Environment Canada posted severe thunderstorm warnings and a tornado watch for Sarnia-Lambton Civic Holiday Monday.
Just after noon, the tornado watch was downgraded to a severe thunderstorm watch with conditions still favourable for the development of severe storms which may produce tornadoes into early evening.
Radar, in the early afternoon, showed severe thunderstorms moving east of Sarnia and northeast of Grand Bend.
They were capable of producing strong wind gusts up to 90 kilometres an hour, pea to dime size hail and heavy rain. Tree damage and power outages were possible.
Late Monday morning, the weather agency posted a tornado watch as threatening storms approached Sarnia and the Lake Huron shoreline from Grand Bend to Goderich.
A watch is an alert that conditions are favourable for the development of severe storms that could produce tornadoes. Warnings will be posted advising of any imminent threats.
Be prepared for severe weather. Take cover immediately, if threatening weather approaches.
In the event of a tornado, or if a tornado warning is issued for your area, it is recommended you take the following actions: Go indoors to a room on the lowest floor, away from outside walls and windows, such as a basement, bathroom, stairwell or interior closet. Leave mobile homes, vehicles, tents, trailers and other temporary or free-standing shelter, and move to a strong building if you can. As a last resort, lie in a low spot and protect your head from flying debris.