A Heat Warning moves into Day 3 for Mount Forest, North Wellington, Listowel and Wingham.
The first heat event of the season continues through Wednesday.
Wellington Dufferin Guelph Public Health says High heat and humidity pose a greater risk early in the season when people are not used to coping with hot weather. Summer sun and high heat bring risks of heat exhaustion and heat stroke, which is a serious condition that can be life threatening.
A cold front will deliver considerably cooler and less humid air to Southern Ontario on Friday.