The most urgent alerts came in the middle of the night.

Hundreds of emergency personnel were searching for stranded people. The Texas National Guard made 237 rescues and evacuations, said Maj. Gen. Thomas M. Suelzer, the commander of the guard, which used helicopters and rescue swimmers to reach people.

Gov. Greg Abbott said rescue crews would work throughout the night. He also signed an emergency disaster declaration encompassing several counties in central Texas. That will expedite state funding for the areas that experienced significant damage.

Lt. Gov. Dan Patrick said Camp Mystic was contacting the parents of campers who remain unaccounted for. He said parents with children who had not heard from camp officials should assume their children were safe. The camp has some 750 campers, he said.

The flooding seemed to take many by surprise. Judge Rob Kelly, the chief elected official in Kerr County, said at a news conference that “we do not have a warning system” and that “we didn’t know this flood was coming.”

Here is what else to know:

• Search for the missing: Downed power lines, flooded roads and spotty cellphone service were among the challenges rescue workers in Texas were facing on Friday as they searched for survivors.

• Federal help: Homeland Security Secretary Kristi Noem has activated the U.S. Coast Guard and the Federal Emergency Management Agency to help search for the missing, Tricia McLaughlin, a spokeswoman for the agency, said in an X post.

• Camp Mystic: The Christian summer camp for girls on the Guadalupe River is nearly a century old. Its facilities include a recreation hall that was constructed in the 1920s from local cypress trees.

• Past flooding: For those old enough to have lived through it, the flooding on Friday surfaced memories of a deadly swelling of the waters along the Guadalupe River on July 17, 1987.

And that’s what happened very early on Friday morning, when the Weather Service issued a series of upgraded warnings in quick succession. Rivers across the region were beginning to rise as storms dropped more and more rain — first, the San Saba River at 1 a.m. local time, and again at 5 a.m., followed by the Concho River and then the Colorado River, each of which are expected to keep rising slowly through the weekend.

The Weather Service also issued warnings for flash floods as the storm’s impacts were becoming more clear. Unlike a slowly rising river, flash floods are sudden deluges that can occur after heavy rainfall with very little notice.

A little after 4 a.m., the Weather Service sent one of its most urgent alerts, a “particularly dangerous situation” warning, reserved for the most urgent and potentially deadly scenarios. It is meant to grab attention, and is most often used when violent tornadoes are nearby, but also for floods and wildfires. The warnings are often shared on the Weather Service’s social media accounts and broadcast by local news organizations.

At 5:34 a.m. local time, a “particularly dangerous situation” warning came for Kerr County: “Automated rain gauges indicate a large and deadly flood wave is moving down the Guadalupe River,” forecasters wrote. “Flash flooding is already occurring.”

It identified Hunt, Kerrville and Center Point, all communities along the Guadalupe River, as places of concern: “Residents and campers should SEEK HIGHER GROUND NOW! Life threatening flash flooding along the river is expected.”

Other nearby areas, including Tom Green County, were also under these warnings starting a little after 6 a.m., one of which noted that “law enforcement reported major flooding and very dangerous conditions across San Angelo,” the county seat. Another warning for the eastern portion of Kerr County as well as part of Kendall County came at 7:24 a.m.

By then, reports were starting to emerge from the banks of the Guadalupe.

The flooding hazards continued through Friday, though the storms were expected to be less intense as the day went on. Weather Service forecasters in San Angelo noted that because of the rain that had already fallen, even small amounts of additional precipitation could exacerbate the flooding hazards.

A flash flood watch was in effect through parts of west-central Texas until 7 p.m. local time on Friday, and forecasters said isolated showers and thunderstorms were likely through the weekend.

Ms. Aldrich, a lawyer from San Antonio, said her two daughters, ages 9 and 12, have not said much about what they endured, and she did not want to press them just yet. But she had a few choice words for the people running the camp, where about 20 out of 750 children remained missing Friday night.

“They should have been watching the Texas Division of Emergency Management and Kerr County,” she said, referring to the authorities who had been warning of potential flash floods. “They were posting stuff yesterday morning. They should have been on top of it.”

