A supercell is a thunderstorm with a persistent, rotating updraft — the mesocyclone. It forms when strong directional wind shear tilts a thunderstorm's updraft from vertical to slanted, separating the updraft from the rain-cooled downdraft and allowing the storm to persist for hours.
The rotating updraft develops through vortex tube tilting: horizontal wind shear generates horizontal vorticity, which the updraft tilts into the vertical, creating cyclonic rotation. Convective Available Potential Energy (CAPE) fuels the updraft intensity; Storm-Relative Helicity (SRH) measures the rotation potential.
Tornado genesis involves concentrated stretching of the mesocyclone vorticity near the surface, forming a wall cloud and then the funnel. Only ~30% of supercells produce tornadoes — the final mechanism involves surface wind convergence and vorticity amplification that is still not fully understood.