What makes a thunderstorm severe?

A severe thunderstorm produces weather capable of causing significant damage, injury, or dangerous conditions. Thunder and lightning alone do not make a storm severe.

In Canada, the main criteria are:

• Large hail: hail 2 cm or larger in diameter.

• Damaging winds: wind gusts of 90 km/h or stronger.

• Tornadoes: any confirmed or suspected tornado makes the storm severe.

Other significant impacts: Environment and Climate Change Canada can issue warnings based on particularly dangerous storm conditions, even when the classic thresholds aren't all met.

What about lightning?

Lightning is not normally the criterion for a severe thunderstorm warning. A storm can produce frequent lightning and still not be classified as severe.

Similarly, heavy rain isn't automatically a severe-thunderstorm criterion. A storm may produce torrential rain and flash flooding while the thunderstorm itself doesn't meet the traditional severe criteria.