Thunderstorms are not considered severe until they meet certain criteria by the national weather service. It is not the amount of rain or lightning that results in the whether the storm is severe, or not. It is the amount of wind and size of hail that the storm produces.
The definition of a severe thunderstorm according to the national weather service:
A severe thunderstorm warning is issued when either a severe thunderstorm is indicated by the WSR-88D(Weather Service Radar) or a spotter reports a thunderstorm producing hail one inch or larger in diameter and/or winds equal or exceed 58 miles an hour. Lightning frequency is not a criteria for issuing a severe thunderstorm warning. They are usually issued for a duration of one hour. They can be issued without a Severe Thunderstorm Watch being already in effect. If the severe thunderstorm is also causing torrential rains, this warning may also be combined with a Flash Flood Warning.
Tornado warnings are a little different. They require strong storms with the proper ingredients to spin a tornado up. Certain signatures on radar will indicate to the meteorologist that the storm may produce a tornado.
The definition of a tornado warning according to the national weather service:
A tornado warning is issued when a tornado is indicated by the WSR-88D(Weather Service Radar) or sighted by spotters; therefore, people in the affected area should seek safe shelter immediately. They can be issued without a Tornado Watch being already in effect. They are usually issued for a duration of around 30 minutes.
