Severe thunderstorm criteria
I use the same criteria as Environment Canada:
- rainfall of at least 50.0 mm in 1 hour or less
- Hail 2.0 cm in diametre or larger (Nickel-sized or larger)
- Winds/wind gusts in excess of 90 km/h
- The storm is producing tornadoes
However, if a storm is causing fairly unusual amounts of flooding or wind damage, then I will consider it severe despite the wind speeds and rainfall amounts being a bit lower than the criteria mentioned above.