is a senior science reporter covering energy and the environment with more than a decade of experience. She is also the host of Hell or High Water: When Disaster Hits Home , a podcast from Vox Media and Audible Originals.
By definition, flash floods are notoriously difficult to warn people about well in advance. They form rapidly, giving forecasters hours of lead time at best to figure out where they might hit with specificity. We’ve seen this with devastating effect in Texas, where flash floods over Independence Day weekend killed over 100 people — many of them children and families who were in bed when officials issued emergency warnings.
Issuing warnings requires a whole lot of weather and water data. Foreseeing how much rain is likely to fall, and then figuring out the flow of that water on land, are both complicated tasks. Climate change adds another risk factor. Meanwhile, the Trump administration’s cuts to federal climate research and weather forecasting could make an already challenging process more precarious in the future.
What makes a “flash” flood? The National Weather Service (NWS) defines it as flooding that starts within six hours of heavy rainfall or another sudden trigger like a dam or levee break. Storms are usually the culprit. And predicting the amount of water that’s going to fall out of the sky — called quantitative precipitation forecasting — is something that scientists are still working on.
“Getting those very precise measurements at those very precise locations is something that we’re still working on”
The shape of a cloud, where water accumulates in the cloud, and how dry the air is between the cloud and the ground in different locations, are all factors that might influence how much rain hits the ground in a certain location, according to Chris Vagasky, a meteorologist and manager of the Wisconsin Environmental Mesonet at the University of Wisconsin-Madison. The size of raindrops falling, along with wind speed and direction, are also contributing factors. For all these reasons, it’s very hard to say exactly how many inches of rain to expect in a specific location.
“Getting those very precise measurements at those very precise locations is something that we’re still working on, improving that science,” Vagasky says. Progress hinges on more advanced computer modeling and a better understanding of how precipitation forms in clouds.
Flash flood forecasting also involves charting the flow of that water on land, which poses its own set of challenges. The type of terrain it falls on — whether it’s sandy or rocky soil or an urban area with a lot of impervious concrete that prevents the ground from soaking up water, for instance — makes a difference. So does the geography of the land and the size of the watershed into which the rain falls. A watershed or drainage basin is a defined area where rainfall and snowmelt generally flow into the same bodies of water. Where there are canyons and hills, even a relatively small amount of rain over a wide-enough area could lead to flash flooding if all the water is funnelled into the same river. The July 4th flooding took place in the hill country of Texas, in an area around an inactive fault zone called Balcones Escarpment that’s dubbed “flash flood alley” because of the heightened risk here.
Forecasts also have to consider the delay time between when the rainfall is most intense over a particular drainage basin and when peak flooding occurs wherever the water converges. The response time is shorter if the basin is small, steep, or highly urbanized, says Claudio Meier, a water resources engineer and associate professor at the University of Memphis.
“[That] means that from the moment that you’re seeing all this rain falling to the moment you get the big flood, you only get a few tens of minutes to a couple of hours. So that’s very little time to warn people or do anything about it,” Meier says.
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