Why it rained so hard Friday: Ophelia returned to hit NYC with an Ida-level storm

New Yorkers and New Jersyans thought they had seen the last of Tropical Storm Ophelia, but the remainder of the weather event that soaked the tristate region last weekend had other plans.

The remnants of Ophelia returned in force on Friday, triggering a state of emergency that shut down the New York City subway, roads and airport terminals. At a press briefing, city officials described the situation as the wettest day since Hurricane Ida.

During the first 24 hours of Ida, more than 7 inches of rain fell in Central Park. Friday had nearly 6 inches the same period, bringing Ophelia close to 2021’s catastrophic rainfall. Ophelia actually dumped more rain on some parts of the city, data from the National Weather Service shows.

JFK Airport, City Island in the Bronx and Park Slope in Brooklyn were all hit especially hard by Friday’s deluge. The National Weather Service posted on social media that JFK experienced the wettest September Day since Hurricane Donna hit in 1960. New Jersey didn’t see a total repeat of 2021’s downpour, but Hoboken and Bergen County both got more than their fair share of rainfall. The Garden State also declared a state of emergency.

Meteorologists said the city’s newest weather disaster had more in common with Ida than many may realize when it comes to how the deluge was born. Similar to Ida, the forecast had called for this rainfall a few days in advance, and the heaviest predictions locked into place by Thursday night.

Yet NYC schools and transit opened, ultimately stranding students, commuters and tourists once the bulk of the rain arrived. Mayor Eric Adams is once again drawing scorn for his emergency preparedness, and disaster experts said the environmental fallout could linger well after the rain weakens on Saturday.

Ophelia strikes back: Did the forecasts give enough warning?

Prevailing winds in the New York City area generally blow from west to east, pushing storms up the coast and out to the middle of the Atlantic Ocean. But Ophelia’s leftovers parked just offshore after its clouds dissipated Tuesday night.

Late Thursday, this moist air — aching to create more rain — pushed back toward the city where it encountered a separate mass of colder air coming from the West. This cold air acted like a wedge, vaulting Ophelia’s moist leftovers higher into the sky, where they would be more likely to form rain clouds and trigger a cascading downpour. The situation created what's known as an inverted trough, and this atmospheric pattern was similar to what happened when the remnants of Hurricane Ida devastated the tristate area in September 2021, as Gothamist reported at the time.

“The combination is not so different than Ida,” Professor Adam Sobel, who leads Columbia University’s Extreme Weather Initiative, said.

This inverted trough got stuck over the region, causing rain clouds to form repeatedly, like one train after another arriving at a station. The downpours were just “trained” above the greater New York City area over and over as we headed through the daytime into Friday night and even into Saturday, said Lauren Casey, a meteorologist at the research group Climate Central in Princeton, New Jersey. Weather stations and anecdotal accounts reported South Brooklyn getting hit hardest in the morning, while some extra bursts waited until the afternoon to strike parts of Queens.

Tropical storm systems are historically known to be flash flood makers when they encounter other weather fronts, through a process known as frontogenesis. But human-driven climate change has added extra moisture to the atmosphere, worsening what would have been manageable storms in the past.

"As we warm our atmosphere, it's capable of holding more moisture — so about 4% [more] for every one degree Fahrenheit,” Casey said, citing research by Climate Central. “In the Northeast, we've seen a 13% increase in the average rainfall intensity since 1970.”

Sobel said rain forecasts have improved tremendously over the past 20 to 30 years, and new advances involving artificial intelligence are under development to offer better resolution toward pinpointing which neighborhoods might experience the heaviest rainfalls during a storm. But he said communicating the broad risks as early as possible is something officials can already do now.

In the Northeast, we've seen a 13% increase in the average rainfall intensity since 1970

Lauren Casey, meteorologist at the research group Climate Central in Princeton, New Jersey.

“It's possible to educate the public that the right way to handle situations of uncertainty is to be prepared even when there's a risk that the preparation might not have been necessary after the fact,” Sobel said.

Casey added the city would benefit from replacing asphalt and pavement with green spaces or infrastructure designed to sop up rain before it causes flash floods, such as the city’s multi-billion Cloudburst initiative.

Sewage overflow watch

While the initial flooding begins to dissipate, the city is still in store for some unhealthy after-effects.

The New York City Department of Environmental Protection has issued an advisory for most of the metropolitan area’s waterways due to the potential for sewage runoff. Public documents state that NYC’s sewer system can only handle about 1.75 inches of rain an hour. Weather stations in Central Park and parts of Queens recorded 2 inches of hourly rainfall around 10 a.m Friday morning, while Brooklyn’s Park Slope area experienced closer to 3 inches.

