Howard and Greshko in National Geographic:
Spring and summer may be the most dangerous tornado seasons in the United States, but twisters can still wreak havoc in winter. At least nine tornadoes barreled through Texas over the weekend, killing at least 11 and damaging up to 1,000 buildings and homes across Dallas and the surrounding counties. Storms and tornadoes across the southeastern United States have claimed the lives of at least 43 people in the last week. “Tornado Alley,” which includes many of the Great Plains states and parts of Texas, is the the most notorious staging ground for U.S. twisters. But in December tornadoes tend to form in the southeast and east Texas, fueled by the warm, moist air coming off of the Gulf of Mexico. Record warm temperatures across much of the eastern U.S. have caused unusually large amounts of water to evaporate into the air, giving recent storms more moisture—and greater potency—than usual. (See why December temperatures have been so freakishly warm.)
Scientists don’t completely understand how tornadoes like this weekend’s form. And meteorologists struggle with forecasting tornadoes, since they’re short-lived, finicky, and relatively tiny compared with other atmospheric phenomena. Here's what we do know: A tornado is a violently rotating column of air that extends between the Earth's surface and a cloud. The most intense tornadoes spawn from supercells, massive thunderstorms with rotating hearts called mesocyclones.