Extreme rainfall frequency in the Atlanta metropolitan area: An analysis of September 2009

Historic, catastrophic flooding resulted across north Georgia from extreme rainfall on September 18-23, 2009. The most intense rainfall occurred in a 24-hour period from September 20-21 when 10 to 20 inches of rain fell. Ten people perished as a direct result of the flooding, with $300 million in property damage. In the National Weather Service's Service Assessment of the event, the United States Geological Survey indicated that they could not accurately characterize the recurrence probability of the event due to the short historical record of area stream gages. This study constructs cumulative distribution functions of rainfall observed from approximately a dozen Atlanta metropolitan precipitation gauges—including Hartsfield-Jackson Atlanta International Airport's 62-year record—and compares them with theoretical distributions for evaluation of statistical significance. A theoretical log-normal distribution is fit to these observational rainfall datasets to estimate the probability of exceedance for extreme precipitation amounts, regardless of the period of record. Ultimately, this analysis supports the conclusion that the rainfall during September 20-21, 2009 was in excess of a 10,000-year rainfall event.