How a confluence of extreme weather, geography and timing created Texas’ flood disaster

While existing weather models can forecast flash flooding in advance, even the best models struggle to represent internal storm structure and to predict where, within a few miles, the hardest rainfall will strike.

In this case, the off-the-charts colors on Friday morning indicated that the south fork of the Guadalupe River was taking a direct and prolonged hit.

Then, instead of moving on, the storms stalled.

Texas state climatologist John Nielsen-Gammon said that the thunderstorms hovered above the Texas Hill Country river, dumping 10-12 inches of rainfall in about six hours. The series of storms “perfectly aligned with the South Guadalupe River Basin,” he said. The area is prone to floods and was filled with campers near the river’s edge. If the storm had been even five miles in another direction it would not have produced as much destruction, he said.

It’s difficult to know exactly how much rain fell. The basin that flooded does not have a rain gauge despite being in an area covered by the TexMesonet monitoring system, which was created after a flash flood disaster that struck Wimberley, Texas, over Memorial Day weekend in 2015. But Friday’s downpour was a record-setting for that location and a storm seen once every 1,000 years, Nielsen-Gammon said.

While National Weather Service forecasters had warned broadly about flash flooding, meteorologists and forecasting experts said the best weather models could not predict precisely where the most intense rainfall would fall, or that the deluge would stall out over a flood-prone basin.

“Even the most detailed weather forecasting models at this point are just barely capable of resolving individual convective storms,” Nielsen-Gammon said, adding that it would be “next to impossible” to predict well in advance whether successive storms would train over the area, stall out and produce intense flooding in such a confined geography.