Ms. Aldrich’s older daughter was in a section of the camp called Senior Hill, and her younger daughter in a section she called the flats, when campers and a counselor were forced to find higher ground to escape the rising waters that were overtaking the campgrounds.

“They came down when the water receded,” Ms. Aldrich said. “And then they made it to one of the buildings that wasn’t flooded anymore. They were up there for a couple of hours.”

The girls were eventually bused to another camp and then brought to the reunification center at an elementary school in Ingram.

Ms. Aldrich said she was notified that her two daughters were accounted for earlier on Friday, but she was desperate to see them in person.

Her good news was tempered by the uncertainty around her. “There still are campers missing,” she said.

Walking away from the shelter, her older daughter told her, “All of my stuff is muddy.”

“I told her, ‘oh, I don’t care,’” Ms. Aldrich said.

Here’s what they say:

For people searching for loved ones

Several reunification centers have been set up for people to connect with their missing loved ones, including:

• Arcadia Live Center in Kerrville, Texas

• Ingram Elementary School in Ingram, Texas

The Red Cross is also assisting with reunification efforts.

For those who want officials to conduct welfare checks, the authorities asked that they call the Kerr County Sheriff’s Office dispatch or the Kerrville Police Department.

For those seeking shelter

People whose homes were swept away or who have otherwise been displaced can find some assistance in these places.

In Kerrville:

• Calvary Temple Church

• Notre Dame Catholic Church

• Schreiner University Event Center

• First United Methodist Church

In Comfort, Texas:

• Comfort High School

• The Immanuel Lutheran Church

• The Lobby Coffee Shop

Other residents

For all residents in affected areas, officials are asking them to stay away from flood zones and downed power lines. Residents should also shelter in place and clear the way for emergency medical workers, the authorities said. Emergency workers in Kerr County have been hindered at some rescue sites by people filming and taking photos of the flood, as well as heavy traffic, they said.

“This incident is not over, and now is not the time to go look at the river,” the Kerrville Police Department said in a Facebook post, referring to the Guadalupe River. “Please help us by steering clear and giving us room to work.”

“We have plenty of resources on the ground,” Freeman F. Martin, the director of the Texas Department of Public Safety, told reporters on Friday afternoon.

Officials said they had received offers from residents who wanted to assist with the search. But they urged people to stay home and warned residents in the area to refrain from launching drones or using private helicopters, saying that doing so could endanger rescue workers.

“We don’t need any more first responders,” Mr. Martin said. “We don’t want anybody to self deploy.”

Mr. Martin said that emergency workers had managed to reach people who were stranded but safe in several locations in the flood zone. Officials were waiting for road conditions to improve before evacuating them.

“We’re able to bring food and water to them,” he said. “It’s just taking time to get them out of there.”

Mr. Martin said that as of 3:30 p.m. on Friday, about 25 roads in the area were impassable. He said that people should not try to drive or wade through flooded areas.

“If you can’t tell how deep it is, the best advice is to stay home,” he said.

Parents of campers who have not been accounted for have been notified, the email said. About 750 girls were at the camp this week, Lt. Gov. Dan Patrick of Texas said at a news conference on Friday.

The camp said that it was assisting with search-and-rescue operations, but that it did not have power, water or Wi-Fi and was struggling to get more help because a nearby highway had washed away.

On social media and in text messages, parents circulated photos of some of the missing girls, and exchanged hopeful stories that they were hearing about dramatic rescues: girls clinging to trees, or floating downriver to a boys camp five miles away.

Camp Mystic aims “to provide young girls with a wholesome Christian atmosphere in which they can develop outstanding personal qualities and self-esteem,”its website said. It draws from families around Texas and beyond, with the youngest campers entering third grade in the fall.

The camp was established in 1926, according to its website, and has been run by generations of the same family since the 1930s. The current owners, Dick and Tweety Eastland, have been with the camp since 1974. They worked alongside the previous owners for years after graduating from the University of Texas in Austin. Camp activities include archery, cooking, cheerleading, fishing and a variety of sports. Videos posted to the camp’s Instagram account show large groups of girls participating in group dance performances this summer.

