All thunderstorms are dangerous. Every thunderstorm produces lightning. In the United States, an average of 300 people are injured and 80 people are killed each year by lightning. Although most lightning victims survive, people struck by lightning often report a variety of long-term, debilitating symptoms. Other associated dangers of thunderstorms include tornadoes, strong winds, hail, and flash flooding. Flash flooding is responsible for more fatalities more than 140 annually than any other thunderstorm associated hazard.

Dry thunderstorms that do not produce rain that reaches the ground are most prevalent in the western United States. Falling raindrops evaporate, but lightning can still reach the ground and can start wildfires.

• They may occur singly, in clusters, or in lines.
• Some of the most severe occur when a single thunderstorm affects one location for an extended time.
• Thunderstorms typically produce heavy rain for a brief period, anywhere from 30 minutes to an hour.
• Warm, humid conditions are highly favorable for thunderstorm development.
• About 10 percent of thunderstorms are classified as severe—one that produces hail at least three-quarters of an inch in diameter, has winds of 58 miles per hour or higher, or produces a tornado.