Tornado outbreak ravages the country for third December in a row

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A terrifying tornado outbreak over the weekend that killed six people, injured dozens and left thousands without power made this the third consecutive December with a spate of deadly tornadoes.

Tornadoes can happen during any month of the year in the United States, but emerging research suggests that as the climate warms, an increasing number of tornadoes may strike during traditionally cooler months.

A powerful storm system struck Tennessee on Saturday, spawning multiple tornadoes across the state. Nashville and the town of Clarksville were hardest hit.

Strong and changing winds at different heights in the atmosphere, also known as wind shear, were a key ingredient that fueled this weekend’s tornado outbreak, promoting “spin” in the atmosphere to form tornadoes. That combined with warmer-than-average temperatures — as much as 20 degrees Fahrenheit warmer than average in some areas — to create tornado conditions.