A three-year average of 47 tornadoes has been reported in the United States during the month of December, according to the Storm Prediction Center. The multi-state outbreak is shaping up to be unusual, especially in how much farther north than usual it occurred. In addition, the number of fatalities that the outbreak may have caused is exceptionally rare, as the month of December rarely sees tornado fatalities at all.


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Photo taken on Dec. 11, 2021 shows houses damaged in tornadoes in Mayfield, Kentucky, the United States. (Photo by Caromirna Sanchez/Xinhua)
FORMATIVE ATMOSPHERE
"The unusual warmth in the north that preceded the outbreak was one of the main ingredients that drove it to occur. It's typically not so warm this time of year for that region, so outbreaks are usually unable to form," reported AccuWeather on Sunday, while quoting DePodwin as saying that "I think the warm December certainly played a role. We haven't had a lot of cold fronts make their way all the way down into the Gulf of Mexico yet."
The Gulf of Mexico being at or above normal this time of year can further aid in the transport of warmer air northward, which is what helped fuel the multi-state outbreak. As the warmer air from the Gulf moved north, a very strong low pressure area was moving out of the Rockies and into the Great Lakes region. This pulled the warm and moist air even farther northward, said AccuWeather.
Four main ingredients needed for severe weather were prevalent to ignite the outbreak, explained DePodwin, adding that "you need moisture, which we had from the Gulf of Mexico. You need instability, rising air. You need colder air aloft, we had that. You need some type of lifting mechanism, a cold front in this case. And then you need some type of turning in the atmosphere or wind shear, as we call it." ■

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