Ice jams are phenomena that obstruct the downstream flow of water in a river during the cold season which can lead to flooding. They occur in settings that facilitate the accumulation of ice floes (e.g., in areas of low streamflow, river bends, near bridges) and are responsible for approximately $300 million (USD) in damage each year in North America (French 2018). Past studies have characterized different types of ice jams as freeze-up, floating, grounded, and break-up (FEMA 2018). Break-up ice jams are often the primary cause of ice jam-related flooding and usually occur in late winter or early spring when warm temperatures and rain cause snowmelt and increased streamflow that breaks existing river ice apart. Warm temperatures and rain that trigger these ice jams typically occur in the warm sector of mid-latitude cyclones that pass over the Northeast U.S. and are often accompanied by an atmospheric river (AR; e.g., Sanders et al. 2020). Break-up ice jams occur with some regularity on the Pemigewasset River in Plymouth, New Hampshire, where they often cause flooding in low-lying areas of neighboring Holderness and are of concern to Plymouth State University, which operates several buildings in the floodplain. Several recent ice jams in this area have motivated ongoing research on ice jams at the university (Sanders et al. 2020). This complementary study provides a climatological analysis of ice jams in the Northeast U.S. and contains meteorological composite analyses that characterize the synoptic-scale environment prior to ice jam events in different states across the region in order to provide additional common-ingredient analyses and improve situational awareness in the ice jam formation process.
The annual distribution of ice jams is bimodal with maxima in late January associated with a mid-winter thaw and in mid-March associated with spring warming. Freeze-up ice jams were observed with maxima in mid-December and late-January and break-up ice jams were observed with maxima in late-January and mid-March, consistent with past research. Rivers in the study area frequently observed ice jams in the same general area (e.g., 41 out of 53 jams on the Pemigewasset River occurred at Plymouth). Antecedent conditions associated with ice jam events typically consist of concurrent maxima in average daily maximum temperature of ~5-10°C and average daily precipitation accumulation of 5-15 mm near the start date of the event, with daily high temperatures that rise above freezing >2-4 days prior to the event and persist for >3-7 days. During the event, temperatures stay above freezing for a total duration of ~24-36 h with average event precipitation totals of 15-25 mm. Ice jams frequently occurred in synoptic-scale environments featuring the characteristics described above that contained ARs with IVT magnitudes >500-600 kg m⁻¹ s⁻¹ in the pre-cold-frontal region of a midlatitude cyclone over New York. The majority of events in New York (88%), Vermont (88%), New Hampshire (97%), and Maine (78%) featured IVT magnitudes ?250 kg m⁻¹ s⁻¹, suggesting that they were associated with ARs.