Hydrometeorological characteristics of ice jams on the Pemigewasset River in central New Hampshire

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Sanders, Matthew Carl
Ice jams that occurred on the Pemigewasset River in central New Hampshire on 26 February 2017 and 13 January 2018 resulted in significant localized flooding in the towns of Plymouth and Holderness. The precipitation events that preceded these floods occurred in association with regions of enhanced moisture and moisture transport known as atmospheric rivers (ARs). ARs are well known to be responsible for wintertime extreme precipitation events, flooding, and variability in annual precipitation on the U.S. West Coast, while their association with extreme precipitation and flood events on the U.S. East Coast is less understood. The role of ARs in flooding associated with ice jams is likely complicated by frozen soils, snow cover, and river ice during the winter, which all can influence the hydrologic response to wintertime liquid precipitation. The goal of this paper is to investigate the hydrometeorological characteristics preceding 20 ice jams on the Pemigewasset River in central New Hampshire and identify the possible role of ARs. The February 2017 and January 2018 events represent two case studies characterized by different antecedent conditions that ultimately yielded the same result. The February 2017 event featured a “long melting period with low precipitation” scenario, with several days of very warm (5°–20°C) daytime temperatures that resulted in xiii extensive snow melt followed by short-duration, weak AR-associated rainfall ~10–15 mm during a 6-h period immediately prior to the formation of the ice jam. The January 2018 event featured a “short melting period with high precipitation” scenario with snow melt that occurred primarily during a more intense and long-duration AR that resulted in >50 mm of rainfall during a 30-h period. Composite analysis of 20 ice jam events support the linkage of these events to ARs: 19 of 20 events were preceded by integrated vapor transport (IVT) magnitudes >250 kg m−1 s−1 in the 2 days prior to the ice jam date and are associated with a composite corridor of enhanced integrated water vapor >25 mm collocated with IVT magnitudes >600 kg m−1 s−1 extending poleward along the U.S. East Coast.