How Ottawa's weather made the perfect recipe for a tornado

The cold air that blew through Thursday after a heat wave is akin to yeast that causes a reaction to happen.

Published Jul 13, 2023 • Last updated 7 hours ago • 2 minute read

OTTAWA – July 13, 2023 – A woman walks through a rain storm in Ottawa Thursday. The city of Ottawa was under a tornado watch at the time. Photo by Tony Caldwell /Postmedia

A recent string of hot, humid days set the table for an Ottawa tornado. Cool air on Thursday gave it the final push.

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“You don’t necessarily get [tornadoes] just with heat and humidity,” said David Phillips, Environment Canada’s senior climatologist, shortly after the tornado hit. But he compared the cold air that blew through Thursday after a heat wave to yeast that causes a reaction to happen.

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“You need winds coming at different directions, shifting winds, and you need the humidity and air temperatures.”

Ottawa had temperatures in the low 20s Thursday, following a string of hot, humid days.

“What you always are concerned about is this cold front coming through because it pushes the warm air up,” Phillips explained. “It gives the system steroids and it picks right up.

“There’s more turbulence, it’s very unsettled, and you have these winds that come in at different directions to give that cloud some rotation.”

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That rotation can tighten into a fast-spinning tornado.

“My sense is that the conditions were right. We saw that cold front and that would trigger thunderstorms, severe thunderstorms,” with heavy rain, hail and strong winds.

Canada is the world’s second most tornado-prone country, with 60 to 80 observed most years. There are likely many more unobserved, deep in forests.

Ontario receives about 12 to 15 a year, said Phillips.

Some touch down in a single farm field; some are much larger.

The most destructive tornado in this region’s modern history swept through on Sept. 21, 2018, touching down in a series of hits from White Lake and Calabogie through Dunrobin, Arlington Woods in Ottawa, and sections of Gatineau including Mont-Bleu.

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Sixty houses in Dunrobin alone were destroyed, along with dozens of low-rise apartments in three- and four-storey buildings in Gatineau. The apartment buildings remained standing for the moment but many lost roofs and had severe wall damage.

Exeter Drive between Wessex and Jockvale in Barrhaven. Photo by Charlie Senack /POSTMEDIA

On Aug. 4, 1994, a powerful F3 tornado hit Aylmer, damaging 385 homes. No one was killed; many of the homes were empty as residents were at work.

A later study by Environment Canada showed the tornado formed almost directly overhead of the radar station at Franktown, which was its blind spot at the time.

Smaller storms also pass through here. In July of 2013, one snapped about two dozen trees at the Pine View Golf Course in eastern Ottawa.

“You (in Ottawa) are not Little Tornado Alley, which would be the area from Windsor to Barrie… between Lakes Huron and Erie,” Phillips said. “But you’re not immune.”