Thunder originates as a supersonic shock wave that quickly decelerates to ordinary sound speed.

Evidence

Traveling at speeds up to 130,000 mph, a lightning bolt is essentially simultaneous from cloud to ground. Thunder is generated when a long, very thin (perhaps a few centimeters), and somewhat jagged column of air is partially ionized and rapidly heated, reaching 30,000°C in a few microseconds. The ionized air has no time to move in so short a period, and the heat causes the pressure to rise to about 10 atmospheres (atm). The violent acceleration that follows as this region expands creates a shock wave traveling initially at perhaps 10 times the speed of sound. The supersonic shock wave is, however, quickly transformed to a “normal” shock wave, traveling at the speed of sound, within a few meters of its origin. — Eric J. Heller, Why You Hear What You Hear- An…, 641

Categorization

• CANDY: Lightning heating air to 30,000°C at 10 atmospheres, then the shock slowing to normal within meters, is a great bar-conversation fact.
• QUESTION: Worth chasing: what exactly converts the supersonic shock into a normal sound wave within only a few meters, and how does that relate to the familiar rolling quality of thunder far away?