From Environment Canada:

12:39 PM EDT Wednesday 10 June 2015
Tornado watch in effect for:

• Sarnia – Petrolia – Western Lambton County
• Watford – Pinery Park – Eastern Lambton County

Conditions are favourable for the development of severe thunderstorms which may produce tornadoes.

Scattered thunderstorms will affect portions of the above regions this afternoon. It is possible that a few of these storms could produce an isolated tornado.

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.

The Office of the Fire Marshal and Emergency Management recommends that you take cover immediately, if threatening weather approaches.

Environment Canada meteorologists will update alerts as required. Please monitor local media or Weatheradio. To report severe weather, send an email to storm.ontario@ec.gc.ca or tweet reports to #ONStorm.

For more information:
https://www.ontario.ca/beprepared.