One in a hundred? Think one in ten

A recent US federal analysis of extreme rainfall is actually significantly underestimating the chances of catastrophic flood events.

Such an underestimation has grave implications for everything from new roads and bridges to the rising cost of flood insurance, according to the new report.

The report, Defining America’s Climate Risk, notes that flooding from heavy rainfall events is a dangerous phenomenon that has become increasingly probable and severe in the United States due to climate change. As discussed in the report, as air temperatures increase, more water vapour may be held in the atmosphere and discharged during rainfall events. For every 1°C increase, 7% more water vapour is carried by the same air volume. As a result, increasing temperatures have created changes in the expectations of the Intensity, Duration, and Frequency (IDF) of rainfall events. 

Rainfall events that were thought to occur only once every hundred years are now occurring with far greater frequency. In some places, these formerly rare events are now occurring as often as every 5 or 10 years, based on the First Street Foundation Precipitation Model (FSF-PM).

The report found half the US population lives in a county where a 1-in-100-year flood is at least twice as likely now as it had been in the past, coming once every 50 years, on average, rather than 100.

The background to this research is the US federal rainfall analysis, which is managed by the National Oceanic and Atmospheric Administration (NOAA). However, according to First Street, this is updated infrequently and has not kept up with the climate crisis, which is now intensifying precipitation events.

Once a decade events

In a shocking statistic, in 20 of the most populous counties in the US, where a combined 1.3 million Americans live – largely in mid-Atlantic states including Pennsylvania, New Jersey and New York – these major events are forecast to happen around once every decade, First Street said.

First Street found in much of the Northeast, the Ohio River Basin, Northwestern California, the Texas Gulf Coast and the Mountain West, the rainfall depths for a 1-in-100-year event could happen far more frequently, which First Street estimates could occur at least every 5 to 10 years.

Inland flooding

While major flooding is often considered a coastal phenomenon driven by hurricanes, intense rainfall also can produce disastrous inland flooding. Hurricanes are producing more rainfall as they move inland after landfall and dissipate, for example.

California was hit with more than 10  so-called “atmospheric rivers” this past winter, bringing destruction as well as relief to the drought-stricken state. Meanwhile, severe flooding from torrential rainfall devastated Eastern Kentucky and Missouri in late July last year, causing some $1.5 billion in damage. 

There are several reasons why NOAA’s data are outdated, according to the report. The first is that it does not adequately account for how climate change is making rainfall events more extreme, First Street contends. Its historical data goes back to before 1900, incorporating years when extreme rain events were far less common.

That time span dilutes extreme rain events that in recent years have gotten bigger and more dangerous with a warming climate, First Street researchers said, noting the top 10 single-day extreme rainstorms have all occurred after 1995.

Measurement metrics

There is also an issue with the rain gauges themselves, First Street noted. Many of NOAA’s measurements come from state-of-the-art gauges, where data is frequently and automatically recorded. But it also has a vast network of volunteer-operated rain gauges with few set standards, and where data is collected less frequently. Because some rain gauges are unreliable, NOAA smooths its data, which can remove or underrepresent some of the more extreme events.

The major rainfall events, and supposedly outdated federal data, could have profound impacts on new infrastructure, as well as the cost and availability of flood insurance to help homeowners protect their property.


As NOAA works on a new rainfall analysis, federal infrastructure funding is already being spent on roads and bridges that are being built using outdated data, according to observers – a scenario that could have serious implications for the safety and survivability of critical infrastructure, especially in increasingly flood-prone places.  

The other big impact is on FEMA’s federal flood insurance overview, which is supposedly underrepresenting the number of people that could fall into FEMA’s Special Flood Hazard Areas, meaning millions of Americans may be unaware of their current flood risk.

As the report says: “The First Street Foundation Flood Model highlights the urgent need for accurate data collection and analysis to protect communities from the impacts of climate change. Policymakers must prioritise the collection and analysis of accurate data to ensure that infrastructure investments provide effective protection against flooding.”

“This includes allocating funding for research, ensuring that climate change and extreme events are consistently taken into account in precipitation models, and updating FEMA’s SFHA designation to consider precipitation. By taking these steps, policymakers can ensure that communities are adequately prepared for the increasingly severe impacts of climate change.”

To access the full report, click here.

In a shocking statistic, in 20 of the most populous counties in the US, where a combined 1.3 million Americans live – largely in mid-Atlantic states including Pennsylvania, New Jersey and New York – these major events are forecast to happen around once every decade, First Street said.