Cities and towns across the northeast turned into lakes on Wednesday night as Tropical Depression Ida wrought a 1-in-200 year flood across an incredibly wide area.
The deluge of historic proportions swept across some of the largest cities in the U.S., including New York where at least nine are dead. Record-setting rainfall prompted the U.S. National Weather Service to issue its first-ever flash flood emergency for New York last night. Though the skies have cleared, some danger remains as rivers continue to climb toward flood stages.
Hurricane Ida was already notorious and due to be retired for the widespread damage it caused in Louisiana. But the storm persisted as it lumbered northward, and left a trail of destruction from southwest Virginia to New York. The storm “re-energised” as it passed over the region, and unleashed further rainfall at rates that stormwater infrastructure simply wasn’t built to handle.
Hourly rainfall records were set at Central Park and Newark Airport; more than 8 centimetres of rain fell at both locations in the span of an hour, and other locations were blitzed as bands of rain crawled over the area. Rainfall totals for the storm maxed out at nearly 23 centimetres in Staten Island with a wide swath of locations receiving at least 15 centimetres.
Sewers were choked by the blitz and pumps that normally keep the New York subway dry were unable to keep up with the rush of water, leading to surreal pictures of subway stations being flooded and roadways becoming rivers.
The sudden onset, huge geographical scope, and severity of the rains left many unprepared. Disaster management agencies across the region reported numerous water rescues of people stranded. Still more terrifying videos showed people riding city buses with knee-deep water inside. It bears repeating that you should never drive into floodwaters. The majority of flash flood deaths occur from people on roadways, and there’s a reason the NWS has coined the phrase “turn around, don’t drown.”
Data from the NWS shows that an area from the DelMarVa border to Connecticut that spans more than 24o kilometres in length experienced what’s considered 1-in-200-year. Put another way, there’s a 0.5% chance of this much rain falling in six hours in a given year. In short, this is unprecedented — at least in the climate of yesteryear.
Climate change has made heavy rainfall more common and intense around the world due to the basic fact that a hotter atmosphere can hold more water. There’s even a simple equation for it that shows for every 1 degree Celsius of additional heat, air can hold 7% more water. When more water goes up into the atmosphere via evaporation, more can come down as dangerous rain.
But the Northeast has been particularly susceptible to this trend. The most recent National Climate Assessment shows the region has seen an astounding 71% increase in heavy precipitation since the 1950s, by far the biggest leap of any region in the country. What we saw on Wednesday night is a prime example of that in a summer of seemingly endless examples. Heavy rains have also fallen in areas as far-flung as Detroit, Tennessee, China, Europe, and India, all with deadly results.
It shows our infrastructure to deal with stormwater is in need of a major upgrade. The mess in the Northeast is still ongoing (as is Ida’s impact on Louisiana, which shouldn’t get lost in this new tragedy). More than 160,000 are without power this morning in Pennsylvania, New York, and Connecticut, and rivers are still rising across the area. That means a soggy and dangerous day is still ahead even as people try to pick up the pieces.