From Environment Canada August 2:

Conditions are favourable for the development of severe thunderstorms that may be capable of producing strong wind gusts, large hail and heavy rain.

Hazards: Wind gusts up to 90 km/h. Nickel size hail.

Risk of a tornado. Locally heavy rainfall.

Timing: This afternoon and evening.

Discussion: Thunderstorms are expected to develop this afternoon and last into the evening. Wind gusts up to 90 km/h and nickel size hail will be possible with these storms. Conditions are also favourable for a tornado risk across the region. Large hail can damage property and cause injury. Strong wind gusts can toss loose objects, damage weak buildings, break branches off trees and overturn large vehicles. Lightning kills and injures Canadians every year. Remember, when thunder roars, go indoors! Emergency Management Ontario recommends that you take cover immediately if threatening weather approaches. Severe thunderstorm watches are issued when atmospheric conditions are favourable for the development of thunderstorms that could produce one or more of the following: large hail, damaging winds, torrential rainfall.