Every year, the US and Canada are struck by more than 1000 tornadoes, quite often resulting in deaths and property damage. Tornadoes, such as the May 2011 tornado in Joplin, Missouri caused 159 deaths, and nearly $3 billion in damages.
Because they tend to emerge quickly, they are difficult to predict. A tornado watch is issued when there is reasonable likelihood of one forming. It is upgraded to a tornado warning when a tornado has been spotted, or if it is extremely likely to form. If one forms near you, there is hardly any time to take shelter before it hits, so learning the cues from the skies can be a life saving skill.
A super cell is often a dangerous thunderstorm with an internal structure that keeps it going for several hours. They are fully capable of producing high winds, large hailstones, and strong tornadoes.
When warm moist air is forced to rise along hills or mountains, or areas where warm, cold, wet, or dry air all meet, thunderstorms and tornadoes can occur.
Most tornadoes form from thunderstorms, but not always. When warm, moist air from the Gulf of Mexico meets the cool, dry air from Canada, it creates instability in the atmosphere. A sudden change of wind direction, and an increase in wind speed creates an invisible horizontal spinning effect in lower atmospheres. The rising air will tilt the rotating air from horizontal to vertical. The strongest, most violent tornadoes will form within that area of strong rotation.