Emergency officials in Texas are struggling with the fact that they appeared to do everything by the book – and yet 21 people are still confirmed dead from torrential Memorial Day weekend flooding.

On Wednesday, authorities performed hundreds of water rescues and searchers continued to look for 13 missing people as more rain fell across the hills and gullies of the Texas hill country, where a stubborn five-year drought has given way to full lakes and soils that can't absorb more water. More rain is in the forecast.

But on Saturday night, hours before the height of the flood, the National Weather Service was trying to get the message out in every way possible: "MOVE TO HIGHER GROUND NOW."

It pinged almost every cellphone within range of a cell tower in the warning area. It went out over television and radio. Police went door to door. But in one tragic case, 12 people in a local vacation rental reportedly got the warning only when it was delivered in person by the homeowner. By then, the flood waters had risen so high that they could not be crossed on foot. The house was swept from its pilings.

What could have been done differently?

The United States has, in fact, dramatically improved its warning technology since hurricane Katrina killed more than 1,000 people in 2005, creating what some say is a global model for giving citizens time to get to safety in the face of disaster. It’s impossible to know the true impact of such improvements, but the number of lives saved by improved and increasingly ubiquitous alerts, officials say, is likely in the thousands.

Last weekend's Texas flooding, however, might have exposed several weak links in the system, such as making visitors aware of the dangers and convincing hardy locals that they shouldn't just "ride it out." Addressing those gaps means education, experts say – teaching residents when they need to be tuned in, alert, and ready to act.

“Modern technology has brought us the greatest level of warning dissemination in our lifetime, but even with all that said there’s always going to be that situation where people may not be aware of what’s going on around them,” says Walt Zaleski, the warning coordinator at the National Weather Service’s southern region headquarters in Fort Worth, Texas.

Houston saw six deaths and hundreds of motorists stranded, some overnight, as culverts turned into torrents. Wimberley, Texas, a scenic bed-and-breakfast town, was arguably hardest hit as a wall of water crashed into the river town late Saturday night and early Sunday morning, damaging and destroying hundreds of homes.

Officials in Houston acknowledge that the warning system may have failed to alert many residents to the potential severity of the situation. And in Wimberley, county emergency officials want to know whether visitors in town for the Memorial Day festivities, in particular, may have missed alarms sent out at 8:23 p.m., about four hours before the height of the flood.

“One issue was [that it] was after dark, people were already starting to prepare for the evening and settling down, putting them in the most vulnerable position of the day, even with all the warning mechanisms out there,” Mr. Zaleski said.

He added that regular flooding is the “nature of the beast in that area this time of the year. Residents always have to be on guard because of the flashy-ness of the rivers, creeks and streams that, due to geography, don’t require much for the water to quickly rise. So, this is not out of the normal, this is the way of life in the hill country.”

Yet the sheer scope of the torrent challenged even seasoned hill country dwellers. A 2013 study by the State University of New York at Buffalo found that personal experiences with “riding out” storms and floods can lead to complacency. Post-disaster surveys, the study pointed out, also showed that many people lack awareness of the seriousness of a situation even when they hear warnings.

“Wimberley had seen floods before, but they were usually slow and widespread, inching into residents’ homes and soaking carpets, but not much else,” writes USA Today’s Rick Jervis from the scene. “The rush of water that descended into town caught everyone by surprise, including those measuring the height of the river.… [T]he river was measured at 44 feet before the river gauge ripped away in the current….”

By some accounts, the alerts did get out to many people.

“We were kept posted, posted, posted,” Gay Sullivan, a Wimberley homeowner, told WFAA-TV. “They were frantic, telling us it was coming … move to higher ground … it was coming.”

In the past decade, the US has dramatically improved disaster detection and warning efficacy, with lead times for tornado warnings growing from four to 13 minutes, a fact that likely saved lives in the 2013 Moore, Okla., tornado. Much of the progress has come from the increased processing capacity of computers and the ability to integrate very granular radar data with on-the-ground observations. The modes of disseminating alerts have also improved.

In Wimberley, local officials used an emergency system to call landline phones with warnings. Meanwhile, a national wireless emergency alert system created in 2012 pumped out flood alarms, as well. Nearly all cellphones under three years old are equipped with the service, which intercepts basic radio transmissions and sounds a special ping. The service has been credited with saving hundreds of lives, says Mike Gerber, the leader of the Emerging Dissemination Technologies Program at the National Oceanographic and Atmospheric Administration.

“Warnings are put into a special format that we push to FEMA’s integrated public alert and warning system, and from there it goes out to wireless carriers, who basically broadcast the alert from cell towers within the warning polygon,” says Mr. Gerber. “If your cell phone is in range of that cell tower, you get the alert.”

As Saturday evening wore on and it became apparent that up to four inches of rain that fell in the Blanco River watershed would create a dramatic rise, the National Weather Service stepped up its alarm. At 11:24 p.m. local time, the weather service sent out its most severe warning, a flood emergency alert: “Rescues are ongoing … Move to high ground now. Act quickly to protect life. Be especially cautious at night when it’s hard to recognize dangers of flooding. Escape to high around to escape flood waters,” the warning read.

Some visitors, however, may have missed out on the local warnings, or misjudged their seriousness, officials say. The fact that some people got caught flat-footed by the powerful flood will “most definitely … be part of our discussion” going forward, Hays County Commissioner Will Copley told the Associated Press.

Disaster experts suggest that the heavy human toll in Texas should sound a warning to county and state emergency managers, universities, and media to not over-rely on technology in the face of disaster.

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“The difficulty is that warning is a process where the warning system itself is really a technological function, but the communication doesn’t occur unless the person on the other side not only receives it, but understands the importance,” says Louise Comfort, director of the Center for Disaster Management at the University of Pittsburgh. “What has happened in the US is that much more importance has been placed on putting into place the best technological warning systems and less on educating the people and making them aware of the suddenness of the risk and the lack of time that they’ve got. The capacity of people to take action in response to warning is what saves lives.”

She says that university extension services and local newspapers, especially, have a role to play in connecting the dots between alarm and action. Those institutions, she says, need to provide the equivalent of "Grandfather telling you stories of floods in the '30s and '40s that make families aware of the potential danger."