Flash flooding: Why this potentially dangerous weather is happening more often on Long Island

In a 24-hour span starting August 18, 2024, as much as 10 inches of rain fell on parts of Long Island, prompting National Weather Service meteorologists to broadcast a rare flash flood emergency. 

Less urgent advisories were sounded for storms in recent weeks, including one on July 29, when more than 2 inches of rain was logged at Long Island MacArthur Airport, the most for the day since the start of recordkeeping there in 1963. Anticipating that storm, which impacted parts of the state from the Catskills to Long Island's East End, Gov. Kathy Hochul warned New Yorkers to “stay vigilant, stay informed, and use caution” in the face of possible flash flooding.

Flash flooding is caused by heavy rainfall in a short period. It's often caused by thunderstorms in the summer, when there's more moisture in the air than in cooler months. It can happen anywhere across the region, as opposed to storm surge flooding, which mostly impacts low-lying coastal areas.

Flash floods killed more than 130 people in Texas' Hill County last year and 33 people in Kentucky, including nearly a dozen who were drowned when their vehicles were swept away, in 1997. The Kentucky flooding destroyed 14,000 homes and caused $500 million in damage, according to the National Weather Service.

It appears to be happening more frequently than in the past, said Jase Bernhardt, associate professor, director of sustainability studies in the Department of Geology, Environment and Sustainability at Hofstra University.

Climate scientists say the changing climate is producing more extreme rainfall events, which can create more flash flooding. In the northeastern United States between 1958 and 2018, extreme precipitation events — defined as those with the top 1% of daily precipitation accumulations — increased by about 60%, the biggest jump of any region, according to the Fifth National Climate Assessment.

Any change in the frequency of flash flooding on Long Island is also a function of the way water moves on the ground and ultimately of urbanization,  Bernhardt said. “Poor drainage, sewer systems" and impervious surfaces like "parkways, parking lots" and other types of development all play a role.

When does flash flooding happen?

MitigateNY, New York State government’s online hazard planning platform, defines a flash flood as one characterized by its “swift onset and high intensity,” which generally occurs within six hours of rainfall.

National Weather Service meteorologists use hourly rainfall and other parameters to assess flash flood risk. Generally, 1 to 3 inches of rain per hour may create nuisance flooding; 2 to 4 may create dangerous flooding impacting roads and other infrastructure; and 3 to 6 inches may create very rare, perilous flooding of areas that don’t normally flood.

How often does it happen on Long Island?

Since 2015, the National Oceanic and Atmospheric Administration has logged 197 flash flood events on Long Island, including nine in 2025, the last year for which data was available. The full list dates to 1996 and includes multiple storms — like one in August 2024 that dumped record rainfall across the region — that generated multiple flash flood events. The events caused no injuries or deaths but combined to cause tens of millions of dollars in property damage. 

Total annual flash flood warnings issued by the National Weather Service’s New York forecast office, covering not just Long Island but the larger metropolitan area, provide a more reliable if less granular way of tracking flash floods over time, Bernhardt said.

According to non-official data logged by Iowa State University and analyzed by Newsday, since 2001 the number of yearly warnings has ranged from 13 to 67, averaging 37. To date this year, there have already been 37.

"The last five, six years has been the most active period that I can remember, at least in the near-term," said Nelson Vaz, the weather service's warning coordination meteorologist. "We have seen an uptick in these extreme rainfall events." 

New York's Climate Impacts Assessment puts the current average annual days of rainfall above 1 inch at 14. 

How does Long Island geography affect risk?

First, the good news: The sandy soil covering much of Long Island tends to absorb rain, and the Island has few major river systems that are especially vulnerable to flash flooding, Vaz said. Also, for at least some of the year, relatively cool ocean waters surrounding the Island dampen storm intensity, though that effect is probably at its weakest by late summer, Bernhardt said.

Now the bad: Parts of the North Shore are hilly, producing rapid runoff that can lead to flash floods. Most of the South Shore is low-lying and vulnerable for a different reason: If it rains near high tide, gravity-fed storm drains may not be able to absorb the runoff, especially if they are impeded by leaves or garbage. 

The August 2024 storm, which led to significant flooding around Stony Brook and the Nissequogue River in northern Suffolk, showed the Island's vulnerability, Vaz said. "We were fortunate to not have something more dangerous. We did see ponds overflow, we saw the dam break at Mill Pond, we saw water rush out to the bay areas. There were homes in that area. It could have been really devastating." 

Where else are we vulnerable?

New York City's recent history has shown that impervious cover — roads, parking lots, rooftops and the like — magnifies flood risk for people living in low-lying areas, especially for people living in basement apartments. 

At a minimum, tens of thousands of city residents, and perhaps many more, live in basement apartments, according to city officials. Many of the city's apartments are unregulated, leaving tenants without legal and safety protections. Most of the 14 people who died from causes related to remnants of Hurricane Ida in 2021 drowned in unregulated basement apartments. Two people also died in basement flooding after a rainstorm last year.

Ian Wilder, executive director of Long Island Housing Services in Bohemia, said there are likely thousands if not tens of thousands of basement apartments on Long Island, though hard numbers are impossible to find because the apartments are illegal in many places.

A 2016 paper from the National Center for Suburban Studies at Hofstra University estimated 14,500 to 16,000 legal accessory dwelling units on Long Island along with 90,000 to 100,000 illegal apartments, though it did not break out basement apartments. 

What are the other risks on Long Island?

The Island has other points of vulnerability. Tens of thousands of buildings worth billions of dollars sit on flood plains, according to MitigateNY, which notes that Nassau is home to one of the largest senior citizen populations in the country, with a number of nursing facilities in flood zones.

Bernhardt said the region’s roads were an area of particular concern. More than half of all flood-related drownings occur when a vehicle is driven into floodwater, according to the Centers for Disease Control and Prevention. “Long Island has plenty of roads and plenty of motorists, parkways that go through low-lying areas,” he said. “That’s definitely a danger.”

Can local infrastructure keep up with heavy rains?

Robert Traver, a professor of Civil and Environmental Engineering at Villanova University who has served on the American Society of Civil Engineers expert review panel on flooding issues, said in an interview that engineers designing storm drains were grappling with similar problems around extreme precipitation events. "Duration and intensities are getting higher, which means it's harder for collection systems to move the water out, which is causing more flooding," he said. "If you have a system built 20 years ago, it's not built to what we're experiencing today." 

How can you manage your risk?

Find out if you live in a flood zone and, regardless of where you live, ensure that government alerts are enabled on your cellphone, if you own one, Vaz said. Weather service meteorologists will use cell networks to broadcast emergency alerts when flash flooding risks reach the "considerable" level, potentially closing major roads and threatening lives, he said. They will not broadcast an alert if the risk is "base" level, which might entail isolated rescues and basement flooding, he said. 

Signing up for local emergency alerts through your town or county is also a good idea, he said. 

If you’re outside during heavy rain, avoid flood-prone areas like stream beds, drainage ditches and culverts. If you’re in a vehicle, follow guidance from public safety officials. 

The weather service also advises motorists not to drive into water of unknown depth or try to pass an area where water is running over the road. If your vehicle stalls, abandon it and seek higher ground.