The weather records for the New York City metropolitan area fell almost as quickly as the rain Wednesday night.
The National Weather Service issued its first flash flood emergency ever for the city, and in Central Park, 3.2 inches of rain fell in an hour, setting a record. Newark, New Jersey, matched it, getting 3.2 inches of rain in an hour.
Wednesday was the wettest day on record in Newark, with total rainfall of 8.4 inches. New York City’s 7.1 inches of rain was its fifth wettest day. Both cities experienced 1-in-500-year rainfall events. The result was one of the deadliest and most destructive flash flood events to hit the tri-state area, with at least 41 people dead by Thursday evening.
The storm was forecast days in advance; the New York office of the National Weather Service issued a flash flood watch as early as Monday. But the intense rainfall still seemed to catch many off-guard, underscoring just how difficult it can be to predict the most dangerous aspects of climate change-fueled storms.
“It wasn’t that it was 6 inches in a day, but most of that fell in a couple of hours,” said Bob Henson, a meteorologist and writer for Yale Climate Connections, an online news service. “That was what really drove the flash aspect of the flooding and what caused the really rapid water rise.”
Even when forecasts predict extreme rainfall, it can be hard for people to grasp just how much water can fall in a short time.
“It can be difficult to visualize what it means when we say ‘life-threatening flash floods,’” Henson said. “Some folks hear ‘this is the remnants of a hurricane’ and think: It’s no big deal. It’s just the leftovers.”
The dangerous disconnect may become even more problematic as climate change supercharges storms and hurricanes. While the frequency of storms is not expected to increase in a warming world, research has shown that climate change is intensifying storms when they do occur — and that can often manifest itself in deluges of rain.
Nine of the top 10 years for extreme one-day precipitation events have been since 1996, according to the Environmental Protection Agency’s heavy precipitation tracker.
Climate change is making storms wetter because a warmer atmosphere can hold more moisture. Scientists have estimated that for every 1 degree Celsius of temperature rise, the atmosphere can hold 7 percent more evaporated moisture. The Northeast is especially susceptible, being the region with the greatest increase in heavy rain events since the 1970s.
Global warming is amplifying the risk of flooding. Storms such as Hurricane Harvey, which dropped up to 60 inches of rain over parts of Texas in 2017, and Ida, when it made landfall in Louisiana on Sunday as a hurricane and as it moved up into the Northeast, show how dire the consequences can be — particularly in cities.
“Rain flows more quickly on pavement than across grass, so runoff can allow water to pool much more readily in an urban landscape than, say, across a meadow,” Henson said. “That’s why urban flash flooding is such a threat.”
The days leading up to the storm highlight what the system got right. Forecast models were predicting high rain totals across parts of the mid-Atlantic and the Northeast five days before the storm hit and offered escalating warnings after that. Two days before the storm, flash flood watches went up for 70 million people, and the day before, forecasters pulled the trigger on a “high” risk for flash flooding.
Considerable to life-threatening flash and urban flooding and significant river flooding were mentioned for the mid-Atlantic and the Northeast on Wednesday. The information was in the National Hurricane Center’s discussion for Hurricane Ida published Monday. That type of designation is reserved for the highest echelon of forecast flood events, issued only about 16 days a years. The flooding on those days accounts for 35 percent of flood deaths and 86 percent of flood-related damage.
As the event unfolded, forecasters raised their alarms, issuing flash flood warnings and flash flood emergencies warning of life-threatening, destructive and deadly flash flooding and imploring people to seek higher ground and not to drive through flooded roadways. For millions, the warnings lasted for hours well into the night.
Another factor in the severity of the flooding is that many parts of the mid-Atlantic and the Northeast have had an exceptionally wet summer, meaning soil is saturated and the risk of flooding is even greater.
The growing frequency of violent storms — which many weather and climate scientists warn will persist — is pushing meteorologists to figure out how to better communicate the risks. As extreme weather continues to exceed the expectations of even the best forecasters, Henson said, scientists will most likely have to adjust how they communicate the threats and how members of the public interpret them.
“As with everything else, I would encourage people to have a balanced weather diet,” Henson said. “I would encourage people to go beyond icons on an app and dig into what the National Weather Service is saying and what local experts are saying.
“Weather apps might be great to know how hot it’s going to be on a bright, clear day but not so great at knowing that a catastrophic flood is coming in 12 hours.”