Thunderstorms are the most common summer storm in St. Louis County, occurring primarily during the months of May through August with the most severe storms most likely to occur in June and July. 

Thunderstorms are usually localized produced by cumulonimbus clouds, always accompanied by lightning, and often having strong wind gusts, heavy rain and sometimes hail and tornadoes. Straight-line winds and heavy rain associated with thunderstorms are one of the greatest concerns for St. Louis County.