Parts of Cincinnati experienced heavily concentrated rainfall on July 17, which caused flash flooding and hundreds of sewer backups. Local officials are calling it a 1,000-year storm, emphasizing how rare a storm of this severity is.
That caused frustration for some residents on the city’s East Side, which has flooded several times over the past decade.
"Calling something a 1,000-year storm over and over again, and saying there’s nothing that could have been done, is not productive," one Hyde Park resident told City Council a few weeks after that July storm.
The phrase "1,000-year storm" sounds like a storm of that size will only happen once a millennium, but that’s not what scientists mean when they classify storms this way.
Here’s how these storm events are classified and what these terms actually mean.
Click to jump to a specific section:
- How local officials collect and analyze rainfall data
- What does ‘XX-year’ storm mean?
- Who decides the criteria for storm probability?
- How climate change affects storm probability
- Cincinnati’s east side and urban heat island effect
- How big that storm really was
- Why does storm probability matter?
- Learn more about sewers and stormwater in Cincinnati
How local officials collect and analyze rainfall data
Reporters often cite rainfall data from the National Weather Service (NWS), which has rainfall gauges at several locations in Greater Cincinnati.
The Metropolitan Sewer District of Greater Cincinnati (MSD) has much more detailed and accurate rainfall data, according to Director of Capital Planning John Barton.
"We are actually the experts on quantifying rainfall in Cincinnati," Barton told WVXU.
That starts with having more raw data. The NWS has several rain gauges in the region, but MSD has over 30 in Hamilton County alone.
"Those are calibrated every year, and are all cross-checked," Barton said. "We have a weather provider, Vieux and Associates — they actually are analyzing that data on the radar and the rain gauges on a storm-by-storm basis for us and have been for over 20 years."
That analysis includes the NWS gauges as well, plus some citizen-owned rain gauges.
Barton says that means MSD has immediate access to pretty accurate rainfall data which is then put through a quality control process involving actual humans and can take about a month to complete. That results in the most accurate possible data.
In cases of extreme weather events, like what happened on July 17, MSD asks Vieux to speed up that process to just a few days.
What does ‘XX-year’ storm mean?
Phrases like 1,000-year, 500-year, 100-year, and 50-year storm simply describe probability.
Each year, there is a 1% chance that a storm classified as a 100-year storm will occur. The probability is the same every year, regardless of how recently a 100-year storm has occurred.
A 500-year storm has a 0.2% chance of occurring in a year; a 100-year storm has a 1% chance of occurring; a 5-year storm has a 20% chance of occurring, etc.
Think of it like rolling a die — the probability of rolling any one number never changes, no matter how many times you roll. You could roll a dozen times and not get a six, but that doesn’t mean you’re more likely to get a six on the 13th roll.
Each year, there is a 0.1% chance that a storm classified as a 1,000-year storm will occur. Those odds are exceptionally low, but there are a couple of additional factors.
First, there are different categories of 1,000-year storms, each with a 0.1% chance of occurring every year. It’s based on not just the total amount of rain, but also how quickly the rain falls.
In Cincinnati, a 1,000-year storm could be:
- 7.46 inches of rain over 24 hours
- 6.6 inches of rain over 12 hours
- 5.79 inches of rain over 6 hours
- 4.83 inches of rain over 3 hours
There are 19 total categories, starting with five minutes and going up to 60 days.
"A 15-minute 1,000-year storm is totally unrelated to a 72-hour 1,000-year storm. They’re totally different types of rainfall," Barton said.
Second, probability is based on location. The likelihood of a storm event in Lincoln Heights is unrelated to the probability of a storm event in Mt. Washington.
Adding all those together means the probability of Greater Cincinnati getting a 1,000-year storm in any given year is actually higher than 0.1%.
If you’d like to dig a little deeper, the NWS recommends this article from the Australian Government: Why do 100 year events happen so often?
Who decides the criteria for storm probability?
The standardized criteria for precipitation frequency comes from the National Oceanic and Atmospheric Administration, or NOAA. The current version is NOAA Atlas 14, which predicts probability of rainfall for specific locations based on data through the year 2000.
You can see the table for Ohio at this link, then select one of the Cincinnati stations from the drop-down menu.
How climate change affects storm probability
As mentioned, the NOAA Atlas 14 does not include rainfall data for the past 26 years. That could mean the effects of a warming climate are not fully accounted for.
