Last fall, the 12 students in the Jackson School of Geosciences' GEO 347G "Climate System Modeling" class set out to understand something that hit close to home: what were the climatological factors that made the July 4, 2025 rainstorm in Central Texas so severe? What they discovered was that the storm — which caused catastrophic flooding that killed at least 139 people — very well could have been worse.
In a paper published recently in Geophysical Research Letters , Research Scientist Edward Vizy and Professor Kerry Cook analyzed the class's work and discovered that above-average sea surface temperatures weakened an air current called the Great Plains low-level jet, thus weakening this enormous rainstorm.
Had sea surface temperatures in the Gulf of Mexico been at the 40-year average for this week in early July, the researchers estimate that rainfall totals could have been 5-8% higher over Central Texas. (Vizy said it would take further work to quantify how exactly this would have impacted flood levels.)
Instead, warmer-than-average sea surface temperatures reduced the contrast between land and ocean temperatures, and this slowed down the jet. The Great Plains low-level jet is a fast-moving current of air that curves from the Gulf of Mexico over Texas, the Great Plains, and eastern United States. As the jet stream's winds hit the higher terrain of the Texas Hill Country, they are forced upward, setting in motion a process that can cause storms to develop and intensify. A weaker low-level jet means weaker storms and less intense rainfall.
There are many factors that contribute to how a storm develops. Vizy and Cook concentrated on the storm's connection to sea surface temperatures and soil moisture because it can inform the prediction of future storms.
"Sea surface temperatures in the Gulf of Mexico, soil moisture — these surface conditions are more persistent than the storm itself. So it gives you a leg up on predictability," Cook said. "We say, 'there's memory there.' That's what can help with lead times."
The models the GEO 347G class and the researchers ran are very similar to models that forecasters run for weather prediction. Cook said that forecasters and climate scientists at the Austin/San Antonio office for the National Weather Service will see the results from this study; her hope is that it will help them better understand the role of the Great Plains low-level jet in storm development when predicting future storms.
Soil moisture also contributed to the development of the storm; soils in the region were wet following Tropical Storm Barry, an on-the-ground condition that can enhance storm precipitation totals. Vizy and Cook found that soil moisture not only acted as a source of moisture for storm development, it also influenced the circulation and the low-level jet strength, which affected the storm rainfall totals.
For the first few weeks of the semester, the students in GEO 347G worked to model the storm exactly as it occurred, working to correctly simulate how, when and where the rain fell in this storm, as compared with satellite observations, radar data and ground conditions. This initial step is not a simple one. For instance, a small spinning circulation in the thunderstorm, known as a mesoscale convective vortex, formed within the larger storm. For this weather event to have been properly simulated in the models, that vortex needed to develop over the Hill Country at the correct time.
The class then made a series of changes to the models' surface conditions, changing sea surface temperatures and soil moisture to their average values to see how the storm would change. These perturbation simulations, as they're called, were run on the high-performance supercomputers at the Texas Advanced Computing Center, which completed them in a matter of hours. This access to TACC is what made it possible for the class to present on their findings in the span of one semester, Vizy said.
Having 12 students be able to complete all these runs added to the robustness of the final study, said Vizy. And according to Elizabeth Chapa, a student in the class, being able to create a climate model that helped illuminate how the devastating storm worked made the research particularly meaningful to the students, too.
"(This storm) directly affected everyone in this class," Chapa said. "The scientific question was so specific to us and to Texans, so I think that makes it even more special that we got to contribute to this specific paper on this specific topic."
