August 9, 1930 was torrid & likely extremely unstable, but also dry air at mid levels wedging in from the Plains hot developing Dust Bowl. Dew point depressions were 35-45 degrees with temperatures 104-111 & dew points 65-70 & such extreme heat may have led to steep lapse rates.
Despite apparent capping, some weakness in the northeastern quadrant of the upper ridge & possible diffusing outflow boundary triggered isolated violent severe storms with microbursts over southern Indiana & around Louisville.
On microburst hit Vincennes hard with hundreds of trees downed or heavily damaged, lines downed & damage to buildings. A similar storm occurred at Louisville with damage to plans & hangars at Bowman Field airport.
Other such storms were also reported farther north. More organized, widespread damaging winds & damaging large hail hit far northeastern Nebraska to Iowa & northern Illinois.
Such extreme heat, dry mid-level air, significant dew point depressions, likely extreme instability with strong cap behave like a boiling pot of water on a stove. The lid may be breached if there is a weakness or trigger within the cap. This was a common occurrence in the summer of 2012. Even though it was a major drought, some of these intense, pulse storms were able to breach often the strong capping.
Organized bows or even derechos can occur in these heat waves, usually on the edge of the most intense heat where small shortwave (cool pockets aloft with lower pressure) act as triggers.
Below are Storm Prediction Center reports on/around extremely hot days of 2012. Notice the pulsey storms (one storm in July produced a 75 mph wind gust at the Mount Carmel Municipal Airport) & the more organized severe storms to the north & northeast.