Descriptions of the atmospheric mechanisms which cause tornados to form ('tornadogenesis') are currently unclear. Similar problems exist in full explanations of dust-devils, waterspouts, landspouts and hurricanes. Currently available computer models (such as VORTEX2) are only partially successful. Tornados usually from in severe storms known as 'supercells' - but the specific conditions which lead to tornado formation - or non-formation - are unknown.
“Despite increased understanding of how environmental profiles of temperature, humidity, and winds affect the tornadic potential of convective storms, much is still unknown about what ultimately differentiates between seemingly similar nontornadic and tornadic supercells. Observations and simulations of nontornadic supercells show remarkable similarity to their tornadic counterparts, with operationally unobservable differences ultimately leading to tornadogenesis failure. While skill in tornado warnings has generally increased over time (Brooks 2004b), the false-alarm rate for tornadoes still hovers around 75% ”
Note: For unknown reasons, some tornados (approx. 1%) rotate in the opposite direction to normal.