basis of their Palmer Drought Severity Index (PDSI) and soil moisture data for the chosen area as wet, dry, or average. This study specifically focuses on wet antecedent conditions, and includes a case study on Hurricane Irene, as it is representative of wet antecedent conditions. These observations were then compared to those made in the case study of Hurricane Floyd in Andrews et al. (2018). Using discharge data from various USGS gauges up and down the length of the Connecticut River and within the watershed, the relationship between streamflow and precipitation was examined. In the presence of wet antecedent conditions, the river exhibited above average discharge levels exacerbated by an extreme precipitation event, with values nearing 50% higher than those exhibited by a dry antecedent event. Upstream versus downstream patterns were also noted in the timing of discharge versus precipitation peaks. The gaps between peaks of discharge and precipitation increased downstream, implying that wet conditions immediately allowed for a spike in river discharge that coincided with precipitation at upstream gauges. As the gap increased downstream, a delay in runoff is apparent. This relationship is likely related to the fact that saturated soil does not allow for as much absorption, resulting in the nearly immediate transfer of the precipitation into streamflow. A sample of 16 storms which impacted the Northeast were also collected, classified on the basis of their antecedent conditions, and composited. These composites served as a baseline comparison for individual storm discharge, with wet antecedent storms generally producing three times as much discharge as dry antecedent storms. It is hoped that this work can be used to better forecast and issue watches, warnings, and advisories for flash or areal flooding events in watersheds.