LONDON, Ont. -- Environment Canada has issued a tornado warning for parts of southwestern Ontario.

The warning is in effect in London, Parkhill and eastern Middlesex County.

The area is also under a severe thunderstorm watch.

Ontario's Office of the Fire Marshal and Emergency Management warns that people in the area should take cover immediately if threatening weather approaches.

It says that people who hear a roaring sound or see a funnel cloud, swirling debris near the ground or flying debris should also take cover immediately.

Warnings are an urgent message that severe weather is either occurring or will occur, whereas watches indicate that there's potential for the severe weather to occur.