Central Iowa Hit by 1,000-Year Rainfall Event, National Weather Service Says

DES MOINES, Iowa — Parts of central Iowa experienced a 1,000-year rainfall event over the course of three days last week, according to the National Weather Service in Des Moines.

Heavy rain fell across the region from the evening of July 2 through July 4, with the hardest-hit areas seeing totals that exceeded both 24-hour and 48-hour rainfall thresholds considered a 1,000-year event, the weather service said.

From the evening of July 2 through July 3, parts of the north side of the Des Moines metro area received 10 to 12 inches or more of rain, exceeding the 24-hour, 1,000-year rainfall threshold for central Iowa, according to National Weather Service data.

Rain continued the following night, with the Ames metro area receiving another 4 to 6 inches or more from the evening of July 3 through July 4. Additional heavy rain also fell in the Des Moines metro area during that period, the weather service said.

Combined, rainfall totals from both nights exceeded the 48-hour, 1,000-year event threshold in parts of central Iowa, according to the National Weather Service.

A 1,000-year rainfall event does not mean that type of rainfall occurs only once every 1,000 years, the weather service said. Instead, the term describes the statistical rarity of the event: In any given year, there is about a 0.1% chance that rainfall amounts of that magnitude will occur during the specified time period.

“Does not mean they occur every 1,000 years,” the National Weather Service said in a graphic explaining the term. “Think annual percent chance, not number of years.”

The National Weather Service in Des Moines released rainfall estimates showing the heaviest totals concentrated over the Ames and Des Moines metro areas, based on 48-hour multi-sensor precipitation data ending at 7 a.m. CDT on July 4.

Information and graphics for this report were provided by the National Weather Service in Des Moines.