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 dependent both on the duration of the storm and what area it impacts. If the rain falls over the course of 3 hours, it takes over 7 inches for it to be classified as a 100-year rainfall. But if those same 7 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 12 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 approaching a 100-year storm over portions of the Bull Creek watershed. The flood events on October 30, 2015, and May 26, 2016, produced rainfall greater that 13 inches in eight hours in portions of the Onion Creek and Dry Creek East watersheds. However, other areas of Austin did not experience as severe a storm in these events. 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.