A heat warning has now been issued for Niagara.
Environment Canada says significant heat and humidity will arrive on Friday.
Forecasters say daytime temperatures will be in the low to mid thirties with afternoon humidex values of 40 or higher for Friday and Saturday.
Night time minimum temperatures will be in the low to mid twenties.
A cooler and less humid air mass is forecast to arrive in the wake of a weak cold front Sunday.
Officials say heat-related risks are greater for young children, pregnant women, older adults, people with chronic illnesses and people working or exercising outdoors.
They are reminding you to never leave people or pets inside a parked vehicle.