This is why there’s been so much extreme rainfall and flooding in the U.S.
This is why there’s been so much extreme rainfall and flooding in the U.S.
In the week and a half since floods killed at least 130 people in Central Texas, heavy rains have filled New York subway stations, submerged busy roadways and drowned two people in a vehicle in New Jersey. Storms have stranded Washington-area motorists. Deadly torrents returned to a flood- and fire-scarred region of New Mexico. And meanwhile, flooding rains have yet to relent in Texas.
Downpours and thunderstorms are a familiar signal of summer — but this one has been different. Barely halfway through, it’s already been a hyperactive season of often fatal flooding, with more than twice as many floods as usual so far this July, according to National Weather Service reports.
While there are varying meteorological forces behind this month’s extreme rainfall, what has connected them all is significant amounts of atmospheric moisture pulsing above the country.
It is flowing from abnormally warm oceans across the Northern Hemisphere that are likely to stretch elevated flood risks into August, data shows — perhaps into record territory. The conditions are allowing plumes of tropical moisture to stretch into middle latitudes and stagnate there, sending flood risks surging and exemplifying a critical consequence of rising global temperatures that researchers have been predicting and tracking for decades.
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“Global warming changes the odds,” said Kevin Trenberth, a distinguished scholar with the National Center for Atmospheric Research who has published on the topic of increasing atmospheric water vapor since the 1990s.
Additional flooding rainfall is likely this week in areas that have already been hit hard, including in Texas on Tuesday, the Midwest and the East through the rest of the week, and along the Gulf Coast from a possible tropical storm starting Thursday.
The drivers of the recent extreme rainfall include tropical storm remnants and frontal systems — but definitely not cloud seeding, a method of weather modification that conspiracy theorists have falsely blamed for the deadly Texas floods but that cannot produce the amount of rainfall and energy that recent storms have carried.
The atmospheric moisture fueling the rain is flowing from oceans that have been unusually warm near the United States, Europe and eastern Asia, causing global subtropical ocean temperatures to reach record highs.
But what happens in the ocean doesn’t stay in the ocean.
Evaporation from the warm waters is sending unusually high levels of atmospheric water vapor across wide swaths of land in populated parts of the Northern Hemisphere, including the central and eastern United States.
Water vapor is fuel for storms. When vapor condenses into clouds, it releases heat energy, causing storms to intensify. Weather systems that have more of this fuel can produce more rainfall.
As the planet warms, that fuel is building up in the skies. For every 1 degree Celsius (1.8 degrees Fahrenheit) of warming, air is capable of holding 7 percent more moisture. Human-caused fossil fuel emissions and the greenhouse effect have caused average global temperatures to rise close to 1.5 degrees Celsius since the Industrial Revolution.
Trenberth said the trend has been too often ignored as high ocean heat content drives increasing atmospheric moisture.
There is still a randomness to the cases where all of that moisture is translating into pouring rain. But as extreme rainfall becomes more frequent and widespread, the odds of encountering extreme precipitation are rising, he said.
“You can be lucky,” Trenberth said, “or not.”
An unusually high number of floods
Flooding rainfall is a normal part of summer weather in the United States, but the number of floods reported so far during July is the highest in more than a decade.
There have been 1,203 reports of flooding across the country, more than double the average of 563.
The rate of flooding events has been most unusually high in New Mexico, Virginia, North Carolina, Pennsylvania and Texas.
In North Carolina, for example, July 6 flooding from the remnants of Tropical Depression Chantal inundated communities around Chapel Hill and Durham, prompting dozens of high-water rescues and displacing dozens of residents.
Two days later, floods in Ruidoso, New Mexico, killed three people after more than 3 inches of rain fell in 90 minutes. The town’s recent history of wildfires — and past floods — means there isn’t enough vegetation to help such an intense burst of rainwater percolate into soil.
The same day as the Ruidoso floods, parts of Chicago received more than 5 inches of rain within 90 minutes, a rate that local Weather Service meteorologists called “staggeringly high.” The downpour flooded viaducts and basements.
In the meantime, most of the nation’s attention was focused on Texas, where more rivers continued to flood over the weekend as storms kept wringing tropical moisture from the air. Flooding on the Guadalupe River, where July 4 floodwaters tore through summer camps and RV parks, returned Sunday, forcing a pause in ongoing efforts to search for more than 100 people who remained missing.
Now, New York and New Jersey are the latest to experience such intense rainfall. New York City officials said that, as severe as Monday’s storm was, it was nothing the metropolis had not experienced repeatedly in recent years.
“It is now the case that five of the most intense rainstorms in New York City’s history have taken place in the last four years,” Rohit Aggarwala, commissioner of the city’s Department of Environmental Protection, told reporters Tuesday.
More than 2 inches of rain fell within an hour at Central Park, the second-highest hourly observation on record there. But unlike the city’s two other most intense rainfalls on record, this one was not tied to a tropical storm, said Matthew Wunsch, a meteorologist at the Weather Service’s New York City forecasting office. When the remnants of Hurricane Ida hit the city in 2021, causing deadly floods in basement apartments, 3.15 inches fell in an hour. That same year, remnants of Hurricane Henri dropped 1.94 inches of rain in an hour.
For a 15-minute stretch Monday, the rainfall rate surpassed 4 inches per hour, Aggarwala said, citing New York state’s mesonet weather observation system.
“This is a level of rain that we never expected over the 400 years we’ve been here,” Aggarwala said.
In places like New York, there’s a continuing effort to adapt to the risks the extreme moisture poses. Monday’s storm caused significant sewer system overflows and sent water erupting from inside one Manhattan subway station, city leaders said, reiterating what has become a refrain after major flood events: The city’s infrastructure was simply not designed for so much rain falling so fast.
“It’s as if New York City moved 500 miles south, and so we have an infrastructure that was designed for an environment we no longer live in,” Aggarwala said.
“The rule books seem to have changed drastically,” Mayor Eric Adams added.
A moisture-laden atmosphere
Globally, atmospheric moisture levels have been steadily rising over the decades, with 2024 setting a record.
While 2025 is running a notch below last year, there have been pockets of enhanced moisture near the most unusually warm ocean regions — including eastern Asia, Europe, and the central and eastern United States.
This excessive moisture has frequently been blown from the western Atlantic Ocean into the country before being harnessed by a stormier-than-usual weather pattern.
Excess fuel for heavy downpours shows no signs of abating over the next two weeks, with additional flooding events likely.
Only a handful of Western states have not reported any flooding this month.
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