With more rain falling, the torrents have already pushed Oklahoma City handily past a rain record, and rescuers have carried out at least 48 high-water rescues.
By late Saturday, 3.15 inches had drenched the city, bringing the total for the month to 17.61 inches. "It ... shatters the all-time monthly record of 14.66 inches set in June of 1989," said CNN weather producer Sean Morris.
And in the town of Claremore, near Tulsa, a firefighter died while attempting a high-water rescue, said local emergency manager Thomas Hudson.
In Hays County, Texas, adjacent to Austin, hundreds of people were rescued or evacuated from their homes, according to sheriff's office spokeswoman Lt. Jeri Skrocki.
Many of the rescues were along the Blanco River, she said. Hays County set up emergency shelters in two schools, a community center and a nursing home.
"We have 25 active rescues that we are working right now," said Kristi Wyatt, communications director for the city of San Marcos, the seat of Hays County.
"We have people on car tops and rooftops awaiting rescue. People in homes are going to higher levels in the homes. Emergency workers are working around the clock trying to get to those people and get them out safely," Wyatt said.
Authorities had to open more evacuation centers because the first one filled up so quickly.
"We had over 300 residents at the first location and the second location is getting filled up as well," Wyatt said.
"These are just people who had no friends or relatives to go to, so there are many more residents than that evacuated," she explained, adding that the National Guard was coming to Hays County to provide air assistance.
An area of about 400 homes around Louis Creek Dam is under mandatory evacuation, according to Miranda Haas with the Montgomery County, Texas, Office of Emergency Management. The dam has not breached and workers continue to pack soil on it.
"Our construction efforts have been phenomenal, they have made tremendous progress, it's just the weather is not letting up at all," she said.
Haas said a Red Cross shelter was opened at 8 p.m. (9 p.m. ET) Friday night and remains open Saturday.
Wichita Falls 'historic flood'
Wichita Falls, Texas, was warned that its river could widely overflow its banks and severely flood broad swaths of surrounding areas, as well as large parts of the city. Officials published a potential flooding map with a red zone nearly the size of the city.
"Predictions from the National Weather Service indicate that significant flooding along the Wichita River is very likely," the town's emergency management agency said. "The National Weather Service is calling this an 'historic' flood event."
The agency called for the voluntary evacuation of 2,177 homes.
Wichita Falls is having the rainiest May ever recorded there and "could set an all-time record for rainiest month ever recorded there," CNN's Morris said.
Broad, muddied flood waters gushed across fields, towns and roads in images from both states, turning land expanses into lakes, half burying cars and houses.
Blue and red emergency vehicle lights bounced off dark, watery surfaces, as rescuers worked through the night.
On the National Weather Service map, chartreuse squiggles signified overflowing rivers and creeks from southern Texas to northern Missouri. Much of the state of Oklahoma was covered in the bright green.
Motorists abandoned cars in streets and parking lots, as rising waters took them over. The weather service put out its usual flood mantra to drivers, "Turn around, don't drown" when encountering flooded roads. "Most flood deaths occur in vehicles."
The weather service also told campers and hikers to seek higher ground.
In addition to the worst-hit areas, flood watches and warnings reached from the Texas and western Louisiana Gulf coasts up through eastern Kansas and western Missouri.
In middle of drought
Despite the heavy rain, western Oklahoma and parts of the Texas Panhandle and central Texas are still facing moderate drought or abnormally dry conditions, according to the U.S. Drought Monitor. The rainfall should put a dent in it, though.
But the current deluge might be a bit much.
"I didn't hesitate telling people... there's going to come a day when we're gonna wish the rain would stop," Wichita Falls Mayor Glenn Barham told CNN affiliate KAUZ. "I think that day is probably here."
In 2011, drought and wildfire brought heavy damage to Texas.
The drought caused at least $5 billion in economic damage, and wildfire damage amounted to tens of millions of dollars, authorities said.