Tornadoes occur primarily in the United States, and most of them happen in "Tornado Alley," a stretch of the country that extends from northern Texas through Oklahoma, Kansas and Missouri, and also includes parts of Iowa, Nebraska, Louisiana, and eastern Colorado. The landscape that includes distant mountain borders, and miles of flat plains, combine to create huge, extreme thunderstorms called supercells. Dixie Alley is an offshoot of the north-south Tornado Alley, which spawns huge storms and tornadoes across the South.
The storms are fed by cold dry air from the northwest, which meets up with warm dry air coming up from Mexico and warm, moist air from the Gulf of Mexico.
A NOAA (National Oceanic and Atmospheric Administration) map of tornadoes between 1950 and 2006 shows more than 30 counties in Tornado Alley that had more than 15 strong or violent tornadoes during this time. About 88% of tornadoes are weak, moving slowly, with relatively slow wind speeds. The rest of them are fast-moving, and about 1% are extremely strong violent killers that can and do devastate entire towns.