Environment Canada has issued severe thunderstorm watches covering all of eastern Ontario and western Quebec, saying a tornado is possible from late Wednesday morning until that evening north of the Highway 401 area.
The highest tornado risk is toward the Ottawa River valley, the agency said.
The next steps would be a tornado watch and if a tornado is happening or seems just about to happen, a tornado warning.
If a tornado forms, people are advised to stay away from exterior walls and windows, ideally in a basement or on the ground floor, and to get out of vehicles.
The storm might also produce strong wind gusts, large hail and heavy rainfall of up to 50 millimetres falling within an hour. There’s no tornado risk mentioned for areas closer to Lake Ontario and the St. Lawrence River.
People are reminded of the dangers of weather like this and asked to take cover if threatening weather approaches.
Severe thunderstorm watches are issued when conditions are likely to produce large hail, damaging winds or torrential rainfall. Like tornadoes, it moves to a warning when one of those things becomes more certain.