A Time Series Analysis of Rain-on-Snow Events at the Summit of Mount Washington, New Hampshire

This study investigates the characteristics of rain-on-snow (ROS) events on the summit of Mount Washington, New Hampshire, using data from the daily weather observations taken at the Mount Washington Observatory from 1981 to 2020 to create two different climatologies (1981-2010 and 1991-2020). Several metrics were used to evaluate ROS events, including the frequency and duration of ROS events, the total amount of precipitation during the events, and their impact on snowpack stability and the surrounding watershed. Based on these metrics, several statistically significant (p < 0.05) trends were found in the data set, and a case study was then performed to investigate the characteristics of a ROS event that occurred on December 18-19, 2023, which resulted in record flooding across the majority of Northern New England. Notably, a positive increase in ROS events was found for both climatologies, but the highest increase happened in the most recent two climatologies. Furthermore, the decadal trends showed that the most recent decade (2011-2020) shifted the maximum number of ROS events from November to December relative to the other three decades (1981-2010), indicating a shortening winter season and weakening snowpack. Additionally, the December 2023 event case study showed how the snowpack characteristics at the beginning of the winter season can significantly contribute to the flooding associated with ROS events, particularly one that produced an 800-year flood.