Environment Canada has extreme cold warnings in place Monday for northern and eastern Ontario, all of southern Quebec and western New Brunswick.
People in southern Ontario could expected some of the coldest air they've experienced in about three winters, said CBC meteorologist Colette Kennedy.
"This cold hasn't been experienced for a while," said Kennedy, noting the brisk winds were the result of an Arctic front from the northwest. "We've had a relatively mild winter in many cases, especially in the East."
With the wind chill, temperatures could feel as cold as –45 C in northern Ontario and parts of central Quebec, while the temperature in southern Ontario and western New Brunswick is expected to fall to around –35 C, Environment Canada said.
Coldest southern Ontario's been since before pandemic
In Ottawa, the wind is expected to aggravate what should already be the city's coldest temperatures of the pandemic, Environment Canada said.
The forecast for the capital city calls for temperatures steadily falling this evening and overnight to between –25 C and –32 C by dawn Tuesday. Wind chill could make that feel like –35 C.
Environment Canada said Toronto is expected to go as low as -18 C tonight.
Kennedy said the last time it was this cold in southern Ontario was before the pandemic started in 2020.
She said temperatures this low mean people should cover as much exposed skin as they can, dress in layers — ideally a wind-resistant outer layer — and watch for cold-related health problems such as muscle pain, numb extremities and shortness of breath.
"The strong winds pull any heat your body is providing, away from you," said Kennedy. "So, that's another reason that people just have to be so cautious because it can be a very, very short period of time where you could get into a situation with frostbite or worse."
Montreal shelters desperate for volunteers
In Montreal, temperatures are expected to dip to about –26 C overnight and wind chill values could feel as low as –38 C tomorrow morning in Montreal, Laval and the South Shore of Montreal, Environment Canada said.
It hasn't been that cold in Montreal since Jan. 14, 2018, when temperatures reached –27 C, according to the agency.
Meanwhile, local homeless shelters say they are desperate for volunteers, with COVID-19 threatening to overwhelm the system as temperatures drop to extreme lows starting Monday night.
Due to COVID-19 outbreaks among people experiencing homelessness in Montreal, there are fewer spaces available in the city's shelters and in warming centres, said Émilie Fortier, the head of emergency services at the Old Brewery Mission.
Could be coldest weather in two years in Maritimes
Temperatures will dip to around -30 C overnight with windchill in most on New Brunswick, but could reach as low as -38 C in the some western areas of the province.
"It appears that this is going to be one of the coldest air masses that we have seen in in two years," said CBC meteorologist Ryan Snoddon. "We have to go back all the way to February 2020 to find temperatures that cold in many places like Halifax, Charlottetown, Moncton and Fredericton."
Weather should warm up by Wednesday
Snodden said extreme cold warnings could potentially be issued further east as the Arctic air mass moves.
Temperatures are expected to continue to drop over the next two days in the Maritimes and will reach their coldest point late Tuesday night or early Wednesday morning, he said.
Both Kennedy and Snoddon said the cold spell should be over by the end of the day Wednesday in Ontario, Quebec and the Eastern Townships.