Why So Many Illinois Tornadoes Were Spawned on a December Day

In Saturday's case, a bullish swirl of low pressure aloft was spinning over the Missouri Valley near St. Joseph, Missouri.
A strong jet stream rounded the base of that upper low from the Red River Valley of Texas, punching northeastward into the mid-Mississippi and Ohio Valley.
Closer to the surface, a complex frontal system was tapping milder and somewhat more humid air northward from the Deep South into central and southern Illinois.
But is it really warm enough in December for severe thunderstorms?
After all, high temperatures in central Illinois only topped out in the 60s – not exactly the hot, humid day you might think of before severe thunderstorms break out in, say, late spring or early summer.
What's important isn't the actual surface temperature or dew point, but rather the relative difference between the air near the ground and aloft, which meteorologists refer to as instability.
Warmer and more humid air near the ground topped by colder and drier air aloft is an environment with more instability to generate thunderstorms.

https://weather.com/storms/tornado/news/2018-12-03-illinois-tornadoes-december-why