KITCHENER -- Environment Canada has issued a heat warning for most of Southern Ontario as high temperatures and humidity levels continue through the weekend.

On Saturday, it's expected to feel like the upper-30s in Waterloo Region, while on Sunday, increasing humidity levels will make it feel like the low 40s. Overnight lows will hover in the low- to mid-20s for most in Southwestern Ontario, providing little relief from the extreme heat.

Sunny skies continue Saturday with the UV Index around 10, or Very High. A cold front will move through Sunday, sparking the risk of thunderstorms along with periods of rain. Behind the cold front temperatures will cool off slightly early next week.

Environment Canada is advising people to watch for the effects of heat illness: swelling, rash, cramps, fainting, heat exhaustion, heat stroke and the worsening of some health conditions.