A thunderstorm that produces a tornado, winds of at least 58 mph (50knots), and/or hail at least 1" in diameter.
A severe thunderstorm warning is issued when either severe weather is reported by storm spotters or is indicated by radar. The criteria for a storm to become severe includes: Winds of 68mph or greater. Hail of 1" in diameter or greater. A severe thunderstorm warning is issued by your local National Weather Service forecast office. [...]
Severe weather refers to any dangerous meteorological phenomena with the potential to cause damage, serious social disruption, or loss of human life.