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.