First of all there are basics elements that are needed for a thunderstorm to form, which include moisture, atmospheric stability, and something that can trigger motion in the atmosphere. Moisture will produce the clouds and precipitation. For atmospheric stability, rising air will be needed to produce clouds, and in order for air to rise rapidly, it will have to become buoyant compared to the surrounding air.
For tornadoes, you will need a large thunderstorm in a cumulonimbus cloud, a sudden change in wind direction and wind speed at high altitudes, which will cause air to swirl horizontally. The rising air from the ground will push up in the swirling air and tip it over.
