Environment Canada has issued an extreme cold warning for southern Ontario and at 7:30 a.m. the city's chief medical officer issued an extreme cold weather alert.
The city's cold weather alert — which comes day after a 55-year-old homeless man died of exposure — makes more shelter beds available.
It was -13 C in Toronto at 6:30 a.m. but temperatures will fall later in the day. With windchill, it will feel like -30 C with winds gusting in the 60 km/h range.
"Bundle up if you have to be outside and try to limit your exposure if you can," Environment Canada meteorologist Mark Schuster said. "Try to not have any exposed skin because frostbite could become a threat after a few minutes if you have exposed flesh."
Schuster said the day's coldest temperatures will come in the afternoon before easing considerably overnight. By Thursday, daytime highs will reach about -7 C.
He said this week's temperatures are well below the normal daytime highs in the -2 C range.
On Toronto's streets, CBC's Michelle Cheung found people doing what they could to keep warm.
"The few people who are walking around are all bundled up and walking fairly quickly," she reported.
Drivers will also need to prepare for the cold. CAA says extreme temperatures can double their normal call volume, with most calls about dead batteries.
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