In the last week I’ve received 5.94 inches of rain in my gauge near Athens. Many areas of the Southeast received more than ample rain in the last few days. Fortunately, it looks like we may have a dry spell mid-week to cheer us up and help the farmers catch up on their field work.
Sometimes I am asked how often a rain of a particular amount occurs. To determine that, we can use tables of return periods produced by the National Weather Service’s Hydrometeorological Design Studies Center at http://hdsc.nws.noaa.gov/hdsc/pfds/. Pick your state and then use the tools to pick your location. By doing this for Athens, GA, I get a table which shows the expected rainfall amounts for a variety of time periods and return periods.
The table for Athens is shown below. For my 5.94 inches over seven days, I get a return period of 2 years. This means that on average, a 7-day rainfall of 5.95 inches occurs once every two years. A more correct way of putting it would be to say that it has a 1 in 2 chance of occurring in any given year. It would take a lot more rain to reach the 100 year threshold, which is 11.6 inches for Athens. Note that areas near Mobile AL, received over 11 inches in the last week; however, their 100 year return period storm for seven days is 20.3 inches. Clearly Mobile gets more heavy rain than Athens does!
When you hear the term “100-year storm” it’s important to note that really this means that particular depth of rainfall has a 1 in 100 chance of occurring in any given year. And it is certainly possible to have two years in a row with 100 year storms at the same place, although the likelihood is quite rare. More often, a 100-year storm occurs somewhere nearby but not at exactly the same location, and that is more likely.
It’s also important to know that while engineers use tables like these to design culverts, roofs, roads, and other infrastructure, local building codes may use different sets of tables as their design standard. For example, in Georgia there is a stormwater management manual for the state which has its own set of tables in Appendix A.