Catastrophic Flooding in NYC, Climate Change & Health

The deadly deluge of flooding rain yesterday in New York City was catastrophic in its scope of impacts. Preliminary storm totals from the remnants of what was once Hurricane Ida, according to the National Weather Service, include a whopping 7.19” in Central Park. 

The latter part of August was already an exceptionally rainy period. Less than two weeks earlier another tropical system, Henri, saturated the city with substantial downpours. On August 22nd, between the hours of 10 and 11 pm, 1.94” of rain was observed at Central Park. At the time, Henri shattered the one-hour rainfall rate record. But for New Yorkers, Ida’s wrath proved to be much worse when it hit on the first day of September.

By the time the remnants of the former Category Four Hurricane IDA advanced on New York, its winds were not the primary threat. It was the relentless rain of the storm that prompted the National Weather Service to issue a rare “Flash Flood Emergency” for New York City. 

A “Flash Flood Warning” may sound more familiar. It means dangerous flooding is happening now or will be soon. Flash Flood Emergencies are exceedingly rare and are only issued by the National Weather Service when a “severe threat to human life and catastrophic damage from a flash flood is happening or will happen soon.” This was the first Flash Flood Emergency ever issued for New York City.

Last night’s storm was particularly dangerous not just because of how much total rain fell - but also how fast the rate at which the water came. Remarkably, between 8:51 pm to 9:51 pm, Central Park observed 3.15 inches of rain, the greatest amount in a single hour ever observed in the city, since record-keeping began. 

This level of rain intensity spells disaster for urban areas, where there is little grass and vegetation to soak floodwaters into the earth. In the case of New York City - more than 70 percent of its landmass is covered by impervious surfaces - from parking lots to streets, sidewalks to rooftops. 

The same thick, grey asphalt and concrete - notorious for holding on to the sun’s heat on sticky summer days has the reverse reaction to water. Rather than absorbing the rainwater, it repels it. Preventing the rain from penetrating its hard surfaces. City streets are designed that way. The objective is to direct water off the roads and into drainage systems to keep traffic running smoothly and pedestrians safe. 

But the aging infrastructure of New York, combined with an extraordinary amount of rain rapidly falling at record rates, resulted in urban roads turning into treacherous rivers. In New York City, which has the greatest population density of any major city in the U.S.- over 27,000 people per square mile, this extreme weather event was high impact, destructive, and deadly. 

While a Flash Flood Emergency in itself is rare, evidence points to climate change bringing more intense rainstorms and super-charged tropical systems in the future. “The number of weather, climate, and water extremes are increasing and will become more frequent and severe in many parts of the world as a result of climate change,” says World Meteorological Secretary-General Prof. Petteri Taalas, in a recent report released in conjunction with the UN Office for Disaster Risk Reduction.

What’s the connection between warmer air and more intense rain? As Climate Signals reports, “for each degree 1°C (1.8°F) of global warming, saturated air contains 7 percent more moisture." The increase in atmospheric moisture content raises the risk of extreme precipitation events.

These climate-related events not only destroy property and disrupt transportation they also can lead to human health threats - some that you may not be aware of. 

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