In Cincinnati, average rainfall intensity increased 10% between 1970 and 2025, according to an analysis of climate data by nonprofit Climate Central. Total annual precipitation has also increased by 6.8 inches.
NOAA is working on Atlas 15, which will include the more recent years of data and will also predict future trends through the year 2100.
“NOAA Atlas 15 represents a shift from a stationary assumption (i.e., extreme precipitation events do not change significantly over time) to a nonstationary assumption (i.e., extreme precipitation events change over time),” says a federal government website about the project.
The site says Atlas 15 will be published in 2026 for the contiguous United States, and in 2027 for areas outside the contiguous U.S.
Cincinnati’s east side and urban heat island effect
A warmer climate means more moisture in the air, which affects the amount and intensity of rainfall during a storm.
A related factor could explain why communities on the east side of Cincinnati seem to get hit with these major storms more often: the urban heat island effect.
“Heat islands occur when a developed area experiences higher temperature than nearby rural areas, or when areas experience hotter temperatures within a city,” says an EPA website on the topic.
In other words, areas with a lot of concrete, asphalt, and other infrastructure are hotter than areas with a lot of green spaces and trees. More heat means more moisture accumulation, ready to drop in buckets as the storm continues.
MSD’s John Barton says, anecdotally, he can see Cincinnati’s heat island affecting the severity of storms.
"We see even in smaller storms, oftentimes they seem to blow up over Cincinnati," Barton said.
Most storms come through the area from the southwest, possibly picking up more moisture as they move over the Downtown heat island. When a storm continues to the northeast, those communities, theoretically, get heavier rainfall than other parts of the area..
"We want somebody to study it and say, okay, look, we've looked at it, and yes, the Oakley/Hyde Park area on the northeast side of the urban heat island is getting hit with more intense storms, or it's not," Barton said, adding MSD is in talks with the University of Cincinnati to commission that research.
How big that storm really was
Cincinnati and MSD officials initially reported that Hyde Park, Oakley and surrounding areas got between 5 and 6 inches of rain in just one hour on July 17, citing the National Weather Service.
That amount of rain is literally off-the-charts — the NOAA Atlas 14 only goes up to a 1,000-year storm event, which would be anything above 3.52 inches in 60 minutes.
Barton says that was likely a miscommunication during the emergency management response because none of the rain gauges — for NWS or MSD — recorded that amount of rainfall in just one hour.
"Possibly, what was originally communicated as an instantaneous peak rate, during recommunication was retranslated into the total for one hour," Barton said in a written statement after a recorded interview with WVXU.
The actual data is still incredible: 5.07 inches fell over the course of three hours, well above the threshold of 4.83 inches for a three-hour 1,000-year storm. That was concentrated over a few east side neighborhoods, including Hyde Park and Oakley.
For context, the average July monthly rain total for the entire Cincinnati area is 3.83 inches, according to NWS data. The highest monthly average for this area is 4.75 inches in June. To get over 5 inches of rain in a small area, in just three hours, is remarkable.
Why does storm probability matter?
Engineers and city planners use storm probability to design all sorts of infrastructure, including bridges, flood control, stormwater and sewer systems, and more.
"A 10-year storm — that's what we design new infrastructure to, that's our design standard," said MSD Deputy Director Reese Johnson at a recent Hamilton County Board of Commissioners meeting.
That sounds low, but designing for larger storms would be incredibly expensive, partly because we’re living with decisions made one or two centuries ago.
"We filled in [a] creek and the floodplain and put houses essentially on top of the creek [or] within a few feet of where the creek is. That's a problem. When you get really big rainfalls, you can't have houses in where the creek used to be," Barton said. "And the ratepayers justifiably say, 'We can't pay 12 times our rates for you to tear down all our houses to make these projects happen.' "
NOAA Atlas 15 may indicate extreme precipitation is more likely than we know now, but the cost of designing and building will only continue to increase.
Learn more about sewers and stormwater in Cincinnati
Check out WVXU's podcast Backed Up to learn more about the history and challenges of Cincinnati's combined sewer system.
In recent severe rainstorms and for the past several decades, sewage has bubbled up in our basements and poured into our waterways. Climate change is making it worse, and the powers that be can't seem to agree on how to fix it.
Backed Up demystifies one of the most complex systems of public infrastructure — our sewers — and tells the stories of the people suffering under decades of mismanagement.
You can listen at wvxu.org/backedup or subscribe wherever you get your podcasts.
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