Of the estimated 100,000 thunderstorms that occur each year in the United States, only about 10 percent are classified as severe.
All thunderstorms are dangerous however.
Every thunderstorm produces lightning, which kills more people each year than tornadoes, and heavy rain from thunderstorms can lead to flash flooding. If spotters report damage from the storm or the potential for large hail or damaging winds are indicated by radar, a warning will be issued. Because of the strength of the winds and the resulting damage, the destruction caused by a downburst is sometimes mistaken for tornado damage. Downbursts are often classified into one of two categories. A Microburst is a downburst which produces a swath of damage less than 2.5 miles wide.