London and parts of Middlesex County including Strathroy and Parkhill are under a thunderstorm watch.
Environment Canada issued the weather bulletin Sunday afternoon as the humidex climbed to 32 degrees.
The agency says conditions are favourable for the development of severe thunderstorms that may be capable of producing strong wind gusts.
It says severe thunderstorms with damaging winds are possible as a cold front approaches from the west.
Sarnia-Lambton, Huron-Perth and Grey-Bruce are also under a thunderstorm watch.
Strong wind gusts can toss loose objects, damage weak buildings, break branches off trees and overturn large vehicles. Locally heavy rain is also possible.