What experts say caused the destructive storm that hit Toronto

A cold front combined with hot and humid air made conditions ripe for what the city has described as a “100-year storm” on Wednesday.

The violent storm, which brought intense periods of heavy rain, destructive winds, and hail to Canada’s largest city, knocked out power and flooded major highways across Toronto, including the Don Valley Parkway.

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“Everything seemed to come together, the huge amounts of rain, the wind, the visibility, everything, the hail, at around 5 o’clock in the afternoon,” retired Environment Canada senior climatologist David Phillips told CP24.com on Thursday.

“Of course, that’s probably the most vulnerable time for Torontonians as they’re leaving work or getting on the road to get home.”

Heat and humidity were ‘fuel’

CP24 Meteorologist Bill Coulter said a tropical air mass filled with heat and humidity fueled the storm.

He added that the cold front descending from the north acted like a “steam shovel” digging under the hot and humid air.

“That ignition was happening late afternoon, at the point of maximum daytime heating, literally when that fuel was most volatile,” he said Thursday.

“The cold front pushed the warm, moist air quickly up to cooler layers of the atmosphere where it rapidly condensed and built into towering cumulous clouds. Powerful updrafts held up the core of water and ice until it became too heavy.”

The rain fell in torrents, Coulter said, and the hail followed.

“The downdraft as the core of both fell created violent winds extending outward from the storms,” he said.

“Any sunshine that returned between waves of storms only served to destabilize the atmosphere further and recharge the atmosphere with that fuel of heat and humidity.”

Canada’s national weather agency sent out a flurry of alerts, including a severe thunderstorm warning, ahead of and during the storm.

Coulter noted that these types of weather events are rare in Toronto.

“Toronto tends to not get a lot of these kind of really powerful storms because often the cold front comes in from the west, interacts with the Niagara Escarpment, that kind of flattens out the cloud,” he said.

“Then as it expands on the eastern side of the escarpment, it dries out a little bit, and then it redevelops on the east side of the GTA and eastward.”

He said what was different about this storm is that it came from the north.

“This coming down from the north had no interactions (with the escarpment) so you had these ripples ahead of the cold front that gave wave after wave of ignition to this tropical air mass,” Coulter said.

“And boy did we pay the price yesterday.”

He said the storm will be “one for the history books.”

‘Becoming more of a pattern’

Toronto’s city manager said Thursday that the storm, which brought more than 110 millimetres of water to some parts of the city on Wednesday, met the threshold of a “100-year storm” due to the rainfall totals.

“The reality is weather is never the same twice. This was a storm that brought a large amount of rain across the city of Toronto,” Paul Johnson said.

“It is a bit unique in that. I’ve talked to some of our folks, particularly in Toronto Water, and we’ve not seen an event like this that’s brought this amount of water across the city of Toronto. It has been isolated.”

Phillips said the storm was reminiscent of previous major storms Toronto has seen over the past 15 years.

“We had one about two years ago that did almost a billion dollars of damage,” he said, pointing to the 2024 storm that also closed down the Don Valley Parkway and caused mass flooding at Union Station.

A summer storm in 2013 also inundated the city with water, flooding streets and transit vehicles.

“We’re seeing 100-year storms don’t take 100 years to come. We’re seeing like a dozen in 100 years,” Phillips said.

“This is just the kind of a sign of the times … It wasn’t a one-off. This wasn’t something you’d see once in a lifetime. This is becoming more of a pattern that we’ve seen in our summers during our hot seasons in the Toronto area.”