Background
Freezing rain is common in Canada and New England, generally occurring at the narrow boundary between cold air from the east and north and moist air from the south. Typically, a warm air mass will travel northward along the Mississippi Valley and overrun a shallow layer of cold air trapped at the surface. Such a favorable cold air damming happens with an east to northeasterly flow in the St. Lawrence and the Ottawa Valley, and along the axis of the Appalachian Mountains.
Snow is produced at upper level in such a winter storm system but it eventually melts into rain as it falls through the warm air layer, above freezing temperature, associated with the overrunning. The rain passes through colder air near the surface and supercools. When that rain touches the ground in the cold air below, the droplets freeze on contact, creating accumulations of ice. If the cold air layer is too thick, the droplets refreeze before hitting the ground and form ice pellets which are less hazardous.
The Montreal area typically receives freezing rain between 12 and 17 times a year, averaging between 45 and 65 total hours of rain. However, a freezing rain storm usually lasts only a few hours and leaves a few millimeters of accumulation. It renders roads and sidewalks slippery, causing minor traffic collisions, but road crews use de-icing material to take care of it. Power lines and other equipment are built according to tough standards since large accumulation events happened many times prior to 1998. In Quebec, standards were reinforced after a storm left 30 to 40 millimeters (1 to 1.5 inches) of ice over Montreal in 1961.
Prior to the 1998 storm, the last major ice storm to hit Montreal (1961) deposited around 30 to 60 millimetres (1½ to 2¼ inches) of ice. However, the 1998 storm left deposits twice as thick, downing power lines all over the region, damaging most of the trees in Montreal, and leaving streets covered in a thick, impassable layer of ice.
Read more about this topic: North American Ice Storm Of 1998
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