Hurricane Ida Flooding in New York: What Happened on 1 September 2021

Originally published 2021-09-04.

On the evening of 1 September 2021, the remnants of Hurricane Ida dumped record-breaking rain on New York City and northern New Jersey. Central Park measured 3.15 inches (80 mm) of rain in a single hour, the most ever recorded there, and the National Weather Service issued the first flash flood emergency in New York City's history. Water poured into subway stations and basement apartments. Early reports counted at least 13 dead; the final toll in the Northeast was far higher, and most of the New York City victims drowned in flooded basement homes.

This page sets out what happened that night, why a storm that had already weakened over land proved so deadly, and what changed afterwards.

From Louisiana to New York

Ida made landfall near Port Fourchon, Louisiana, on 29 August 2021, exactly 16 years after Hurricane Katrina, as a powerful Category 4 hurricane with sustained winds of 150 mph (240 km/h). It caused severe wind damage and long power cuts across southeast Louisiana before weakening as it moved inland.

By the time it reached the Northeast three days later, Ida was no longer a hurricane. But its leftover circulation was still carrying vast amounts of tropical moisture, and it merged with a frontal system. That combination wrung out extreme rain in a narrow band over Pennsylvania, New Jersey and New York. Overall, Ida killed more than 100 people and caused around $75 billion in damage, making it one of the costliest tropical cyclones on record.

The night of 1 September

The heaviest rain fell after dark. Between 8:51 and 9:51 p.m., Central Park logged 3.15 inches, breaking an hourly record that had been set less than two weeks earlier during Tropical Storm Henri. LaGuardia Airport had its wettest day on record at 6.80 inches (173 mm), and Newark also broke a decades-old rainfall record.

The National Weather Service office for New York first issued a flash flood emergency for northeastern New Jersey, and about an hour later the first one ever for New York City. Mayor Bill de Blasio declared a state of emergency that night, describing it as a historic weather event, and banned non-essential road travel until 5 a.m. New York's governor Kathy Hochul and New Jersey's governor Phil Murphy declared states of emergency for their states.

I’m declaring a state of emergency in New York City tonight.

We’re enduring an historic weather event tonight with record breaking rain across the city, brutal flooding and dangerous conditions on our roads.

— Mayor Bill de Blasio (@NYCMayor) September 2, 2021

Across the river, an EF3 tornado with winds near 150 mph tore through Mullica Hill in southern New Jersey, flattening homes. It was one of several tornadoes spawned by the system.

Subways, roads and airports underwater

Videos shared that night showed water cascading down subway stairs and flooding platforms. Seventeen trains were trapped and service was suspended until the next morning; everyone on board was evacuated without loss of life, but repairs to the subway cost around $75 million. Large parts of NJ Transit, the Long Island Rail Road, Metro-North and Amtrak also stopped running.

Flash floods hit New York City as the remnants of Hurricane Ida battered the region with rain and subway stations were flooded. pic.twitter.com/lOnXGtsVJ1

— USA TODAY (@USATODAY) September 2, 2021

Major roads turned into rivers, and drivers abandoned cars. At Newark Liberty International Airport, terminals flooded and all departures were grounded, with more than 400 flights cancelled.

Residents described how fast the water rose. One New Yorker told the BBC that water began bubbling up through his shower drain during dinner, and by the time he had checked the utility room the living room was under almost a foot of water.

The human toll and the basement problem

The first count, on the morning of 2 September, was at least 13 dead across New York and New Jersey. Among the early reported victims was a man in his 70s whose car was swept away in Passaic, New Jersey, and a two-year-old boy in Queens. The toll rose sharply over the following days.

In the end, 13 people died in New York City, 11 of them in flooded basement apartments, including a family of three in Woodside, Queens. New York State recorded 18 deaths in total, New Jersey around 30 and Pennsylvania 5, with further deaths in Connecticut and Maryland. Many of the New Jersey victims died in vehicles caught by rising water.

The basement deaths exposed a long-standing housing problem. New York has tens of thousands of basement and cellar units, many of them illegal conversions rented at lower prices to low-income and immigrant families. City inspectors later found that five of the six flooded basement homes they investigated were illegally converted. These spaces often have a single exit and no way to escape when water pours in from the street or up through drains.

Why the flooding was so severe

Several factors came together that night:

• Rain rate, not just total. Much of New York's sewer network was reportedly built to handle about 1.75 inches of rain an hour. Ida delivered far more than that, so water had nowhere to go.
• Hard surfaces. Paved streets and roofs send rain straight into sewers instead of letting it soak into the ground.
• Warning gaps. Many people did not grasp that a weakened hurricane far from the coast could be deadly. Forecasters had warned of heavy rain, but some officials, including Rep. Alexandria Ocasio-Cortez, said the region needed more advance notice and clearer alerts.
• A warming climate. Warmer air holds more moisture, and scientists say this makes the most intense downpours heavier. Two record hourly totals in New York within a fortnight fit that pattern.

What changed afterwards

President Biden approved a major disaster declaration for New York on 6 September. Hochul and de Blasio set aside $27 million for undocumented residents who could not receive federal FEMA aid. The city created an Extreme Weather Response Task Force and a Climate Driven Rain Response Plan, promising earlier travel restrictions, door-to-door warnings for basement residents, phone alerts and faster evacuations when flash flooding threatens.

Total damage estimates ran to $7.5–9 billion in New York and $8–10 billion in New Jersey. Ida showed that inland flash flooding, not only coastal storm surge, can be the deadliest part of a hurricane.

Practical flood safety

• If you live in a basement unit, know a second way out and move upstairs when a flash flood warning is issued.
• Never drive or walk into floodwater; a few inches of moving water can knock you down, and about a foot can float a car.
• Sign up for local emergency alerts (in New York, Notify NYC) and take "flash flood emergency" messages seriously.

Sources: Wikipedia: Effects of Hurricane Ida in the Northeastern United States; Wikipedia: Hurricane Ida. Original reporting: BBC News; embedded posts by Mayor Bill de Blasio and USA TODAY.