Over the weekend, nearly 10 million people across the Southwest U.S. were under flood watches, even amid what scientists are calling the worst drought to hit the western part of the country in 1,200 years. The deadly combination meant flash flooding hit parts of Texas, Utah, and New Mexico, stranding hikers in Zion National Park and Carlsbad Caverns National Park and flooding homes and roads in Dallas.

But What Exactly Is a Flash Flood? 

A flash flood is characterized by torrents of water that tear through river beds, mountain canyons, and city streets, sweeping clean their path, according to the National Weather Service. They occur within minutes or hours of a short period of sustained and heavy rainfall, typically lasting six hours or less. In some cases, flash flooding may result from other causes like the failure of a levee or dam.
And while it may seem unlikely, drought conditions, like those affecting the Southwest, can exacerbate flooding. During extremely dry conditions, topsoil hardens, making it difficult for it to absorb high levels of precipitation, the Washington Post reported, and while rain can temporarily mitigate extra-dry conditions, it’ll take more than a few days of intense rain to put a stop to the so-called megadrought. According to the lead author of a study, published in February in Nature Climate Change, it could take multiple high-precipitation years to repair the effects of the drought lasting two decades.
Just as climate change has exacerbated these conditions, it is also making flooding more frequent and intense, according to the United Nations Environment Programme. Higher temperatures increase the odds of extreme weather events, cloud formation, and precipitation, bumping up the risk of flooding, especially when combined with changes in the topography like the removal of vegetation.
All this means that more extreme flooding can be expected, especially in places built on flood plains or where the ground cover has been altered. And in places where flooding has already occurred, incidences of flooding are expected to happen more frequently, in some cases with disastrous consequences. A study, published in August in Science Advances, for example, states that climate change has doubled the risk of a megaflooding event in California that could displace millions of people and wreak $1 trillion worth of damage.

How Can You Safely Navigate Flash Flooding?

Although prepping for a California megaflood isn’t something individuals can likely help with, there are some best practices experts recommend for keeping safe as the risk of flooding rises. 
The Bureau of Land Management (BLM) recommends checking the weather in advance when visiting national parks. Zion National Park, where a woman is still missing after hikers were washed away by a sudden deluge over the weekend, is particularly vulnerable to flash flooding. BLM authorities recommend avoiding the park’s picturesque slot canyons carved from rushing water in the past if there is any amount of rain in the region. They also recommend hiking with buddies, hiking at or below your skill level, and alerting fellow explorers of your planned course and return time.
When encountering flash flooding outside of a national park environment, the National Oceanic and Atmospheric Administration (NOAA) advises drivers "turn around, don’t drown." The saying was devised as a national campaign in response to vehicular deaths during flash flood conditions. According to NOAA, nearly 100 people drown every year in floods, a majority due to attempts by drivers to drive through waterlogged areas. According to NOAA, drivers can lose control in as little as six inches of water, and 18 to 24 inches will sweep a car off the road.