The National Weather Service urged people to avoid travel if a flash flood warning was in place for their area.

A flash-flood warning is issued when a flash flood is imminent or occurring, the Weather Service said. If you are in a flood-prone area during a flash-flood warning, move immediately to high ground.

Even when floods can be anticipated, communicating their severity to the public is a tall order. Because flash floods are very localized, even neighborhood-level warnings may seem like false alarms to some residents, leading to what the journalist Zoë Schlanger has dubbed “ alert fatigue .”

Flash flooding is weather’s No. 2 killer, claiming more lives than anything but heat, so it’s important to understand what causes it and how to stay safe.

If there’s a “flash flood watch,” according to the service, flooding is not guaranteed, but conditions are favorable enough for it to be possible, so be prepared to change your plans.

Flash floods often manifest as walls of water that can wash away most things in its path and carry debris; they can reach heights of 30 feet or more and trigger equally dangerous mudslides.

A desperate search continues for dozens of people still missing after catastrophic flash flooding in Central Texas, with one Bay Area family among them. The death toll is at least 81 people as of Monday morning,