This rotating air, created by speed shear and directional shear, are lifted into the convective storm to develop a mesocyclone, which is a cyclonic flowing column of air within the thunderstorm. That is, counter-clockwise flowing air (in the northern hemisphere) being lifted into the storm cause the the storm to take on supercell characteristics. A thunderstorm with a persistent mesocyclone is considered a supercell thunderstorm. Sometimes, a supercell thunderstorm takes on very amazing features, like this tornado warned supercell near West Point, Nebraska in 2013.
Once the supercell storm is matured, meaning that precipitation is reaching the ground, a few things start to take place. First, , as the precipitation begins to fall to the surface, the momentum of the falling rain drops begin to cause the air from higher up in the storm updraft to accelerate downward along with the rain. As this rain reaches the earths surface bringing down the air from aloft, a baroclonic boundary is formed, known by storm chasers and scientists as downdraft. You may even start to see lightning in the storm, too.
Supercell thunderstorms have two types of downdrafts without any sort of gap in between them. Rear-Flank Downdraft (RFD), located at the rear or western side of a supercell, and Forward-Flank Downdraft (FFD), usually north and north east of the mesocyclone updraft. They are part of the same descending air but the names are derived from their location relative to the storm updraft. This falling/sinking air is thought to have a significant role in tornadogenesis. In most cases, the FFD now causes the rotating storm to ingest cooler and drier air. In turn, as this inflow air is ingested into the base of the rotating updraft, the ambient temperature(s) are cooled to the dew point. This is often when the “wall cloud” becomes visible.
How can you find a mesocyclone? Typically, the best detection of mesocyclone formation is Doppler radar that shows wind velocities within a thunderstorm.
Storm Relative Velocity with highlighted mesocyclones.
Now the stage is set for tornadogenesis. Keep in mind that fewer than 20% of all supercells actually develop tornadoes so just because you can find a mesocyclone on radar or you see a wall cloud while you are out spotting or chasing, does not mean that you will actually see a tornado.
If all conditions are achieved and no outside disturbances occur, a tornado can likely develop from the above conditions. The actual process of tornadogenesis is still, for the most part, unknown. However, what we do know at this point in time is that a supercell feature known as rear-flank downdraft (RFD) has a lot to do with tornado formation than previously thought.