Read some information from Internet:
This area often has the three ingredients necessary for tornadoes to form: a lot of warm, moist air close to the ground; atmospheric instability, a condition that promotes the vertical movement of air; and clashing air fronts that act to propel moist air upward.
In Tornado Alley, warm, moist, low-elevation wind from the Gulf of Mexico collides with cool, dry, higher-elevation wind coming over the Rocky Mountains. This unstable interaction causes lots of warm air to quickly rise and cool air to fall, which leads to the formation of a supercell, a type of thunderstorm with a long-lived, swirling updraft of air.
But not all supercells result in tornadoes.
Scientists believe that if the two opposing winds move at different speeds, the air in between them will rotate around a horizontal axis. If one end of the horizontal air column gets caught in the supercell’s updraft, it will tilt vertically, forming a funnel cloud. The continuous upward energy of the supercell elongates the funnel cloud, and causes its spin to tighten and speed up — similar to the way ice skaters spin faster when their arms are pulled close to their bodies.
Rain and hail from the thunderstorm push down on the tail end of the funnel cloud. When the bottom of the funnel cloud finally touches the ground, it becomes a tornado.
This description is very close to the real conditions,… without reasons for under surface horizontal rolling on my view. But if it is happened it could add more rotation to the formation…
Rotation of new formation is the result of summering TWO VECTORS of two streams, which have few main factors of depend…and create the main direction of rotation under circumstances of air streams speeds and resistant to move…
The “Theory of Tornado Formation” is only start to research and develop. I did my conceptual basic theory about it in special blog…
The best images of beginning of tornado formation
I found on Internet: