Sounds like these people didn't have much warning. One of the factors that contributes to flooding is that so many local and state governments refuse to update their floodplain maps - because then developers won't be able to build, and they won't want to contribute to the politicians anymore:

LODI, Ark. — At least 16 people were killed and dozens more were unaccounted for after flash floods raged through campgrounds in western Arkansas early Friday morning.

As many as 300 people, including families with vehicles and off-road backpackers, may have been camping along the Caddo and Little Missouri Rivers as waters surged by 20 feet between midnight and dawn, according to Red Cross and state emergency officials.

Terrified families tried to outrace the churning, swiftly rising water, some fleeing up hillsides as tents vanished, recreational vehicles flipped over and rental cabins were demolished.

As of Friday afternoon, 16 bodies had been recovered and dozens more people were still missing, said Chad Stover, a public affairs officer of the Arkansas Department of Emergency Management.

Kayla Chriss, 22, of Vivian, La., and her family had been camping in the area since Monday. “Without warning everything started washing away,” she said.

Around 2:30 a.m., Ms. Chriss, her 3-year-old daughter and her 4-year-old son were pummeled into the floodwaters. She held her son between her legs, but she watched in horror as her daughter floated out of reach.

Miraculously, a man — “I only know his name is Jerry,” she said — grabbed the little girl and lifted her onto a tree. Ms. Chriss said she started to black out when her hair got caught on a jutting limb, rousing her so she could pull herself and her son onto tree branches where they waited, wet and scared, until daylight.

“I was just singing to my son, telling him everything is going to be O.K.,” she said Friday evening, shortly after being discharged from a hospital with only minor sprains; her son had a black eye. “I was just trying to find a way to keep him out of the water. If it wasn’t for him being there, I wouldn’t have made it. He kept me going.”

The National Weather Service issued a flood warning around 2 a.m., after the heaviest rains had started, according to The Associated Press. By then, the disaster was already unfolding, and in any case, state officials said, the terrain and lack of cellphone service in the valleys made communications difficult.