About 100,000 thunderstorms develop over the United States during an average year. The vast majority of those storms don’t leave behind any damage in their wake.

While lightning and flash flooding are the two leading causes of injury and death during thunderstorms across the U.S., a storm is only considered "severe" based on winds, hail, and tornadoes.

The U.S. National Weather Service considers a thunderstorm to be severe if it produces wind damage, a measured wind gust of 58+ mph, hail 1.00 inch in diameter or larger, or a tornado.

The Storm Prediction Center (SPC) collects storm reports for wind, hail, and tornadoes throughout the year. Accurate tornado reports date back to 1950, while reports for both wind and hail date back to 1955.

Winds are severe when they either 1) produce wind damage, or 2) produce a measured gust of 58 mph or stronger. A wind gust is considered significant when it measures about 75 mph or stronger.

Hail is severe when it measures at least 1.00 inch in diameter, or the size of a quarter. Most hailstones are measured against common objects to make reporting easier. Golf ball size hail measures 1.75 inches in diameter. Hen egg size hail is 2.00 inches in diameter. Baseball size hail measures 2.75 inches in diameter, while softball size hail comes in around 4.00 inches in diameter.

The SPC also receives tornado reports. Preliminary tornado reports are often duplicates for the same storm, and some turn out to be reported damage from straight-line winds rather than actual tornadoes. Experts with the NWS survey suspected tornado damage to confirm if a tornado occurred and rate the damage on the Enhanced Fujita Scale. Final data only reflect confirmed tornadoes.

Between 2014 and 2023, an average year saw the SPC receive about 16,600 wind reports, 7,000 reports of large hail, and about 1,100 confirmed tornadoes.