Wind shear as the initiating mechanism
Tornadoes begin with wind shear: a change in wind speed or direction with altitude. When wind speeds increase rapidly upward, or when wind direction rotates, a layer of air can begin rotating about a horizontal axis. This is the source of the rotation, not instability or vorticity directly from updrafts. In supercell thunderstorms, wind shear creates horizontal vortex tubes that the main updraft can tilt into the vertical.
Once tilted vertically, the stretching updraft intensifies the rotation through conservation of angular momentum, similar to a spinning ice skater pulling in their arms. The pressure at the center drops rapidly, feeding back to accelerate the circulation. Within minutes, a rotating column can intensify from weak circulation to violent tornado-strength winds.
Supercells and predictability
Not all strong storms produce tornadoes. Supercell thunderstorms are the primary producers because they maintain organized rotation and sustained updrafts. The mesocyclone visible on radar identifies the potential for tornadogenesis. The rotation is typically strongest between 2-6 km altitude, and the tornado occurs when that rotation tightens and extends downward to the surface.
Tornadogenesis remains incompletely understood, and individual tornadoes are difficult to predict beyond about 30 minutes lead time. Storm-relative helicity, wind shear, and instability are necessary ingredients, but no combination of these parameters guarantees a tornado will form. This uncertainty is why tornado watches cover large areas and tornado warnings require rapid detection from radar and spotters.