One person died and another remains missing after heavy rainfall triggered floods in the San Antonio area on Thursday.

The unidentified male was one of three homeless men taking shelter in a culvert under a roadway when floodwaters overtook them, according to the San Antonio Express-News. The other two men were able to swim to safety, the report added.

In a separate incident, four people were attempting to cross a creek near Interstate 10 when floodwaters swept them away Thursday evening, the San Antonio Express-News also reported. A woman swam out on her own, but two others had to be rescued. The missing man fled from rescuers and was then swept away, the report said.

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"Radar indicated a cluster of thunderstorms moving southeast off the Texas Hill Country was intercepted by a westward-moving outflow boundary, giving a boost to those thunderstorms just as they moved over the San Antonio metro area," said weather.com meteorologist Jon Erdman.

In just 22 minutes Thursday afternoon, San Antonio International Airport picked up 1.01 inches of rain, according to local reports.

San Antonio, as well as much of central Texas, is part of "flash flood alley." This region is susceptible to flooding that can develop very rapidly, posing a deadly threat to anyone in its path.

"San Antonio is notoriously flash flood-prone, so these heavy rain rates, however brief, still managed to trigger deadly, local flash flooding," Erdman said.

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