A multiscale analysis of an ice jam flood on the Pemigewasset River in central New Hampshire on 26 February 2017
ice jam , flood , atmospheric river , Pemigewasset , Student Showcase of Research & Engagement 2018
Ice jams and their floods occur during winter and spring in the northeastern U.S. and can have locally high impacts. A high-impact ice jam flood formed on the Pemigewasset River between Ashland and Plymouth on 26 February 2017. The rain from the cold front combined with precursor above freezing temperatures and a deep ripening snowpack to produce enhanced streamflow, ice break-up, damming, and flooding in Holderness that resulted in dozens of Plymouth State University students losing their vehicles. The first two weeks of February in featured below freezing temperatures and a series of heavy snowfall events in New Hampshire that led to frozen rivers and a deep snowpack. A large-scale regime transition by mid-February resulted in temperatures exceeding 10°C above normal during 18-25 February, which ripened the snowpack and led to total snow-water equivalent losses >10 cm over the Pemigewasset watershed. In the six hours leading up to the cold frontal passage, deep-moist southerly flow and snow-to-rain melting levels at 2000-2200 m were present over the region. The 3-5 cm of rainfall associated with the cold front in addition to the increased snowmelt led to the enhanced streamflow, ice dam, and flooding on the Pemigewasset River.