Environment Canada has issued a severe thunderstorm watch for Guelph, Waterloo Region and Wellington County.
The agency says scattered thunderstorms are in the forecast Thursday afternoon through the evening.
They say hourly rainfalls totals have the potential to exceed 50 mm.
It says conditions are favourable to potentially develop storms capable of producing strong wind gusts, large hail and heavy rain.
Environment Canada is warning residents to go inside if threatening weather approaches.
The agency says the ongoing heat watch which was issued last week is still in effect and will continue through the rest of the week.