Environment Canada said conditions were favourable for the development of thunderstorms on Thursday afternoon and evening.
A severe thunderstorm watch was posted for north and east Lambton County just before 3 p.m., and upgraded to a warning for the entire county about 40 minutes later. The warning was later downgraded back to a watch.
A severe thunderstorm warning was also posted in London-Middlesex, Chatham-Kent and Windsor-Essex. The warning had expired as of 7 p.m. Thursday.
Environment Canada said the storms could be capable of producing strong wind gusts and torrential rain.
"Rapidly rising rivers and creeks can sweep away bridges, culverts, buildings, and people," the advisory read. "Strong wind gusts can toss loose objects, damage weak buildings, break branches off trees and overturn large vehicles. Heavy downpours are likely to cause flash floods and water pooling on roads. Water-related activities may be unsafe due to violent and sudden gusts of wind over bodies of water."
Emergency Management Ontario advises people to take cover immediately if threatening weather approaches.