The atmosphere became increasingly unstable as the day progressed. By 4 o'clock, the weather service was calling in extra people to track the developing storms and issue tornado warnings. As the warm air rose, it naturally cooled, and as it cooled, the moisture in the air condensed from vapor back into a liquid, a process that releases lots of energy. The condensed water fell to the ground as rain. The released energy is what powered the thunderstorms and tornadoes.
Most storms pass harmlessly.
A few, like Monday's, spawn tornadoes. They varied from multiple vortex tornados to big cones. They were changing by the second. A conservative estimate might be a dozen. One particularly caught his attention.