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 3/4" in diameter or larger
  
  and./or
  
  

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.