Environment Canada said at 4:15 p.m. ET that meteorologists were tracking a dangerous thunderstorm near London, Ont., that is likely to produce a tornado.
"Doppler radar indicates strong rotation about 20 [kilometres] north of London and it is moving eastward at 60 km/h," Environment Canada said. "This is a dangerous and potentially life-threatening situation.
Tornado warnings were also posted for areas between Brockville and Ottawa, but they have subsequently been ended.
Many watches and warnings for severe thunderstorms were also posted across the same parts of the province.
People are advised take cover immediately if threatening weather approaches, Emergency Management Ontario recommends.