What Does A One-In-A-1000 Year Storm Look Like — And Will We Be Seeing Them More Frequently?

Hurricanes have always been taken seriously by residents who live on coastlines. After all, there are historical precedents that make clear how much damage can accrue from a hurricane’s wrath. What’s not so well understood is how what used to be referred to as a one-in-1000 year storm generates wind exposure, tornadoes, record rainfalls, storm surges, and wind waves — and how we’ll likely be seeing many more of these than ever before, both along the coasts and inland.

Anyone who questions the need for better education and information about the risks of extreme storms need look no farther than Hurricane Helene, which was one of two-1,000 year storms to hit North Carolina last month. A combination of an unnamed tropical storm and Helene within two weeks brought 1-in-a-1000 year rains each time, hammering the state. If you’ve experienced one of these storms, our hearts go out to you.

Rains as severe as Helene’s now occur about once every 70 years due to global warming. Estimating the probability of rare events requires increasingly large amounts of data. NOAA’s Atlas 14 was last updated for the Carolinas in 2006, and those calculations only used data through 2000. Russ Schumacher, state climatologist at Colorado State University, explains the connection between extreme storms and climate change.

“Statistics assume the climate isn’t changing, but we know that it is. Especially in regions near the coasts, the frequency of heavy rainfall has increased as a result of human-caused climate change. Warmer air can hold more moisture, and warmer oceans provide that moisture as the fuel for heavy rainfall.”

A one-in-1000 year storm like Hurricane Helene in one region doesn’t mean it will take another thousand years for a comparable event to reoccur. Instead, for example, in the case of a 1000-year rainfall event, the term actually describes a one in one thousand chance or a 0.1% chance that X amount of rain can fall in X amount of time. It’s a measure of the statistical probability a certain amount of rain can fall in a certain timeframe.

The frequency of heavy rainfall has increased by 10% in the last 50 years, bringing more frequent and severe flooding across the world. What was once considered a once-in-a-1,000-year disaster is now a recurring reality, one that is not going away anytime soon. The common result of such storms today is power outages, coastal and inland flooding, beach erosion, infrastructure and structural damage, and direct human deaths. Importantly, these heavy precipitation events are quickly becoming accepted as a part of life, and with that acceptance comes the reality that what had once been one-in-1000 year storms are misnamed. Their increase in severity and frequency in a continually changing climate stresses everyday life.

But don’t be lured into thinking that you’re necessarily done: these estimates merely state the odds of another one-in-1000 year re-occurrence.  The magnitude and frequency of extreme precipitation events in the early 21st century have already proven to be increasing at a rate more quickly than previously anticipated. Over the past decade, evidence for increasing short-duration rainfall extremes in a warming climate has mounted.

In any given year, there’s a 1% chance of a storm arriving that drops that much rainfall on your area, Mari Tye, a civil engineer who works on resilience at the National Center for Atmospheric Research in Boulder, Colorado, told Bloomberg. The risk also increases the longer you’re in a specific place. So if you wanted to know the likelihood that you’ll someday experience a hundred-year rainfall, Tye said, “you’d be looking at a probability of more like a 1-in-4 chance within the lifetime of your mortgage.”

Experts explain that the increase in storm intensity since the 1980s can be  ascribed to robust variations in atmospheric circulation as well as sea surface temperature rise.

Rainstorms aren’t randomly distributed; they are a result of atmospheric processes like thunderstorms and hurricanes, which are produced by local and regional climate patterns. With so much of the geographic region of the US population likely susceptible to this new common occurrence of extreme precipitation, the time has come to redefine what it meant to experience the current definition of a one-in-1000 year storm. Now more than ever before it is crucial to understand changes in response to hurricane processes to better inform and prepare coastal communities.

How much of the population is affected by the underestimation of this risk in the contiguous US? Are current hydraulic and adaptation structures any longer adequate to manage increased hourly extreme precipitation rates? How fully do communities understanding new extreme weather patterns and the likelihood of short-duration heavy precipitation?

The change in extreme precipitation comparisons against the current national standard for precipitation climatology has been under recent scrutiny. Quantifying the evolving frequency of short-duration rainfall extremes is a challenge for climate change assessments, as the effects of climate change is not generally included in current estimates. Some researchers call for revising the NOAA Precipitation Frequency results as a critical step for addressing flood hazards. Researchers at NOAA have begun the difficult task of updating these precipitation frequency estimates, based on a shifting climate. They’re aiming to issue a new version sometime in 2027. Climate-adjusted national standards are needed to take into account these recent increases that could be used to prevent life and property loss from catastrophic precipitation-driven floods.

Sudden, intense, and unexpected storms like this are no longer rare, says the ocean advocacy group Save the Sound. Instead, “they are becoming the new normal due to climate change.” The group calls acknowledges that “flooding is complex in both causes and effects,” but they call for urgency in creating a resilient infrastructure plan that includes bridges, culverts, and road stream crossings with flooding in mind. “We should be increasing the use of green infrastructure and protecting our region’s forests and marshes that naturally slow and filter floodwaters,” the group argues.

Engineers use estimates of the probability of rare events to design large critical facilities, such as dams, that can withstand the flood that would occur with the worst-case scenario rainfall at their sites. It is time to look at current precipitation standards for designing transportation infrastructure and urban stormwater drainage systems as well, in order to protect the public, property, and municipal infrastructure from the effects of extreme storms.