The death toll from the storms and flooding in Texas and Oklahoma climbed to 21, authorities said late Wednesday. Torrential rain over the weekend gave way to floods, prompting the evacuation of hundreds of people.

Authorities reportedly said that 17 people died in Texas and four in Oklahoma, and the toll is expected to rise as at least 11 others remain missing. Houston Mayor Annise Parker reportedly said two people, whose boat capsized during a rescue operation, were among the missing. Many cities in Texas remain in danger even after deadly thunderstorms that hit the region this week tapered off, officials said, according to the Associated Press (AP). 

In suburban Houston, areas along the San Jacinto River were expected to flood, authorities reportedly said, adding that many residents of Wharton, located 60 miles southwest of Houston, were asked to evacuate due to the predicted rise of the Colorado River.

More rainfall on the hard-hit Houston area hindered cleanup efforts, while other parts of Texas continued to face extreme conditions. Texas' Memphis city received 1.45 inches of rainfall in one hour amid flash flooding, authorities reportedly said.

Hundreds of people living west of Fort Worth were urged to evacuate their homes along the rising Brazos River. Flooding was also reported on Interstate-35W in the northern portions of Fort Worth.

"The river is coming up fast and flowing at dangerous volumes," Parker County Judge Mark Riley told a news conference, according to Reuters.

Tornado warnings were issued for Randall County, Pecos County and Armstrong County in Texas, and for Ellis and Roger Mills counties in Oklahoma until Wednesday night, according to the Weather Channel

Meanwhile, the deaths of an Oklahoma firefighter and a Texas teenager, who were swept into openings in drainage systems and drowned, raised concerns over the dangers posed by storm drains that help protect neighborhoods during flash flooding. Wide drain openings risk the lives of residents and rescue workers as the drainage channels create powerful currents that can suck people in. The two were among the 21 who died in the Memorial Day weekend flooding, AP reported.