Flooding in Houston from Hurricane Harvey in 2017. Jabin Botsford/The Washington Post

Kim Schultz still struggles to describe how hard and fast the rain fell in her corner of southern Indiana that afternoon last September.

“I’ve never seen anything like it. Never,” she recalled.

Relentless rainfall inundated parts of the area – including 4.59 inches in a single hour at one spot in nearby Switzerland County, according to the National Weather Service. The deluge transformed the creek that runs near her home near Madison, Ind., into a raging river that swept away a calf and hundreds of hay bales, she said.

The floodwaters tore the asphalt off the top of a nearby bridge, carried away numerous vehicles, ripped trees from their roots, left roads impassable. One house was carried from its foundation and destroyed, killing a 61-year-old woman who lived there.

“You just don’t realize the power of water until you see it for yourself,” Schultz said.

The torrential rains that struck this corner of southern Indiana were rare. But in this area and others across the country, such devastating precipitation is becoming more common as the world grows warmer, according to new data released Monday by the nonprofit First Street Foundation.

In a new peer-reviewed model, the group says the U.S. government’s current precipitation frequency estimates, considered the authoritative source for planning and infrastructure design nationwide, do not fully capture the frequency and severity of extreme precipitation in a changing climate. What now qualifies as a “1-in-100 year storm” – in short, an event with a 1% chance of happening any given year – is already happening more often in some places.

In the county where Schultz lives, First Street’s figures show such storms could happen five times as often. Fifty miles southwest in Louisville, heavy downpours could come about nine times as often, according to the group’s model.

Because the government’s official precipitation estimates don’t yet account for climate change, “it is underpredicting what the actual risk is today,” said Matthew Eby, First Street’s founder and chief executive.

By First Street’s estimates, more than half of Americans now live in an area that is twice as likely to experience such a rain event than is predicted by Atlas 14, produced by the National Oceanic and Atmospheric Administration and considered the gold standard of precipitation frequency estimates across the United States.

Roughly 20% of the country can now expect a 1-in-100-year storm to happen every 25 years, First Street finds. And in the case of about 20 counties that are home to more than 1.3 million people – including parts of Indiana, Kentucky, Pennsylvania, North Carolina and elsewhere – such extreme rain events could happen on average once a decade, if not more often, the group said.

“The magnitude of the changes in expected rainfall intensity are startling for many areas in the United States,” Jungho Kim, First Street’s senior hydrologist and a lead author on the new study, said in announcing the findings. “And it is important that Americans are fully aware of this consequence of climate change that can impact their lives and homes.”

The warming atmosphere is supercharging various weather-related disasters – wildfires, hurricanes, crippling heat waves. But it also is fueling once-unthinkable amounts of rain in single bursts. The resulting deluges pose serious challenges in a nation where aging roads, antiquated storm-water systems and other infrastructure are increasingly outmatched.

Over and over, scientists have documented a central explanation driving the shift toward more frequent devastating rains. For every degree Fahrenheit that the air temperature increases, the atmosphere can hold about 4% more water.

The world already has warmed at least 1.9 degrees Fahrenheit since preindustrial times. That increased heat means more moisture in the air – in the United States, much of it comes off the Gulf of Mexico – and more fuel for more intense rainstorms.

“There is a growing acceptance that as the globe warms, extreme precipitation will increase,” said Kenneth Kunkel, an atmospheric sciences professor at North Carolina State University who has studied precipitation trends for decades.

As that happens, having accurate data about changing precipitation patterns is critical for communities nationwide.

In recent years, most notably in the 2021 Bipartisan Infrastructure Law and in last year’s Inflation Reduction Act, Congress set aside hundreds of billions of dollars to fund historic upgrades to the nation’s drinking water systems, bridges, rail systems, interstates, ports and other key infrastructure.

But Eby and others worry that there is a risk that large sums of money could be wasted on projects that are not actually designed to stand the test of time, given that the climate of the future will look very different from the past.

Chad Berginnis, executive director of the Association of State Floodplain Managers, said that for a long time, the government’s approach toward precipitation estimates assumed that the probabilities of extreme events remained relatively constant in a given place.

“For decades, that probably worked fairly well,” Berginnis said of relying on historical data alone. But, he added, “over the last couple decades, that has changed pretty significantly.”

In Harris County, Tex., home to Houston, First Street estimates that 60% more rain could now fall during a 1-in-100-year storm compared with what Atlas 14 shows for the area. The same goes for the areas around Baltimore and Philadelphia.

As climate change fuels more extreme rain events, particularly in places such as the Northeast, the Ohio River Valley and along parts of the Gulf Coast, the old assumptions are no longer sufficient. While some jurisdictions might be able to pay to undertake more elaborate studies of potential risks in a hotter future, many local officials and planners must rely primarily on NOAA’s existing precipitation frequency estimates when deciding how to build a new project.

“Every year we are spending millions and billions of dollars on infrastructure, and we are doing it only looking to the past for guidance, and not looking into the future,” Kunkel said.

NOAA is keenly aware of the issue, but until recently did not have the resources to construct forward-looking, nationwide estimates of precipitation that also account for the impacts of climate change. Last year, however, Congress gave the agency new funding to create a more robust, comprehensive set of data for every corner of the country – and to update its estimate no less frequently than once every 10 years.

The agency is working steadily toward that goal. But it has said the work will take years and probably won’t be complete until 2027.

In a statement, NOAA declined to comment on First Street’s research. But the agency said the “scientific rigor” behind its precipitation frequency estimates is why they are the standard relied on by engineers and city and regional planners across the country.

The agency’s methodology “undergoes a rigorous peer review process and employs broad stakeholder engagement” to ensure accuracy, transparency and trust, it said.

NOAA is working to develop Atlas 15, which will incorporate historical changes in rainfall intensity as well as estimates based on models of future climate conditions. The agency said it also “is concurrently undertaking experimental research and testing of methodologies to use its extensive climate modeling capabilities to provide accurate estimates of future precipitation on a global scale.”

As engineers, public works officials and planners wait for updated, authoritative federal projections, First Street is hoping to help fill the gap by offering what it calls “an early understanding” of what NOAA’s estimates are likely to produce.

“The First Street data might have the greatest value in the immediate planning environment,” said David Conrad, senior water resource policy adviser to the flood plain managers group. “People can look at that and say, ‘Are we on track? And should we make any adjustments?'”

The answers matter. In the case of a city expanding a storm-water system, underestimating how often heavy downpours might occur could mean pipes won’t drain fast enough to prevent flooding on streets or in homes. A new road could flood more often than expected, shortening its useful life.

First Street, which has previously examined how climate change is deepening risks posed by floods, wildfires, heat and hurricane wind strength, created its latest analysis using data from nearly 800 federal weather stations. Researchers focused on the past 20 years of rain gauge data, in an effort to highlight how the warming atmosphere has increasingly played a role in more intense rain events.

As with the group’s previous analysis, the latest findings underscore how significant amounts of risk are likely unaccounted for in parts of the country.

Americans should consider the rising number of catastrophic rain events in recent decades not as outliers, but rather “as the ‘new normal’ for many areas,” Monday’s report finds. “What this means for communities today is that their understanding of risk is often underestimated,” the group wrote, “and in many locations the infrastructure in place or that is currently being built to protect communities, property and individuals is built to an insufficient standard.”

For his part, Eby said he hopes the findings will help officials think carefully about how they spend taxpayer dollars on major projects, and to have the foresight to grasp how the atmosphere in many places is changing.

“This data hopefully will open people’s eyes,” he said. “[To] realize what the true risk is today, and also what it will be in the future.”