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.