Environment Canada issued a severe thunderstorm watch for Sarnia-Lambton Civic Holiday Monday.
The weather agency says conditions are favourable for the development of severe thunderstorms that may be capable of producing strong wind gusts and heavy rain.
Thunderstorms were developing Monday afternoon over portions of southern Ontario.
Forecasters say some of these storms will have the potential to produce significant rainfall amounts as well as wind gusts to 90 km/h.
Some hail is possible as well.
Additional thunderstorms may affect the area Monday night in advance of a cold front that will bring relief from the heat and humidity Tuesday.
A heat warning remains in effect for holiday Monday with the humidex nearing 40 C (104 F).
The temperature at Sarnia's Chris Hadfield Airport reached 31 C (88 F) Sunday afternoon with the humidex peaking at 41 C (106 F). -With files from Dave Dentinger
