In a month, 5 natural disasters occurred in the United States, which should occur once every 1000 years: it will only get worse

As climate change increases the frequency and intensity of heavy rainfall in the US, the National Oceanic and Atmospheric Administration (NOAA) may soon update its thresholds for what constitutes a record-breaking downpour event, reports FoxWeather.

Photo: IStock

On July 26, St. Louis received 24 cm of rain in 23 hours, breaking an all-time record with over 30 cm of rain in some suburbs.

Two days later, dozens of people died in eastern Kentucky when more than 25 cm of rain fell, causing rivers to rise 60 cm in a few hours to levels well above historical records and wash houses off their banks.

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In early August, another heavy downpour hit southeast Illinois, radar estimates of 25-33 cm of rain falling in just a few hours. That same week, normally dry Death Valley received a year's worth of rain in three hours, inundating the region with flood waters that blocked all roads inside the national park, some of which sustained moderate to severe damage.

Just this week, the Dallas-Fort Worth Metroplex was hit by extensive flooding as 7 cm of rain fell in an hour, part of a series of thunderstorms that brought 30 cm of rain to Dallas-Fort Worth International Airport in 24 hours. East Dallas suburbs report more than 38 cm of rainfall.

All five extreme events reached the local threshold, which was classified by the National Weather Service as a "1 in 1000 year" shower event, but they all occurred within one month. Last year, the deadly flooding that hit New York and New Jersey from the remnants of Hurricane Ida and the catastrophic flooding in west-central Tennessee were also classified as extremely rare 1 in 1000 year rainfall.

What does rain fall "once every 1 years" mean?

The term "rainfall or flood with a period of 1 in 1000 years" may sound like an event that is predicted to occur only once in a thousand years, and that by surviving such an event, you expect to remain safe until 3022.

This is not entirely true. Instead, the term is given to an event that forecasters think can only happen 0,1% in any given year - a 1 in 1000 chance. A 0,5% chance is considered a "1 in 500 year" event, and a 1% chance - the event "1 in 100 years" and so on.

The National Oceanic and Atmospheric Administration's Office of Water Forecasting monitors historical rainfall events throughout the country to classify how much precipitation must fall in certain periods (such as an hour, a few hours, or days) relative to their climate averages to achieve each probability. For example, it took much less rain in Death Valley (just under 3,8 cm) than in St. Louis (over 22 cm) to reach the 0,1% probability threshold.

Because so many of what are thought to be extremely rare events happen with much greater frequency, you might wonder if the calculations are wrong.

According to a study by the US Global Change Research Program, climate change is increasing the amount of heavy rainfall in the US, both in intensity and frequency, especially in the Northeast and Midwest.

Warmer ocean temperatures increase evaporation and increase the amount of water vapor available. Meanwhile, warmer air can hold more water vapor. As the Earth's temperature rises with climate change, storms will have access to even more moisture when heavy rains occur, according to the USGCRP.

Nine of the 10 years with the most one-day rainfall between 1910 and 2020 have occurred since 1996, according to the U.S. Environmental Protection Agency. The EPA added that "the incidence of extreme one-day precipitation remained fairly stable between 1910 and the 1980s, but has increased substantially since then."

NOAA may soon adjust criteria to account for global warming

The Hydrometeorological Design Research Center of the Office of Water Forecasting has been researching and updating rainfall frequency thresholds across the country for many years and has created interactive maps to allow users to calculate the frequency of specific rainfall in regions where the study is completed.

However, these maps will most likely be updated in the coming years.

The recently passed Infrastructure Act includes funding to allow NOAA to further analyze any changes and trends in observed precipitation due to climate change and include them in a national update of precipitation frequency estimates. This research is already underway.

Record flooding is not the same as record summer rain

Because these extreme and rare rainfall and floods made headlines, NOAA emphasized that 1 in 1000 year rainfall is not the same as 1 in 1000 year floods.

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An extreme rainfall that meets these criteria may not result in a 1000-year record flood, just as a 1000-year record flood may not result in a record-breaking summer rain. Thus, according to NOAA, these terms should not be used interchangeably.

NOAA is responsible for determining the probability of extreme precipitation, and the USGS is responsible for determining such probability when it comes to flooding.

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