Obviously, this is a very simplified explanation of the physics behind the development of tornados, but I just want you to understand the basics.
Conditions have to be just right for a tornado to develop.
Here is a short step-by-step explanation: (an interesting tornado article)
-Moist warm air is overrun by much denser, cold dry air
-The inversion should be drastic (big difference between the warm and cold air).
-If the inversion is big enough, the re-adjustment will be explosive, thus creating tornadoes.
http://images.usatoday.com/weather/photos/capmap2.jpg