Clusters of thunderstorms formed repeatedly over the same area of central Texas on Friday, moving slowly and delivering torrential rain that triggered deadly flash flooding. Some locations saw a month’s worth of rain in only a few hours.
“It’s the prolonged excessive rainfall over one area that makes them so dangerous,” said Emily Heller, a meteorologist with the Austin-San Antonio National Weather Service office.
The catastrophic weather in Kerr County occurred with a steady stream of moisture flowing in from the Gulf and pulling in remnant moisture from a former tropical storm that soaked Mexico’s Yucatán Peninsula earlier in the week.
It was as if a completely saturated sponge overhead was wrung out.
“When there’s a lot of moisture in the air, like there was over the last couple of days, it can initiate these clusters of storms that are pretty small in spatial scale but can have really intense rain rates,” said Russ Schumacher, a professor of atmospheric science at Colorado State University.
Earlier in the week, the forecast for July 4 called for a chance of rain on Thursday and potentially drier weather on Friday. But by early Thursday morning, it had begun to shift, and a chance for thunderstorms with torrential rainfall had entered the forecast.
Early Thursday afternoon, the National Weather Service issued a broad flood watch for parts of south-central Texas, including Kerr County, warning that a slow-moving storm system was expected to bring anything from scattered showers to intense storms through Friday morning.
“It really became apparent early Thursday as models were finally coming into agreement,” Ms. Heller said. “We already had some rain, that was the push to send out the watch.”
The first flash-flood warning, this one for Bandera County, went out just before midnight on Thursday. A watch is a heads-up that conditions are favorable for flooding, while a warning is an order to take immediate action because flooding is expected to occur or already happening.
Just after midnight on Friday, the Weather Prediction Center, an arm of the Weather Service that forecasts heavy rainfall, issued a forecast that clearly showed an extreme rainfall event was expected to occur in central Texas. The agency warned that hourly rainfall rates exceeding two to three inches of rain an hour could lead to significant flash-flooding, especially considering the terrain.
The Hill Country, where the Guadalupe River spilled over its banks on Friday, is often referred to as “flash flood alley” because rainfall gets funneled through the hilly terrain and canyons into the valleys. “This can cause devastating flash flooding in a hurry,” Mr. Schumacher said.
By 1 a.m. Friday, the Weather Service said a very dangerous flash flood event was unfolding in Kerr County and rainfall rates were up to three to four inches an hour with no indication of easing. A flash-flood warning was issued.
“When you’re getting eight or ten inches of rain in a few hours, the soil can’t take up that water, and its going to run off quickly and the rivers come up extremely quickly,” Mr. Schumacher said.
At about 4 a.m., a “particularly dangerous situation” warning came for Kerr County. This rare level of alert is used only in the most extreme weather events.
It identified Hunt, Kerrville and Center Point, all communities along the Guadalupe River, as places of concern: “Residents and campers should SEEK HIGHER GROUND NOW! Life threatening flash flooding along the river is expected.”
The Guadalupe River had risen 22 feet in three hours. “This is a very dangerous and potentially deadly flood wave moving down the river,” the Weather Service said.
Many people were asleep during the predawn storms, and the Weather Service said the warning was set up to set off mobile phone alerts and wake up people in the path of the flooding.
Erin McCann contributed to this story.
Amy Graff is a Times reporter covering weather, wildfires and earthquakes.
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