About 20 million people remain under flood watch as severe storms cause lingering downpours across the Deep South.

Hailstones the size of golf balls accompanied by as much as 100 millimetres of rain pelted the US Gulf coast from Texas to Louisiana, flooding highways, downing power lines and closing some schools, officials said.

The Houston area braced for more flooding as heavy rain and hail pelted the nation’s fourth largest city for the second time this week, threatening to send some streams over their banks.

The National Weather Service said some areas received as much as 76mm of rain an hour on Thursday.

It issued a flash flood advisory for five counties in southeastern Texas including Harris, which includes Houston.

Local media reported that the storms left as many as 100,000 customers without electric power and dozens of local roads covered with water, making driving treacherous.

Houston’s 209,000 public school students got the day off as the city’s Independent School District, the state’s largest school system, shut down its 280 campuses on Friday because of the inclement weather.

The Houston Chronicle reported that parts of the US Interstate 10 highway in the city was closed late Thursday in east Houston, stranding at least 40 motorists.

Officials said the worst of the storm cleared away early on Friday, but flash flood warnings and flood watches remained in effect from east Texas to Knoxville, Tennessee.

Houston has repeatedly faced flooding in recent years because the city has insufficient drainage and experienced rapid development that reduced wetlands.