As a succession of thunderstorms fed by the remnants of Tropical Storm Barry pummeled Texas' Hill Country, tools used by the National Oceanic and Atmospheric Administration to detect extreme rainfall began "maxing out the color charts." The forecasting models — a flash flooding guidance system calledFLASHand a program called theMulti-Radar/Multi-Sensor Systemthat detects heavy precipitation — use a cascading series of colors to communicate the severity of rainfall and flood risk, said David Gagne, a National Center for Atmospheric Research scientist who is focused on using machine-learning to improve weather models. The floods that battered Texas' Hill Country and have killedmore than 100 peoplewere the result of a confluence of factors related to storm dynamics and the local topography. Ultimately, they culminated in what Gagne called a "worst-case scenario." "All the ingredients came together at the wrong place, at the wrong time, at night on a holiday weekend," Gagne said. "This was at the top end of the scale." 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 disasterthat 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 modelscould 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. Gagne reviewed the modeling tools available to NWS forecasters. The most urgent warnings went out, he said, when the two NOAA tools showed rainfall rates beginning to spike. That gave people just a few hours to flee, assuming they received the warnings through cellphone alerts, weather radio broadcasts or by other means.Some residents likely didn't get the alertand Kerr County does not have a siren. Texas leads the nation in flood deaths. The state counted 1,069 flooding deaths from 1959-2019,according to research published by Hatim Sharif, a professor of civil and environmental engineering at the University of Texas at San Antonio. That's 376 more deaths than the next closest state over that time period — Louisiana. During the six-decade period of study, the National Weather Service's San Antonio/Austin area forecasting office counted the second-highest number of flooding-related deaths in the country. Many of those took place in the Hill Country, which some call "flash flood alley." The region is created by the Balcones Escarpment, a fault zone of weather-beaten rock from Waco to Del Rio that divides the state and can set off and trap severe storms. "The Hill Country is basically the first terrain obstacle that moist air from the Gulf experiences as it moves farther north. That provides extra lifting and allows thunderstorms to trigger across southern Hill Country," Nielsen-Gammon said. Sharif said steep hills, narrow canyons carved out of limestone, rapidly funnel water from smaller creeks into larger ones and then into swollen rivers. In many areas, there's only a few inches of shallow soil on top of bedrock. Narrow creeks are etched into bedrock. "It doesn't absorb a lot of water," Sharif said. "It doesn't take a lot of time to flood those creeks … you have this dissected area that moves water efficiently to the main stream." In just over 12 hours on Friday morning, the Guadalupe Riverrose about 20 feet, according to a river gauge in Hunt, turning a mellow stream into fast-flowing rapids.
