Environment Canada has placed parts of the Greater Toronto Area (GTA) under a severe thunderstorm watch on Sunday as a heat warning continues for the region.

The federal weather agency says the area could see strong wind gusts, hail up to nickel size and torrential rain. 

Rainfall amounts could reach close to 50 millimetres, Environment Canada said. 

The thunderstorm watch is expected to continue into Sunday afternoon. 

People are advised to go indoors when they hear thunder and avoid driving through flooded roadways. 

Heavy rain may cause flash flooding and reduce visibility, while strong wind gusts may damage trees, buildings and overturn vehicles, Environment Canada said. 

The thunderstorm watch comes as much of southern Ontario remains under a heat warning that is expected to last until Thursday. 

Daytime highs could be between 29 to 32 C, with the humidex making it feel like it's 35 to 40 C. 

Overnight lows are expected to provide little relief, ranging from 19 to 23 degrees.

Environment Canada says the heat is expected to ease somewhat on Monday, although more humid conditions could return Tuesday. 

People are advised to drink water often, watch for signs of heat exhaustion and to check on older adults and those at risk of heat illness.

The heat warning and thunderstorm watch are in effect for the following areas: Toronto, Vaughan, Richmond Hill, Markham, Uxbridge, Beaverton, Pickering, Oshawa, Durham region. 

Meanwhile, the following areas are only under a heat warning: Mississauga, Brampton, Newmarket, Georgina and northern York Region.