Hydrometeorological Characteristics of Ice Jams on the Pemigewasset River in Central New Hampshire

November 15, 2020

Matthew Sanders, M.S., of Plymouth State University, has published a paper in the Journal of Hydrometeorology, along with his advisor, CW3E affiliate Jason M. Cordeira of Plymouth State University, and Nicholas D. Metz of Hobart and William Smith Colleges (Sanders et al., 2021). This study contributes to the goals of CW3E’s 2019-2024 Strategic Plan to support monitoring and projections of climate variability and change by advancing understanding and projections of extreme precipitation events.

The study considers two impactful ice jams, both of which resulted in flooding, in order to illustrate the hydrometeorological characteristics of ice jam events on the Pemigewasset River in central New Hampshire. The two case studies show the importance of short term antecedent conditions to the ultimate impacts of the ice jams. Antecedent conditions from each event, one which included a long melting period with low precipitation and the other which included a short melting period with high precipitation, contributed to the formation of the ice jams, along with the presence of an atmospheric river (AR). In addition to the case studies highlighted, a longer period of record (>25 years) with composite analysis is employed in order to understand the circulation patterns associated with these events. Composite analysis of 20 ice jam events during 1981–2019 allows for the construction of a schematic summarizing common synoptic-scale characteristics (Figure 1). All but one of the events were preceded by environments characterized by ARs along the U.S. East Coast. Five general phases associated with ice jams and flooding were identified and described in order to support situational awareness, beginning 1-2 weeks before the event with cold temperatures and periodic snow accumulations, and continuing through a few days before the event with warming temperatures and some melting of existing snowpack. Directly before the event, a midlatitude cyclone almost always associated with an AR brings periods of heavy precipitation. At the event, the ice aggregates and blocks rising river flow; and just after the event, there is potential for freezing in place especially if a cold front passes. This work provides an important assessment of the synoptic-scale characteristics and effect of short term antecedent conditions before ice jams that cause significant local impacts in the Northeast United States.

Figure 1: Figure 13 in Sanders et al., 2020: Overview schematic for meteorological features associated with ice jams on the Pemigewasset River in Plymouth, New Hampshire. The schematic highlights 1) the location of Plymouth (yellow star), 2) upper-level (e.g., 300-hPa) jet streaks (“J”; purple shading with dashed contours), 3) upstream trough in mid-level (e.g., 500-hPa) geopotential height contours (solid black lines), 4) warm-period temperature anomaly (red oval and shading), 5) tropospheric moisture (green shading for IWV values >25 mm) and moisture transport (black arrow representing AR and corridor of IVT values >600 kg m−1s−1), 6) surface frontal features, and 7) surface cyclone and anticyclone locations (red “L” and blue “H” symbols, respectively) and associated lower-tropospheric flow (red arrows).

Sanders, M. C., J. M. Cordeira, and N. D. Metz, 2020: Hydrometeorological Characteristics of Ice Jams on the Pemigewasset River in Central New Hampshire. J. Hydrometeorology, 21, 2923-2942 https://doi.org/10.1175/JHM-D-20-0027.1 .