Living With Climate Change: What makes flash floods so dangerous? Here’s how to protect yourself.

Plus, this summer, the early arrival of an expected El Niño ocean weather phenomenon is adding to heat and other weather extremes, which can mean more water in places.

Read: Here comes El Niño: It’s early, likely to be sloppy and add even more heat to a warming world

Over the weekend, several people were missing after intense thunderstorms dumped record amounts of rain across a wide swath of Canada’s Atlantic-coast province of Nova Scotia, causing flash flooding, road washouts and power outages.

And dangerous flash floods were responsible for at least six deaths in Bucks County, Pa., in recent weeks. In that area, the search for a missing baby resumed this week after his toddler sister’s body was found. Pennsylvania flooding followed similarly dangerous conditions in New York and Vermont earlier in July.

China, India and other parts of the globe are dealing with their own flooding extremes.

Read: Record monsoon rains have killed more than 100 people in northern India this week

Read: Homeowners can check a property’s flood, heat and wildfire risk for free with this high-powered app

Flooding, like most disasters, involves a number of competing factors that may impact frequency and intensity, sometimes in opposing ways. Climate change, which is worsening extreme rainfall in many storms, is an increasingly important part of the mix, scientists say.

How does a flash flood differ?

According to the National Weather Service, a flash flood is a flood caused by heavy or excessive rainfall in a short period, generally less than six hours. Flash floods are usually characterized by raging torrents after heavy rains that rip through river beds, urban streets or mountain canyons, sweeping along everything in their path. They can occur within minutes or a few hours of excessive rainfall.

One of the biggest differences between flash floods and generalized flooding is that flash floods can be more dangerous than people think. Even a few inches of water can have a current that knocks people down and just a few feet of water can send a car floating. Flash flooding at night is particularly dangerous, because unknown depths are disguised, looking simply like wet surfaces.

Flash floods can also occur even if no rain has fallen, such as after a levee or dam has failed, or after a sudden release of water by a debris or ice jam. What’s more, the inability of some municipal storm sewers to keep up with the water removal can aggravate the situation, and that’s been especially true as the nation’s infrastructure has aged.

Flash flooding isn’t unheard of in the Southwest U.S. during monsoon season, when a seasonal change in winds draws moisture into the area and can lead to more showers and thunderstorms. But exceptionally dry years mean that the bare ground can also be as hard as concrete in some places, allowing for restricted absorption.

Related: Flash floods, like in Las Vegas, are deadlier than hurricanes, tornadoes or lightning

Sometimes both flash flooding and ongoing rising waters from rivers and creeks can devastate an area at the same time. In 2017, after the very dangerous and expensive Hurricane Harvey swamped coastal areas, it stalled around southern Texas for days as a weakening hurricane that brought catastrophic flash and river flooding, the NWS archives say.

And during 2021’s Hurricane Ida, the NWS office in New York declared its first-ever set of flash flood emergencies in the region, an alert level that is reserved for “exceedingly rare situations when a severe threat to human life and catastrophic damage from a flash flood is happening or will happen soon,” officials said at the time.

Flash flooding is expected to increase

Most worrying, perhaps, is that flash flooding is expected to increase as there are more extreme precipitation events brought on by climate change and man-made atmospheric warming, meteorologists and climate scientists say. This is because warmer temperatures increase evaporation, which puts more moisture into the atmosphere that then gets released as rain or snowfall.

As global warming continues to exacerbate sea level rise and extreme weather, our nation’s floodplains are expected to grow by approximately 45% by century’s end.

Already, inland U.S. flooding risk increasingly factors into interactive warning maps and real estate and insurance decisions, as homeowners, business operators and politicians grapple with all the ways climate change should change how we talk about and prepare for weather extremes, including the frequency of events and extended seasons. Because flood insurance is handled through the federal government’s Federal Emergency Management Agency, and not individual private policies, there’s often more to understand about paying for damages when you are impacted. 

Planning and engineering trade groups say cities must dedicate more of their infrastructure budget to fortifying storm sewers. And cities might think of leaning on nature to fight nature.

The Natural Resources Defense Council argues that populated areas, especially, must think of permeable pavement and rainwater harvesting (instead of simply allowing for too-slow runoff), green roofs, rain gardens and additional tree planting. 

Here’s more from the NRDC.

For now, the Red Cross has some tips for flood and flash flood safety, such as getting out of your vehicle sooner versus later, and moving to higher ground, if you get caught on a flooded road.

Read: ‘I started just connecting the dots’: Iowa TV meteorologist hounded off the air over climate-change reporting

See additional coverage on the MarketWatch Living With Climate Change page.

The Associated Press contributed.