Almost every week, at least one tornado occurs in the US, most of which occur in the Central United States between the Rocky and Appalachian Mountain Ranges. "Tornado Alley" is part of the Central US, ranging from South Dakota to northern Texas, and most tornadoes occuring there are at least an EF2. But research has shown there are more "Tornado Alleys" in different parts of the Central United States.
2 Weeks after People: Several tornadoes rip through northern Texas. Only the average damage of an EF2 tornado has been caused.
75 Years after People: An EF4 tornado hits Oklahoma. A lot of damage is caused. In the time of humans, the damage would be cleaned up, and destroyed buildings would be rebuilt, but tornadoes continue to damage the area until there is no sign of plant life anywhere in the area. The entire "Tornado Alley" is now empty of plant life.
500 Years after People: Not a trace of plant life remains in the Central United States. The area looks like a warzone, and only the strongest of the strongest man-made structures have survived.
1000 Years after People: The Central United States is completely unrecognizable. Only 0.00000000000000001% of the stuff in the area has survived. More tornadoes continue to devast the area.
4950 Years after People: Due to the strong impact of tornadoes, the Appalachians are too weak to block the tornadoes from spreading across them. Tornadoes, even EF5 ones, start to occur east of the Appalachians. Fortunately, the Rockies are still strong enough to block tornadoes from spreading west across them.
10,000 Years after People: The Central United States is 100% destroyed and even the strongest of the strongest man-made structures have collapsed. The worst comes when the same tornado that hit Oklahoma in 1999 and was almost an EF6 returns, now as an EF6. Developing in Oklahoma, the tornado moves east through the states of Missouri, Illnois, Indiana, Ohio, Pennsylvania, and New Jersey, ripping apart everything in its path. When it reaches the Atlantic, the tornado hides in the clouds, coming out once in a while as a waterspout. The EF6 tornado will never die.