An Anomalously High Accumulation of Ice Pellets in St. John's, Newfoundland, on 16 January 2018: A Case Study

Located at the convergence of most of North America’s storm tracks, St. John’s, Newfoundland has some of the continent’s most active winter weather, specifically with respect to explosive cyclogenesis and heavy warm frontal mixed precipitation. An anomalous precipitation event occurred in the city on 16 January 2018 that resulted in the accumulation of 6-8 inches of ice pellets, which exceeded the city’s previous highest ice pellet amount documented in literature (4-6 inches on 1-2 February 1992). A subsequent warm spell caused partial melting of the accumulated ice pellets, allowing liquid water to permeate through any gaps; this was followed by a freeze that resulted a thick layer of dense ice across the city, posing serious threats to transportation and safety. Such high amounts are unusual for a hydrometeor whose formation and sustenance depend on a delicate balance between opposing thermodynamic processes such as the latent heat release associated with the freezing of rain drops in the low levels of the atmosphere and the cold air advection at the surface. The climatological factors that make eastern Newfoundland’s environment favorable for long-lived mixed precipitation events will be discussed. With the goal of improving the difficult prediction of mixed wintry precipitation, we studied the atmospheric and oceanic environment that allowed such a unique event to occur by examining synoptic conditions, upper-air soundings, cloud condensation nuclei distributions and backward parcel trajectories at the time. We found that the near-freezing marine environment of eastern Newfoundland at the time favored a long-lived ice pellet event by providing unlimited cold moist air supply at the surface, as well as sea-salt condensation nuclei in the sub-freezing surface layer, which likely favored ice pellets over freezing rain. The synoptic environment was also found to be conducive with blocking high pressure over northern Quebec providing cold air at the surface and slowing down the warm front’s progression, allowing a thin band of ice pellets to stall over St. John’s.