Multi-scale analysis of the extreme April 2023 ice storm over eastern Canada

On 5–6 April 2023, a high-impact winter storm produced up to 40 mm of freezing rain and ice pellets over southern Quebec, Canada, the highest amount since the historical 1998 ice storm. More than 800 trees were damaged in Montreal alone, causing some areas of the city to be without power for up to five days. The ice storm occurred exceptionally late in the season and was unusually short-lived for an ice storm of that magnitude. The goal of this study is to examine, at multiple scales, the factors leading to the relatively large and rapid ice accumulation during this high-impact storm. This study focuses on a northwest–southeast transect along which changes in precipitation type were observed, consequently influencing the accumulation of freezing rain at the surface. Air-parcel trajectory analysis indicates that along the length of the transect, relatively warm air aloft was associated with low-to-mid-tropospheric southerly flow while the relatively cold air mass near the surface was associated with northerly flow. Despite similar large-scale patterns along the transect, the microphysics processes differed: areas northwest of Montreal saw up to 9 cm of ice pellets compared to the 40 mm of liquid precipitation that led to 14.6 mm of ice accretion in the city. In Montreal, the air temperature remained near 0°C shortly after the storm, before warming caused accumulated ice to melt 16 h after the end of the precipitation period. The storm’s impacts were mitigated by the occurrence of ice pellets, especially to the north of Montreal, and by the subsequent warm temperatures.