Synoptic and Mesoscale Conditions Associated with the Major Ice Storm of April 5 and 6, 2023 in eastern Ontario and southern Quebec

A major ice storm affected eastern Ontario and southern Quebec, including the major cities of Ottawa and Montreal, on April 5 and 6, 2023, with up to 30 mm of ice accretion. It led to two deaths and more than a million customers without power, the largest since the January 1998 ice storm, the most extreme and impactful ice storm recorded in this area. It occurred exceptionally late in the season and was unusually short-lived for an ice storm of this magnitude, with the bulk of the ice accretion occurring over <12 hr due to heavy precipitation rates. We examine the synoptic and mesoscale conditions leading up to and during the ice storm. On the synoptic-scale, we examine the surface pressure systems, mid-upper level troughs and ridges, and air masses. On the mesoscale, we investigate the horizontal and vertical structure of terrain-induced channeling of cold surface northeasterly winds and associated occurrence of a terrain-induced front in the St. Lawrence River Valley. We also provide an overview of precipitation type and amounts throughout the area, as they relate to the aforementioned mesoscale features. We investigate how the mesoscale terrain effects relate back to the synoptic-scale features and how they all contributed to the strengthening and maintenance of the ice storm. We also compare this case study to the historic January 1998 ice storm.