Temperatures reaching near 30C and increasingly humid weather is being forecast for Monday into Tuesday.
Environment Canada has issued a special weather statement in effect for Newmarket and York Region for "the first heat event of the season."
Temperatures Monday night are forecast to briefly dip to the mid to upper teens.
However, maximum temperatures on Tuesday will be in the low 30s, and with forecast minimum temperatures in the low 20s in many areas, with little relief from the heat expected.
Cooler air is expected to arrive Wednesday.
These conditions pose a health risk when you are not used to the heat, Environment Canada advises.
Everyone is at risk from heat, especially older adults, infants and young children, and people with chronic illnesses.
You are advised to drink plenty of cool liquids before feeling thirsty and to keep cool by dressing for the weather and spending more time in a cool place
