Severe thunderstorm

Posted March 1, 2009 4:02 p.m. EST

The National Weather Service classifies a thunderstorm if it's severe if it produces a tornado, hail three-quarters of inch (about the size of a penny) or wind gusting above 50 knots, or 58 mph. Lightning, due to its unpredictability, does not play a part in whether the NWS considers a storm to be severe.