New analysis shows the rainiest days are often getting rainier

·6 min read

Cities across the contiguous U.S. saw record rainfall during 2021. A new analysis shows that the heaviest rainfall events in a city each year are increasingly seeing more rainfall. (Climate Central)

The heaviest single-day rainfall events of each year have had an increase in precipitation over the past 70 years, especially for locations in the Gulf Coast and mid-Atlantic, according to a recent study by Climate Central, a consortium of independent scientists and journalists based in Princeton, New Jersey.

In an analysis of 246 locations across the contiguous U.S., 72%, (178) of the locations studied by Climate Central saw an increase in the amount of rain that fell on their annual wettest day since 1950. Cities along the Gulf Coast and mid-Atlantic experienced the greatest increase by volume, according to the analysis, with Houston seeing an extra 2.8 inches, an 83% increase in rainfall, on its annual wettest day compared to its wettest day in 1950.

Heavy single-day rainfall events can be -- and have been -- caused by hurricanes, atmospheric rivers and slow-moving thunderstorms. The heaviest rainfall event in Houston during 2017 occurred on Aug. 27 amid Hurricane Harvey's landfall. More recently, the atmospheric river that trailed into Northern California during October of this year delivered more than 5 inches of rain in one day in Sacramento -- the most rainfall the city has seen in a single day between 1950 and 2021.

Dating further back, the extreme rainfall from a single day in Atlantic City during 1997 serves as an example of how thunderstorms can potentially cause an extreme rainfall event. The city recorded 11.10 inches -- nearly 5 inches higher than the city's second-largest single-day rainfall event since 1950. Jen Brady, a senior data analyst at Climate Central, said that this event was unrelated to tropical activity.

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Taking this into account, the data suggest that events such as these are increasingly delivering more rain across the U.S. each year, in turn increasing the risk of flooding.

"We expect that the type of high-intensity event we saw across the country this year will increase," Brady said. "And the intensity of these events are magnitudes above what would be experienced in what you would traditionally think of as a heavy rainstorm."

Flooding is one of the costliest weather-related hazards in the nation, having caused $48 billion in damages from 2016 through 2020, according to the National Oceanic and Atmospheric Administration's (NOAA) list of $1 billion disasters. Climate Central estimates that precipitation extremes are responsible for more than one-third of flood damages. The aftermath of flood events is magnified in poor and rural communities of color due to outdated infrastructure and discriminatory housing and relief policies, according to Climate Central.

The analysis points to climate change, specifically a warming atmosphere, as a reason for the increase in heavy rainfall. With every 1-degree Fahrenheit increase in temperature, the atmosphere is able to hold about 4% more moisture, which can lead to more frequent and intense, heavy rainfall events. Past studies project that the frequency and intensity of these heavy rainfall events will increase with additional warming over the century.

Climate scientists have also pointed to how the warming atmosphere plays a role in fueling hurricanes. As more moisture is available in the atmosphere to feed the storms, they in turn are able to dump more rainfall over the areas they cross, increasing the risk of flooding.

"Hurricanes ... and tropical systems in general ... are very efficient rain producers," Brady told AccuWeather via email. "So, many rainfall records can be tied to tropical systems. And because a warmer atmosphere and warmer water work together to evaporate more water into the storm ... more is available for rainfall."

Hurricane Ida delivered a devastating blow to Louisiana in late August 2021 with Category 4-strength winds, and the rain it delivered days later as a tropical rainstorm proved disastrous for the Northeast, claiming more than 50 lives. While the impact of Ida in the Northeast is not specifically linked to climate change in the Climate Central study, AccuWeather forecasters drew a link to climate change as a contributing factor in the development of a phenomenon known as a marine heat wave, which helped fuel some of the major rain events in the New York City area this year.

To date, the 7.13 inches of rainfall that Ida dumped over New York City on Wednesday, Sept. 1, has been the wettest day the city has recorded this year, serving as the city's 2021 heaviest rainfall event. Despite that staggering rainfall total, Sept. 1, 2021, is not the rainiest day in Big Apple history. In fact, two days in the city's history were rainier.

April 15, 2007, stands as the rainiest day in New York City history, thanks to a major nor'easter. On that day, weather officials measured a whopping 7.57 inches of rain. And about 30 years earlier, on Nov. 7-8, 1977, the record 24-hour rainfall total was set when 7.40 inches fell in New York City, according to a report at the time by The New York Times. The storm's total rainfall over a 36-hour period amounted to an astonishing 9.19 inches the Times reported, citing National Weather Service (NWS) measurements.

Workers pump water from a flooded section of Interstate 676 in Philadelphia Friday, Sept. 3, 2021, in the aftermath of downpours and high winds from the remnants of Hurricane Ida that hit the area. The cleanup and mourning continued as the Northeast U.S. recovers from record-breaking rainfall from the remnants of Hurricane Ida. (AP Photo/Matt Rourke)

While a majority of the cities studied reported an increase in intense rainfall totals, others such as Washington, D.C., and Chicago had relatively little change over the years, with the nation's capital even seeing a slight decrease in record rainfall days over the years. Little Rock, Arkansas, Dayton, Ohio, and Erie, Pennsylvania, also recorded a slight decrease in record rainfall days.

The warming can lead to both more intense periods of rainfall and drought, like what has been gripping places across the American Southwest, as high temperatures lead to greater evaporation.

Thus far, 2021 has been a record-breaking year for extreme rainfall events from coast to coast. Of the 2,568 stations evaluated across the contiguous U.S., 15% of the wettest days of 2021 ranked in the top-10 annual wettest days on record for those locations. Among these locations, Sacramento, Newark, New Jersey, and La Crosse, Wisconsin, all experienced their wettest days since 1950 as of Oct. 31.

The increase in rainfall also looks different for each region. In the West, Sacramento had a record-setting 5.4 inches of rainfall on Oct. 24 as an atmospheric river doused the Northwest. The measurement accounted for 48% of the city's year-to-date rainfall. Meanwhile, in the Northeast, Newark measured a record-setting 8.4 inches of rainfall on Sept. 1, which accounted for only 16% of the city's year-to-date rainfall.

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