Last night was a doozy for those of us living right in Athens, GA. From about 9:30 pm to just shy of 11:30 pm, the airport received 4.96 inches of rain. Strong thunderstorm cells rained continuously over the area with only minor movement over the 2-hour period. I live about a mile south of the airport and got 3.53 inches during the same time period. Lightning was frequent and there was a lot of street flooding as well as some interior flooding in the area. The roof of a bowling alley partially collapsed. (You can read about that and more at https://onlineathens.com/breaking-news/2016-08-05/rain-collapses-roof-bowling-alley-car-repair-shop-floods-townhomes-athens). It happened so fast that the National Weather Service did not even have time to get out a flood warning until it had almost stopped raining.
Since that is lot of rain, I decided to figure out how often something like that happens. To do that, you need to look at rainfall statistics, and the place to do that is NOAA’s Hydrometeorological Design Studies Center at https://hdsc.nws.noaa.gov/hdsc/pfds/. Pick your state of interest and move the cursor to your location and the site will provide you with a table of rainfall depths by duration (length of time it rained) and return period (how often on average a storm of this size occurs). You can pick your duration and read across the table until you hit the rainfall amount you measured. In my case, 4.96 inches is just shy of the 5.06 inch value the table lists for the 500-year storm. You can also get it in graphical form, which I show below, and in intensity instead of rainfall if you need that for an engineering design.
Keep in mind that a 500-year event can happen two years in a row; it’s better described as having a 1 in 500 chance of occurring in any given year, and each year is independent, so it’s not impossible to have two close by in time (or in space, for that matter). Also keep in mind that high-intensity rains are increasing in the US, so these statistics, which rely on “stationarity” or long-term stability in the statistics, are not as reliable as they used to be.
