The mercury had already hit 30 C at Pearson airport by the early afternoon, with the Humidex making it feel like it was 39 C. By the late afternoon, the temperature climbed to 31 C.
The CBC's Colette Kennedy reported that it was just the fifth time this year that the temperature had reached 30 C.
Environment Canada said the temperature will fall as a cold front sweeps into southern Ontario on Tuesday evening.
The weather agency also warned that a series of thunderstorms may develop later in the day in parts of southwestern Ontario, including in Toronto. Those storms may bring heavy winds, downpours and hail with them.
Long stretch without a heat warning
"It's sad that it took almost to the end of August, almost back to school, to get our first heat warning of the season," said CBC meteorologist Jay Scotland on Tuesday.
He said Toronto hasn't seen temperatures over the 30 C mark since June.
"It's been nearly two months since we've seen any significant heat," he said.
The forecast for a brief spell of hot weather after a cool summer prompted CBC News Network host Andrew Nichols to call this weather event a "one-heat wonder."
Heat warnings are issued when very high temperature or humidity conditions are expected to pose an elevated risk of heat illnesses, such as heat stroke or heat exhaustion.
While extreme heat can put everyone at risk from heat illnesses, health risks are greatest for:- Older adults.
- Infants and young children.
- People with chronic illnesses such as breathing difficulties, heart conditions or psychiatric illnesses.
- People who work in the heat.
- People who exercise in the heat.
- People without access to air conditioning.
- Homeless people.