Much of southern Ontario is under a severe weather watch as forecasters warn humidity coupled with a moving cold front could cause treacherous conditions.

Environment Canada issued numerous weather warnings in the province Monday and says the current conditions can cause torrential downpours, large hail, violent thunderstorms and severe winds.

Large hail was already reported this afternoon in the London and St. Thomas regions.

The Air Quality Ontario website is reporting the cold front is forecast to move southward to the eastern Great Lakes and then move north as a warm front on Tuesday.

A smog alert remains in effect as the GTA boils under squelching temperatures. The mercury rose into the mid-thirties this afternoon with the humidex advisory making it feel like the mid-forties.

Two westbound lanes on Highway 401 at Islington were closed on Monday afternoon after hot and humid conditions caused the pavement to buckle.

Work crews were on the scene to try and patch the affected area.

Environment Canada is investigating whether tornadoes touched down last night near Kitchener-Waterloo.

No significant damage or injuries were reported, however, trees were uprooted and farm machinery was tossed around.

Thousands of people remain without power after Sunday night's storm that toppled power lines in the Huntsville and Parry Sound areas.