last from 10 to 30 minutes long.
death toll is from 0 to 20
sever thunderstorms produce up to 30 inches of rain
about 10 percent of of thunderstorm are classified as severe.
the radar tells the weather man and he tells us to prepare
a severe thunderstorm Is produced by a cumulonimbus cloud, usually producing gusty winds, and heavy rain
"according to https://eo.ucar.edu/kids/dangerwx/tstorm4.htm If the warm surface air is forced to rise, it will continue to rise, because it is less dense than the surrounding air.