The huge and deadly flash floods that swept through Texas Friday morning were extremely rare, meteorologists say. But recent weather and climate played a role. Here's a look at what likely contributed and how.
Once-in-1,000-years flood, even for 'flash flood alley'
The devastating floods took place in the Texas Hill Country in the central part of the state nicknamed"flash flood alley" because it has one of the greatest risks of flash floods in the United States, according to the Lower Colorado River Authority, which credits the steep terrain, shallow soil and unusually high rainfall rates.
The flooding took place along both the Colorado River and the Guadalupe River, which runs parallel to it in this part of Texas, about 60 kilometres south.
Floods along the Guadalupe previously killed 10 people in 1987 and 12 people in 1998, though that's far less than the death toll so far of at least 82 in Friday's event.
More than 30 centimetres of rain fell within a few hours early morning Friday. Colorado State University climatologist Russ Schumacher posted on Bluesky that six-hour rainfall totals made this a once-in-1,000-years event. The waters of the Guadalupe River rose eight metres in only 45 minutes before daybreak, washing away homes and vehicles, and rain continued through the weekend.
How heat in the ocean and atmosphere plays a role
Scientists haven't yet done the detailed modelling to see how much of a contribution climate change made to this unusual event, or how much more likely such a flood would be with human-caused climate change — a type of research called an"attribution" study.
However, climate change has led to warmer oceans, which evaporate more water into an atmosphere. In places such as the Gulf of Mexico, where the sea surface temperatures were up to a degree Celsius above average, that fuels storms such as tropical storm Barry. Barry's remnants brought heavy rain into central Texas, where it stalled, dumping water all in one area, Climate Central reported.
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"But that's just not over Texas currently," Shel Winkley, a meteorologist with Climate Central, told The Associated Press. Even more moisture came from areas over the Pacific Ocean to the west.
Warmer air can also hold more moisture.
"In a warming climate we know that the atmosphere has more moisture to give, to hold on to and then to release. But also the thing that we know about climate change is that our rain events are not as uniform as what they used to be," said Winkley."So, you'll get these big rain events happening in localized areas, tapping into the historic level of moisture in the atmosphere."
How drought made things worse
The region hit by the flooding had been in a drought. Climate change is expected to lead to more frequent, intense and longer lasting droughts with climate change, especially in regions such as the U.S. southwest, which has already seen a decrease in annual precipitation since 1900.
Eighty-five per cent of Kerr County was in an "exceptional drought" — the highest level — over the past three months, while the entire county was listed as being in"extreme drought," the second-highest, reports the U.S. Drought Monitor at the University of Nebraska-Lincoln in collaboration with the U.S. government.
That can increase the risk of flooding by killing off vegetation and making soil less able to absorb water.
Brett Anderson, a senior meteorologist at AccuWeather, told The Associated Press the water"just runs right off of it. It's like concrete."
Robert Henson, a meteorologist and writer with Yale Climate Connections, told AP that the combination of many factors, including weather and geography, all contributed."As is often the case with the worst disasters, many things came together in a terrible way."