During heavy rainfall, flooding can push large amounts of water and contaminants into waterways. Flooding can also overwhelm sewers and wastewater treatment facilities. The resulting overflow can cause untreated polluted water to flow directly into rivers and creeks.

Pollutants such as oils, sewage, fertilizer and street litter can find their way into local lakes and shores, which can be harmful to wildlife and contaminate drinking water. Swimming and boating in these areas can be risky if humans come in contact with the polluted water.

The DEP’s advisories are based on real-time rainfall measurements at 14 wastewater treatment plants and models of how this precipitation influences water quality across local bodies of water. When heavy rain and flooding occurs, the city issues warnings to residents to be cautious when using lakes, canals and rivers.

Some waterbody advisories will remain in place for the next 80 hours or so.

As of Friday afternoon, 39 of the 45 waterways were under advisories — with the highest risk levels at Paerdegat Basin, Spring Creek, Flushing Creek, Coney Island Creek and Alley Creek. These warnings will remain in place for all of these locations for the next 80 hours or so, as well as parts of the East River and Newtown Creek, which is a toxic Superfund site. The Gowanus Canal, another of the city’s Superfund sites, is under an advisory for the next 40 to 55 hours.

Direct exposure to sewage or flood water can cause an infection in eyes, skin, stomach or intestines. The city advises anything exposed to these contaminants such as drinking water, food or even clothing should be thrown out if it can’t be disinfected or dried.

The flooding of water bodies can also have a significant impact on nutrient load, which is when runoff releases a large amount of organic matter from land into local waterways.

“This can result in short term (hours to days) of hypoxia and fish kills with broader impacts to ecosystem health,” said Dr. Edwin Cowen, director of the DeFrees Hydraulics Laboratory at Cornell University.

A deluge can make conditions ripe for harmful algae blooms, which can make it difficult for freshwater treatment facilities.

“The treatment process itself can lead to hazardous by products if algae is in the water,” Cowen said. “[This is] usually avoided by initial filtration, but absent that filtrating you have potential problems.”

What FloodNet is detecting

New York City has a system of 72 flood sensors attached about 10 feet from the ground to sign posts called FloodNet. These sensors record water depths and flood risks in real time, even while it's pouring out, by sending an ultrasonic pulse to the ground. The project is run by researchers from NYU and Brooklyn College.

One early flood hotspot was located on Carroll St. and 4th Avenue in Gowanus, which peaked at just over three feet Friday morning according to the sensor.

“The worst of it has been in the Gowanus neighborhood – some of it is stormwater driven and some of it is coastal flooding,” said Brett Branco, a professor of earth and environmental sciences at Brooklyn College and a lead scientist for FloodNet. “We’ve seen photos people have shared on social media from this location.”

A full moon made the high tide Friday morning worse than normal, which deepened coastal flooding in neighborhoods around Jamaica Bay. Hamilton Beach, located on the Jamaica Bay peninsula, had 15 inches of water.

In flood-prone Williamsburg, the sensor at Lee Avenue and Middleton Street had a reading of nearly two feet, so far.

To check the reading of your nearest sensor, the data is available online.

FloodNet received a $7.2 million grant in February to expand its system of sensors to 500 in the next five years, across the five boroughs.

How over-soaked soil contributes to flash floods

Ophelia’s first pass through New York City and New Jersey last weekend may have exacerbated flash floods on Friday due to saturated soil.

Healthy soil can be like a super sponge when it comes to absorbing rain and holding it. Typically, soil is filled with tiny pores. When it rains, these small voids fill with water, and gravity will help push the water down even further into the soil. Once this soil exceeds its capacity, its draining power diminishes, and flash flooding is possible.

When it reaches this point, Mark Bradford, soils expert at the Yale School for the Environment, said: “Stay off the soil.”

“When it gets this wet, the soil gets incredibly mushy,” Bradford said. “You can damage it [the soil]. Treat it gently, and hold off any yard work or major projects to keep it healthy.”

Much of the soil in New York City is considered less healthy compared to what you might find in a forest or farm according to Bradford. Urban soil has been compacted, so it has fewer pores and less capacity to retain water — that includes most city backyards and some park areas that have experienced erosion as a result of use. Eroded soil is characterized by having fewer roots in the ground as a result of having less vegetation and less trees. Trees and vegetation contribute to healthy soils.

“Soil slows down the rain,” Bradford said. “Most of the soils are exceeding capacity because they’re not properly managed so they don't give us protection against flooding.”