Why achieve this many historically rare storms hit the Carolinas? The geographical location endangers these states, and climate change is putting pressure on the situation

Hurricane Helene caused deadly and destructive floods because it swept through the Southeast from September 26 to 29, 2024. Across much of western North Carolina, where The worst floods occurredThe amount of precipitation exceeded the common expected amount just once every 1,000 years.

But it wasn't the primary 1,000-year rainstorm to hit North Carolina this 12 months. An unnamed, slow-moving storm in mid-September produced greater than a foot of rainfall closer to the Atlantic coast. This storm flooded areas had already been drenched by Tropical Storm Debby in August.

As Atmospheric scientist And State climatologistsWe imagine it is crucial that the general public understands the danger that extreme events may occur. This is very true as climate change alters the conditions that produce and promote storms. Here's how scientists calculate storm probabilities and why events like a 1,000-year storm can occur way more steadily in some places than that term suggests.

Predicting the long run based on the past

Estimates of rainfall return periods – how long, on average, it should elapse between storms of a given size – come from the US National Oceanic and Atmospheric Administrationthe house of the National Weather Service. NOAA publishes these forecasts in a series of reports entitled Atlas 14. Architects and engineers use them to design buildings, dams, bridges and other structures to face up to heavy rainfall.

The estimates use past rainfall data to calculate how steadily rainfall of various sizes occurs in specific locations. In locations where historical rainfall observations have been collected over a long time, it is feasible to calculate with very high reliability the quantity of rainfall that’s exceeded on average a couple of times per 12 months.

Experts then use statistical methods to estimate how steadily larger amounts of rain would occur. The larger the amounts, the less precise the calculations develop into. Nevertheless, it is feasible to reasonably estimate very rare rain events.

The results are average probabilities that a given location will receive a given amount of rain in a given 12 months. If the probability that a storm that produces 6 inches (15 centimeters) of rain in 24 hours occurs in a 12 months is 1%, we’d expect such a storm to occur once in 100 years, so its return period 100 is years. An event will be assumed to have a 0.1% likelihood of occurring in a given 12 months on average once in 1,000 years, hence it is named a 1,000-year event.

It's not “one and done”

The problem with terms like 100-year event or 1,000-year event is that many individuals hear them and assume that they mean that one other storm of that size shouldn't occur for an additional 99 or 999 years. This is an affordable conclusion, however it is incorrect. Every storm is a person event. Just because one becomes unusually large doesn't mean one other storm a 12 months later can't even be more likely.

Imagine you’re rolling the dice. The probability of hitting a pair of sixes is slim – close 1 of 36or barely lower than 3%. However, should you roll the dice again, the chances don’t change – they’re the identical for that roll as they were for the previous one.

A more accurate technique to communicate storm probability is to think concerning the annual exceedance probability – the probability that a rainstorm of a given size could occur in a single 12 months. The probability of a 1,000-year storm occurring in any 12 months is 0.1% and the probability of it occurring again the subsequent 12 months and the 12 months after which can be equal.

Since the US is a big country, we should always expect a 0.1% likelihood of a series of rainfall events every year. The probability of such a storm occurring in any given location is incredibly low, however the probability of it occurring anywhere is way higher.

In other words, even should you are unlikely to experience a 1,000-year storm where you’re, there are more likely to be 1,000-year storms somewhere within the country yearly.

Different areas experience several types of storms

In the true world, actual rainstorms are usually not randomly distributed; They are the results of atmospheric processes corresponding to thunderstorms and hurricanes brought on by local and regional climate patterns. So a map of actual 1,000-year rainstorms would show clusters reflecting hurricanes along the East Coast, atmospheric rivers along the West Coast, and thunderstorm complexes within the Great Plains where thunderstorm systems form.

Types of storms are vital because they last for various lengths of time. Almost all rare one-hour extreme rainfall events are related to thunderstorms, while those lasting 48 or 72 hours are sometimes brought on by hurricanes or their remnants.

North Carolina and South Carolina, that are steadily hit by hurricanes and tropical storms, have experienced quite a few extreme rainfall events in recent times. This includes Record rainfall in October 2015 in South Carolina; Hurricane Matthew in 2016; Hurricane Florence in 2018; the nameless storm mentioned above in September 2024; and now Hurricane Helene.

In fact, since 2002, the three U.S. storms which have caused 1,000-year rainfall in the most important areas have all hit the Carolinas: the October 2015 storm, Florence and Helene.

Loading the weather cubes

Why have so many storms struck the Carolinas in only a number of years, which historically and statistically ought to be extremely rare? There are two predominant reasons which can be related to one another.

First, estimating the probability of rare events requires increasingly large amounts of information. NOAA's Atlas 14 for the Carolinas was last updated in 2006, and only data through the 12 months 2000 were used for these calculations.

As more storms occur and more data is collected, estimates develop into more robust. Given that reliable measurements of precipitation only return about 100 years, the true likelihood of that much rain falling within the Carolinas is can have been underestimated until now.

Second, these statistics assume that the climate is just not changing, but we realize it is. The frequency of heavy rain is increasing, especially in coastal regions increased resulting from human-caused climate change. Warmer air can hold more moisture, and warmer oceans provide that moisture as fuel for heavy rainfall.

As a result, climate change is causing rainfall that was previously extremely rare somewhat more likely. Although the Carolinas have been particularly unlucky in recent times, the chances are also tipped toward heavier rainfall – a trend that poses significant challenges to emergency preparedness and recovery.

NOAA is currently evolving Atlas 15which can update current estimates with newer data and bear in mind the impacts of climate change. The agency also plans to modernize its estimates of an equivalent amount probable maximum precipitationThis is an estimate of the worst rainfall a location could experience.

Engineers use these estimates to design large critical facilities corresponding to dams that may withstand the flooding that might end in worst-case rainfall at their sites. North Carolina has developed its own version of Atlas 15 due to the urgent have to plan transportation infrastructure to handle more events like Florence and Helene.

These updates provide information that will be used for higher planning and decision making. However, extreme rainfall will proceed to pose a serious threat and have significant impacts on many U.S. communities.