December 2021 North American storm complex and derecho
Right ahead of these components, out-of-season air masses were predicted to take shape across several states. In Iowa, Minnesota and Wisconsin, temperatures were from between 30°F and 45°F higher than December averages. Dew points reached into the lower 60s Fahrenheit. Given these ingredients, along with a weakening cap inversion, forecasters called for severe thunderstorms to get started and become widespread over the Upper Mississippi and Mid-Missouri valleys. Strong, damaging winds were the most predicted threat. Tornadoes, though, were also possible. One to two EF2 and greater were not ruled out.