A severe thunderstorm watch has been issued for St. Catharines, Grimsby and the rest of northern Niagara.
Along with a heat warning, Environment Canada says conditions are favourable for the development of dangerous thunderstorms that may be capable of producing damaging wind gusts, large hail and heavy rain.
Severe thunderstorms are possible this afternoon or early this evening.
Heavy downpours can cause flash floods and water pooling on roads.
The Office of the Fire Marshal and Emergency Management recommends that you take cover immediately if threatening weather approaches.