The mercury fell to -13 C by the early evening, according to data published by Environment Canada. But it felt almost twice as cold as a result of the wind chill.
David Phillips of Environment Canada said there may be more in store as the week progresses.
"It could actually get colder on Wednesday than it is today," he told CBC News in an interview.
Click on the video above to see a full report from the CBC's Natalie Kalata.