The Sun heats the surface of the Earth. Warm air at the surface is less dense than the cold air above it, causing convection. The rising air expands and cools – while the sinking air compresses and heats. If the lapse rate is greater than the adiabatic lapse rate (10C per km) the air is unstable and storms can form. If the lapse rate is less than the adiabatic lapse rate, the air is stable and storms don’t form (i.e. inversion.)
When cold air overrides warm moist air (like in North Carolina over the weekend) the lapse rate gets very large and violent storms form.
Blaming global warming for tornadoes is freaking stupid (i.e. Time Magazine) because tornadoes usually require passage of a strong cold front.