Just unbelievable. That is all that I can say about the unprecedented and historic flooding in South Carolina. This flood situation was predicted by meteorologists days in advance, but I don't think people and decision makers could fully grasp the magnitude of the threat potential. Hurricane Joaquin's future track dominated the headlines while many in my profession were begging the media to stay focused on the rainfall-flood threat that was going to emerge irrespective of the hurricane. Lee Grenci discusses the multiple factors linked to why this "1000-Year rain event" event happened, and I offered some discussion in Forbes prior to the event on the possible connections to a "hybrid" Atmospheric River. However, as I watch the media and social media, it is apparent to me that many people still do not understand the concept of what 100- or 1000-Year rain event means. Many people literally assume it means this event "can only" happen every 1000 years (in the case of a 1000-year event).
Here is what it actually means as described on the NOAA National Center for Environmental Information (NCEI, but formerly NCDC) webpage:
...it is a statistical way of expressing the probability of something happening in any given year. A "100 year" storm event has a one in one hundred or 1% chance of happening in any given year. A "500 year" event has a one in five hundred or .2% chance of happening in any year.
So this means a "1000-year event has a 1 in 1000 or .1% chance of happening in any year. However, the events do not have to be in 1000 year intervals. As Bob Henson and Jeff Masters writes in Weather Underground:
the same amount of rain could conceivably occur the very next year, or might not occur until thousands of years later. The three-day 1-in-1000 year rainfall amounts for Charleston and Columbia are 17.1" and 14.2", respectively. The 24-hour 1-in-1000 year rainfall amounts for Charleston and Columbia are 14.8" and 12.5", respectively.
Hydrologists are more likely to say that a 1000-year event has a 1000-year recurrence interval. The United States Geological Survey website further provides details on the procedure:
Statistical techniques, through a process called frequency analysis, are used to estimate the probability of the occurrence of a given precipitation event. The recurrence interval is based on the probability that the given event will be equalled or exceeded in any given year. For example, assume there is a 1 in 50 chance that 6.60 inches of rain will fall in a certain area in a 24-hour period during any given year. Thus, a rainfall total of 6.60 inches in a consecutive 24-hour period is said to have a 50-year recurrence interval. Likewise, using a frequency analysis there is a 1 in 100 chance that a streamflow of 15,000 cubic feet per second (ft3/s) will occur during any year at a certain streamflow-measurement site. Thus, a peak flow of 15,000 ft3/s at the site is said to have a 100-year recurrence interval. Rainfall recurrence intervals are based on both the magnitude and the duration of a rainfall event, whereas streamflow recurrence intervals are based solely on the magnitude of the annual peak flow. Ten or more years of data are required to perform a frequency analysis for the determination of recurrence intervals. Of course, the more years of historical data the better.
If you are a weather geek like me, you may be interested in the National Weather Service's Flood Return Period Calculator at this link. Meanwhile, our thoughts and prayers are with anyone affected by the flooding. And remember, "Turn Around, Don't Drown" when it comes to a decision to drive across a flooded roadway.
