Flash Flood Triggers Blackouts, Transport Chaos in Toronto: AIR Analysis

July 10, 2013

Catastrophe modeling firm AIR Worldwide reports that “after days of heavy precipitation, a torrential thunderstorm on July 8th dropped an additional ten centimeters [app. 4 inches] of rain on Canada’s largest city, Toronto, in just four hours.

“The storm triggered flash floods that cut power to at least 300,000 and the entire public transportation system was shut down for several hours, although many stations have since reopened. Many major roadways were also flooded, submerging cars up to their rooftops, and numerous flights out of Toronto’s airports were delayed or cancelled. The record-breaking rainfall caused widespread flooding and power outages throughout Toronto and surrounding regions.”

According to AIR, a few non-severe thunderstorms were forecast for southern Ontario today, but additional flooding is not expected to occur. The floodwaters have already receded in most areas, making previously submerged roadways passable, flooded subway lines operational, and enabling electricity to be restored to most of the affected areas.

AIR noted that at “this early stage, there is no indication yet of how many homes and businesses have been damaged by the flooding. Since the city’s sewer system was overwhelmed by the amount of precipitation, much of the building damage will likely be attributable to sewer backup. This is typically offered as an optional coverage on most standard residential policies.

“Mississauga, a city of more than 700,000 southwest of Toronto, saw the worst of the blackouts, with 80 percent of the community plunged into darkness after a transformer there was flooded. An estimated 90 percent of Toronto-area residents have since had their power restored, reported a local utility. The worst flood damage was in North York and Toronto’s historic center.

“The thunderstorm produced the heaviest rainfall in Toronto’s recorded history, easily topping the city’s previous single day rainfall record of 3.6 centimeters [1.42 inches] that was set in 2008.”

According to AIR, the warm surface temperatures and humidity needed for thunderstorm formation in this region of Canada “typically begins in late May and early June. During this time, the jet stream is positioned over the northern plains of the United States and Central Canada, providing the necessary conditions for hail, tornadoes, and strong straight-line winds. While these storms can occur at any time of day, they most frequently occur in the late afternoon, during the time of warmest temperatures.”

AIR’s report also explained that “southeastern Ontario and southern Quebec, where Toronto is located, is a region of Canada where severe thunderstorms are common during the summer months. There the jet stream provides favorable wind shear, while fronts frequently separate cooler air from the north and warm, humid air to the south.”

Source: AIR Worldwide

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