Severe Thunderstorm

A thunderstorm (sometimes referred to as a "convective storm" is defined as SEVERE if it produces

straight-line winds 50 knots or greater (57 mph)

and/or
hail 1" in diameter or larger

and./or

tornadoes

The definition was made to highlight thunderstorms liable to pose an enhanced hazard to life and property.

A good indicator that thunderstorms will occur is looking at the dewpoint, and specifically, dewpoint temperatures of 60F or higher is a good indicator that thunderstorms will occur, but is not a criteria for severe thunderstorms.

The government agency responsible for apprising the public about the RISK and OCCURRENCE of severe (convective storms) thunderstorms is the National Weather Service's Storm Prediction Center.