New Delhi, July 27 -- MILWAUKEE - When tornado warnings swept across southeast Wisconsin on Monday morning, workers inside the Waukesha County Courthouse were ordered to shelter in place as a multi-county storm corridor arrived on one of the most suffocating days of the year.

What followed across the state's two most exposed regions was a storm system with an unusual formation story. Gravity waves, atmospheric ripples that can push air upward even when weather models show nothing building, drove the first wave of severe thunderstorms. As CBS 58 reported, the storms formed around 10 a.m. and moved east across counties already baking under a heat advisory, catching forecasters by surprise.

The National Weather Service issued tornado warni...