Why it's so hard to warn people about flash floods

The Verge

a day ago

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.
A river gauge along the Guadalupe River at Hunt in Texas showed how fast water levels rose to deadly levels. The flow of water climbed from 8 cubic feet per second at 1:10AM to 120,000 cubic feet per second at 4:35AM — just before the gauge failed from the inundation. 'Essentially, at 1:10 am the river was a tranquil almost dry riverbed, and by 4:30 am it was a raging flood with more water flowing than the average flow over Niagara Falls,' meteorologist Alan Gerard wrote in his Balanced Weather blog on July 5th.
Prominent scientists have defended the NWS forecasts following claims from some local officials that they didn't have enough advance notice. Forecasters can warn of excessive rainfall days in advance, but pinpointing precise locations for flash floods requires real-time observations that only allow for hours of notice at most.
The last hurdle is to get these messages in front of people
The NWS issued a flood watch Thursday at 1:18PM to notify people that heavy rainfall could cause flash flooding across portions of eight counties in south-central Texas. A 'watch' is an early alert indicating that flooding 'is possible.' About 12 hours later, at 1:14AM, the NWS escalated its message, issuing a flash flood warning, which is typically sent out when a flash flood is imminent or already taking place. Thunderstorms were creating 'life threatening flash flooding,' the warning said.
The last hurdle is to get these messages in front of people, which Vagasky and other experts call 'the last mile.' The catastrophe in Texas arrived in the dead of night when many people were already asleep, making it more difficult to get these warnings out to them.
Getting past that last mile also seems to have gotten tricker recently. X has become a less reliable source of vetted information. And the San Antonio Office of the NWS that played a key role in forecasting was missing a 'warning coordination meteorologist' after DOGE cuts to the agency.
'All forecasts and warnings were issued in a timely manner. Additionally, these offices were able to provide decision support services to local partners, including those in the emergency management community,' a spokesperson for the NWS said in an email to The Verge.
Experts The Verge spoke to emphasized how crucial it will be to continue gathering the robust datasets needed to forecast flash floods. The Trump administration's proposed budget for NWS' parent agency for the 2026 fiscal year would shutter laboratories and research programs vital to flash flood forecasting, scientists warn. A global Flash Flood Guidance System that helped other countries develop their own warning systems lost funding when DOGE dismantled USAID.
The Trump administration has also dismissed scientists working on a new national assessment of how climate change affects the US. Climate change intensified the heavy rain that led to deadly flash floods in central Texas on July 4th, according to a preliminary study completed by the ClimaMeter project funded by the European Union and the French National Centre for Scientific Research. In a warmer environment, more water can evaporate and then get wrung out in thunderstorms, Vagasky explains.
'It's absolutely important to rethink how we communicate early warning systems,' says Mireia Ginesta, a research associate at the University of Oxford and one of the authors of the ClimaMeter study. 'People should take this more seriously and there absolutely shouldn't be cuts in funding for research.'