How weather conditions set the stage for deadly Texas flash floods

San Francisco Chronicle​

14 hours ago

WASHINGTON (AP) — Incredible amounts of moisture in the air fueled a storm that barely moved over central Texas, creating conditions for fatal flash floods in the early morning hours when it was easy to catch people by surprise, meteorologists said.
More than 12 inches (30 centimeters) of rain fell in the Texas Hill Country over a span of several hours early Friday, causing water levels to quickly rise, according to the weather forecasting company AccuWeather. The area is naturally prone to flash flooding, but this was an especially bad storm that hit during the vulnerable overnight hours, experts said. More than two-dozen deaths have been reported along with numerous rescues.
Meteorologists said that an atmosphere warmed by human-caused climate change can hold more moisture and allow bad storms to dump more rain, though it's hard to connect specific storms to a warming planet so soon after they occur.
'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 Shel Winkley, a meteorologist with Climate Central. 'So, you'll get these big rain events happening in localized areas, tapping into the historic level of moisture in the atmosphere.'
In recent weeks, flash flooding driven by bursts of heavy rain turned deadly elsewhere in Texas and in West Virginia. In San Antonio in June, more than 7 inches (18 centimeters) of rain fell over a span of hours, prompting dozens of rescues from the fast-rising floodwaters and killing at least 13. And in West Virginia that month, at least nine people died when as much as 4 inches (10 centimeters) of rain fell within 40 minutes and caused flash flooding in the Wheeling area.
Staggering rain runs off hard ground
Robert Henson, a meteorologist and writer with Yale Climate Connections, said this latest Texas rain storm was roughly a once-in-a-generation event. It fell in the Texas Hill Country where water quickly shoots down rugged hills into narrow river basins that swell quickly.
'As is often the case with the worst disasters, many things came together in a terrible way.' Henson said.
Plus, the area had been in a drought, so the water ran down the dry, hard land fast. That made it more dangerous for children attending camp.
'A sudden surge of rain like that is going to have a harder time getting absorbed,' said Brett Anderson, a senior meteorologist at AccuWeather. 'It just runs right off of it. It's like concrete.'
The sheer amount of rain was overwhelming. Former NOAA chief scientist Ryan Maue, a private meteorologist, calculated Saturday morning that the storm had dropped 120 billion gallons of water on Kerr County, which received the brunt of the storm.
A storm with plenty of fuel
Moisture fueled the storm from many directions. Tropical Storm Barry formed briefly last weekend, moving over Mexico and then its remnants continued up into Texas. But the jet stream, a current of air that moves weather patterns, wasn't there to push that moisture away.
'Normally weather systems and the remnants of tropical systems will get picked up by the jet stream, and that's just not over Texas currently,' said Winkley, the Climate Central meteorologist. 'It's essentially a weather system without a road to get away from the Lone Star State.'
The warm water of the Gulf fueled the moist atmosphere. Even more moisture came from areas over the Pacific Ocean to the west. The combination gave the storm plenty of fuel once it got started.
Winkley said in the area that flooded, climate change might mean that rain storms are less common, but when they do occur they can be more severe.
Generally a warmer atmosphere can hold more moisture, creating conditions for storms to drop more rain.
'With climate change we have a warming atmosphere. A warmer atmosphere holds a lot more moisture, and we are seeing obviously much more total atmospheric moisture across the globe in recent years than we normally have,' said Anderson, the AccuWeather meteorologist.