An unusual brew of meteorological factors conspired to make the weekend ice storm that swept across Southern Ontario, Quebec and the Maritimes one of the most hazardous and damaging in recent memory.
Ice storms can arise whenever warm air forms a wedge between two layers of cold air – one high up in the atmosphere and one at the surface. Precipitation that begins as snow quickly turns to rain as it falls through the warm air. Then, as the raindrops re-enter the cold, they become supercooled, dipping below the freezing point even though they remain liquid. In such a state, droplets of water freeze on contact when they touch a surface, forming an icy glaze on roads, sidewalks and everything else.
That's what typically happens, said Marie-Eve Giguere, a warning preparedness meteorologist based at the Environment Canada's Downsview office in Toronto.
In this case, though, two back-to-back weather systems brought large amounts of precipitation into the region between Friday and Sunday morning, while the slow movement of the combined storm allowed for ice accumulations of as much as three centimetres in some of the worst hit locations from Niagara to Kingston.
"That's what made this storm so unusual," said Ms. Giguere.
The sheer weight of accumulating ice is often enough to bring down vulnerable trees and power lines.
Ice can also short out transformers, wreaking further havoc on the power grid.
The storm unfolded along a line from northern Texas to Nova Scotia.
The line roughly defines the boundary between a mass of cold air that has lately been stationed over the Prairies and a region of much warmer air that has brought unseasonably mild temperatures to the mid-Atlantic states.
New York's Sunday high topped 20 C. As some of that warm air was drawn northward on Saturday, the conditions for a classic ice storm were realized.
Although the system is now moving east, conditions will continue to be treacherous as temperatures plummet behind the storm front.
"This ice is not going anywhere.
"It's staying with us." Ms. Giguere said.
Although no single weather event can be attributed to climate change, researchers have projected more rain during the winter months in southern Ontario as global temperatures continue to rise. That means there will more opportunities for the kinds of conditions that led to the weekend ice storm, or the more severe ice storm of January 1998 which saw ice accumulations in the range of 7.5 centimetres and left more than 30 people dead.