Dark skies, strong winds, hail, lightning - things most of us associate with severe thunderstorms. But there is actually an "official" definition of a severe storm that the National Weather Service uses to decide whether to issue Watches and Warnings.
A thunderstorm is classified as "severe" if it contains hail of 3/4 of an inch or larger (roughly nickel size), or produces wind gusts of 58 mph or stronger.
Many severe thunderstorms in the Heartland occur as part of a "squall line" (a line of thunderstorms characterized by very strong winds) or "super cells". Super cell thunderstorms always produce severe weather: high winds, hail, or tornadoes. Super cells are huge, often isolated thunderstorms that appear as giant rotating columns of clouds towering 50 to 60 thousand feet into the atmosphere. Whereas a garden variety thundershower may have a life cycle of an hour or less, super cell storms can survive for hours, spinning their way hundreds of miles across the countryside. It is the violent rotating updraft of moist air within super cell thunderstorms that usually produces large hailstones or strong tornadoes.