Tornadoes are most often associated with spring weather, but the reality is that twisters in autumn — while rare — have a tendency to be more violent and of longer duration.
That's because one of the ingredients needed for tornadoes, a strong wind field, is stronger in the fall than at other times of the year.
Thus, on those autumn days when the atmosphere is warm and moist enough to generate storms, winds are in place to twist the atmosphere into explosive violence.
On Nov. 17, more than 100 tornadoes were reported in Illinois, Indiana, Missouri, Michigan, Kentucky, Tennessee and Ohio, according to the National Storm Prediction Center.
The National Weather Service said the stage had been set for storms by two days of warm moist air streaming into the region.
That warmth, an arriving cold front and traditionally strong autumn winds combined to spawn these massive tornadoes. At least eight people died in the storms.
Examples of the tornadoes' power and duration:
In Illinois, 24 tornadoes have been confirmed, including an EF4 that killed one person and was on the ground for about 46 miles.
In Illinois and Kentucky, an EF4-strength tornado killed two people while on the ground for 42 miles.
Kentucky also had three EF3 tornadoes that were on the ground for 8, 14 and 19 miles.
The EF4 tornado that struck Oct. 4 in Wayne, Neb., was on the ground for 19 miles.
No one died in Wayne, but one person was critically injured and more than a dozen others suffered lesser injuries.
Source: National Weather Service