Strong winds, large hail and even possible tornadoes were forecast for Texas and the Plains on Monday, after at least six people died during a weekend of heavy flooding.
Rescuers were due to continue searching for at least three people swept away by rising floodwaters in Texas and Kansas. Six bodies already have been recovered in the Lone Star State since Friday following days of torrential rain.
Meteorologists said Monday would bring little respite.
Severe thunderstorms were possible from the Texas-Mexico border up to North Dakota, The Weather Channel reported. Parts of Oklahoma, Kansas, Colorado, Nebraska, and South Dakota were also in the firing line.
"Large hail and damaging wind gusts are the biggest concerns," The Weather Channel reported, adding that isolated tornadoes "can't be ruled out."
Moisture from the Gulf of Mexico meant there was a risk of more heavy rain and flash flooding across many of the same areas.
The National Weather Service issued flood warnings in several of these states, as well as severe thunderstorms and flash flood warnings for parts of Texas.
At Dallas-Fort Worth International Airport, 34 flights were canceled and 104 were delayed on Sunday, according to the tracking website FlightAware. There had been six cancellations as of 4:30 a.m. ET Monday.
Meanwhile, authorities were due to continue searching Monday for a 10-year-old boy who fell into the Brazos River in Texas' Parker County while fishing, NBC Dallas-Fort Worth reported.
And the Coast Guard in Texas joined the search for a 37-year-old man missing after being swept out to sea while trying to rescue a woman.
Stephen Espedal was one of several people who attempted to save the woman at San Luis Pass, between Freeport and Galveston, on Sunday evening. The woman managed to make it ashore and the other rescuers were picked up in a boat — but Espedal has not been found.
Separately, officials in Kansas were still searching for an 11-year-old boy who was swept away by Wichita's fast-moving Gypsum Creek on Friday night, according to NBC station KSNW.
"We're basically in a body recovery mode right now," Wichita Fire Department Battalion Chief Scott Brown told the station.
NBC News confirmed the recovery of six bodies in Texas over the weekend. Among those who died in the floods was 21-year-old National Guardsman Darren Mitchell, whose body was recovered Saturday after his vehicle was swept off the road.
Meanwhile, Tropical Depression Bonnie was continuing to drench parts of the Carolinas early Monday after making landfall Sunday. The system was "meandering" near Charleston, according to the latest National Weather Service update at 11 p.m. Sunday ET, and was set to move up the coast through North Carolina on Tuesday and Wednesday.