The ongoing catastrophic rainfall and flooding in Texas as the result of Hurricane Harvey have pushed often misunderstood and misused terms like “500-year storm” into the headlines. Probably the most important thing to keep in mind is that the time period (i.e., 100-year, 500-year, 1000-year) has nothing to do with the amount of time between events!
Instead, the terminology is the result of statistical calculations called “return period”, “return frequency” or “extreme period” analysis. These calculations estimate the probability of an event happening in any given year; and not the interval between similar events. For example, a 100-year storm has 1 chance in 100 of occurring, or one percent probability of happening in any given year.
It should also be noted that a 100-year rain event does not necessarily equate to a 100-year flood. Due to changes in a watershed, like the of filling in of channels or the paving over of permeable areas, the extent of flooding may change for the same amount of rainfall.
Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services