The 100-year storm is an event that has a 1% chance of occurring in any given year. To put that in perspective, during the span of a 30-year mortgage, there is a 26% chance that a 100-year event will occur.
The amount of rainfall necessary to produce a 100-year storm is partially dependent on the duration of the storm. If the rain falls over the course of 3 hours, it takes about 6 inches for it to be classified as a 100-year rainfall. But if those same 6 inches fall over the course of 3 days, it would be considered a much smaller rainfall event. The standard 100-year design storm for the City of Austin has a duration of 24-hours and produces a total rainfall of over 10 inches. To learn more about rainfall return periods in Austin, see section 2 of the Drainage Criteria Manual.
During a large storm, it is normal for the intensity to vary widely across the city. In September 2010, Tropical Storm Hermine produced rainfall totals equivalent to a 100-year storm over portions of the Bull Creek watershed. However, other areas of Austin did not experience as severe a storm. Keep in mind that even if a large storm has recently occurred, there is the same percent chance of an equally large storm occurring the following year.