The United States records about 1,200 tornadoes each year—more than any other country. The U.S. is especially tornado-prone because warm, humid air from the Gulf of Mexico frequently collides with cold, dry air from Canada and the Rocky Mountains, while hot, dry air arrives from the Southwest. Strong winds changing speed and direction with height can make these unstable air masses rotate, producing powerful thunderstorms and sometimes tornadoes. The broad, relatively flat central plains allow the air masses to meet with few geographic barriers.For much of American history, tornadoes were recorded only through eyewitness accounts and newspaper reports. The first successful tornado forecast was issued in Oklahoma in 1948, and the modern U.S. tornado database begins in 1950.