Each of these separate masses collide, there is a drastic clash of air pressure and density irregularities. In an effort to equalize itself, the atmosphere reacts the only way it knows how - stormy weather.
The equation is simple- Wind blows, air rises and falls, and lightning strikes due to contrasts in temperature, pressure, and air density (and they are all related according to the Ideal Gas Law in physics).
Today's severe weather, by no surprise, has occurred exactly where multiple irregularities in air masses meet...from the Front Range of the Rocky Mountains through the Smoky Mountains. Multiple storm reports can also be found through Northern Michigan as differing air masses, combined with Great Lake effects, has caused a widespread thunderstorm outbreak.