A two-year storm is one that has the statistical likelihood of occurring every two years on average. In other words, it has a 50% chance of occurring in any given year. It is based on statistical analysis of long-term precipitation data.
Similarly, a 10-year storm has a 10% chance of occurring in any given year, and a 100-year storm has a 1% chance of occurring in any given year.
It does not mean that if a 10-year rainfall comes in 2006 we won't see one again until 2016. There could be three 10-year storms in three years, however unlikely. Similarly, there could be a 30-year period without a 10-year storm, which is also unlikely.