The affected stretch of the Guadalupe River is home to several summer camps for children, including Camp La Junta for boys, about five miles from Camp Mystic. The boys camp informed families on Friday that all campers there were safe and fed, but evacuations would not begin until at least 7 p.m. local time on Friday.

The Facebook page for another nearby girls camp, Heart o’ the Hills, reported that it was not in session when the flooding took place.

Ten of the teenagers were swept away and killed; 33 others, and four adults, were rescued. Some of the survivors held on to the upper branches of cypress and pecan trees, praying until helicopters arrived to carry them to safety, The New York Times reported. At the time it was the worst flooding of the Guadalupe River in 55 years, The Times reported.

Rob Kelly, the Kerr County judge, said at a news conference on Friday morning that the flooding this time might exceed what took place in 1987, based on the waterline at his property along the river.

As in Friday’s flooding, in 1987, five to 10 inches of rain fell in the upper headwaters of the Guadalupe River basin. In Hunt, Texas, where the Guadalupe River forks, over seven inches had fallen since Thursday afternoon, which was the highest total at the site since the early 1990s.

The rapid rainfall led to a fast rise of the river that accelerated to over 29 feet before sunrise on Friday, the second highest crest ever recorded. Still, it might have even gone higher. As the waters rose 20 feet in two hours, they flooded the instrument box that the U.S. Geological Survey uses to send river gauge data.

The box stopped transmitting at 4:35 a.m. local time on Friday.

In 1987, though, warnings were issued early in the morning to summer camps along the river, and it was unclear why the church vehicles crossed the swollen river. All but one body was recovered.

More than a year later, a sheriff’s deputy in Kendall County, Texas, said of the missing camper that a cresting river would create sandbars, and a body could end up buried, possibly until another big flood comes to “turn things upside down again.”

In Kerrville, some people huddled inside a church’s activity center, and others looked distraught, shivering under blankets. Brian Eads, 52, was hoping for information about his wife, Katherine, after aggressive floodwaters ravaged their trailer at around 3:30 a.m.

“I have no idea if she’s made it,” Mr. Eads said. “We both got swept away, and then I lost her.”

The couple were awakened by rushing waters, and managed to escape with a man driving a recreational vehicle. But the water caught up with them about 20 feet away, Mr. Eads said, and the vehicle’s engine died. Both he and his wife were swept underwater. He tried to swim toward her voice, he said, but lost her when he was struck in the head by debris. He survived by holding onto a tree and making his way to dry land.

Outside Ingram Elementary School, about seven miles west of Kerrville, people hoped to find missing loved ones, including girls who were staying at Camp Mystic nearby. Some hugged each other outside the main entrance, while others stood waiting to hear from their relative.

Randy Bush, 59, said he had not heard from his fiancée Charlotte Huff, 55, since last night. He had already been at a local Walmart, where others were searching for relatives.

“I have no idea what happened to her,” he said.

His fiancée lives at an R.V. park in the Kerrville area. As soon as he heard about the floods, he rushed to the park, but was stopped by road closures and emergency vehicles.

“When I was there this morning, they were doing water rescues with helicopters,” Mr. Bush said. “From what I saw that park was gone,” he added as he made his way to the school. “It was just all water. It didn’t look like there was anything there. That whole park was done.”

Parents of campers were hoping for the best Friday afternoon. Betty Gerlach, whose 14-year-old grandson is a camper at Camp La Junta, a boys camp about five miles along the river from Camp Mystic, said the boys camp had informed families that all campers were safe and fed. But an evacuation plan was still in development, and would not begin until at least 7 p.m. local time on Friday.

The camp asked families from Houston, the Dallas-Fort Worth area and out of state to begin traveling to the area. But families in nearby Austin and San Antonio were told to “stay put for now,” to avoid overcrowding.

With several camp building washed away in the flooding, the campers had taken shelter in two small cabins while they waited for evacuation, Ms. Gerlach said.

By midafternoon, emergency crews had started to bring some of the stranded girls to Ingram Elementary. One man saw his daughter sitting in the passenger seat of an emergency vehicle and ran after it with a smile.