The storms erupted because of unusually warm moist air from Louisiana to Michigan that was then hit by an upper-level cold front. That crash of hot and cold, dry and wet, is what triggers twisters.
Like most November storms, this one was high in wind shear and lower in moist energy. Wind shear is the difference between winds at high altitude and wind near the surface.
Because it was high in wind shear, the storm system moved fast, like a speeding car, Brooks said. That meant the system hit more places before it petered out, affecting more people. But in places where it hit, the system may have been slightly less damaging because it was moving so fast, he said.
About 90 minutes after the tornado plowed through Washington, rain and high winds slammed into downtown Chicago, prompting officials at Soldier Field to evacuate the stands and order the Bears and Baltimore Ravens off the field. Fans were allowed back to their seats shortly after 2 p.m., and the game resumed after about a two-hour delay.