Temperatures are expected to surge Tuesday as a hot air mass moves over southwestern Ontario, prompting Environment Canada to issue a heat warning for a swath between Windsor and the eastern reaches of the Greater Toronto Area.

While the weather agency predicts a high of 30 C, it expects humidex values to reach 40.

Environment Canada is warning people to stay hydrated and out of the heat to avoid heat stroke.

The hot afternoon will be followed by cooler temperatures by Tuesday night.