Heavy rain and flooding forced schools to close in the Houston area Friday after an overnight deluge swamped the nation's 4th-largest city.

The sodden city was at the epicenter of a sprawling storm system that brought soaking rain to much of Texas, Louisiana and Mississippi. In all, some 21 million people live where flooding is possible Friday and Saturday, according to the National Weather Service.

The Houston fire department, which fielded more than 75 water-related calls late Thursday, urged motorists Friday to stay off the roads to avoid more flash flooding from a storm system lingering in the western Gulf of Mexico.

"There's no sense in putting yourself, first responders, firefighters or anybody in danger needlessly," Houston fire chief Samuel Peña said in update on Twitter.

"The ground is saturated," he said. "It is perfect conditions for flash flooding. We encourage you to stay home."

Among the scores of calls of stranded motorists was a report of a vehicle upside down in a a water-filled ditch. Teams were able to extract a lone occupant, who was alive but unconscious.

The rains stem from a cold front in the western Gulf that is setting up a multi-day period of heavy, excessive rainfall, the National Weather Service says.

"Soils are already saturated across the region, which includes the Houston metro area, and additional rains will only exacerbate flooding concerns," the weather service said

The forecasters said the greatest concern is from east and southeast Texas into parts of Louisiana and Mississippi.

"Portions of the south-central U.S. have been inundated with flooding rain, large hail and continuous lightning this week, and the threat is not yet over," AccuWeather meteorologist Brandon Buckingham said.

Farther west along the coast, The Galveston/Scholes Field Airport received nearly 3 inches of rain in one hour Thursday night, with 2.14 inches of rain falling in a span of just 32 minutes, AccuWeather reports.

To the east, the severe storm generated possible tornadoes in Huntsville, Alabama, and McComb, Mississippi.

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

Contributing: The Associated Press