"If we do get the amount of rainfall that's expected, you can and probably should expect major flooding," Rusty Cornelius, administrative coordinator for Harris County Emergency Management told Reuters.

The National Weather Service predicted thunderstorms could return to the Houston area and continue until midday Tuesday, dumping as much as another 5-10 inches (13-25 cm) onto the already water-logged ground.

That would likely cause the already swollen bayous that crisscross Houston to overflow their banks.

The Houston Fire Department rescued more than 500 people from flood waters with no serious injuries or fatalities reported. "Most of them were rescues of civilians from their vehicles after they drove into flooded areas," said Assistant Chief Omero Longoria.

The worst of the flooding hit South Houston, which declared a state of emergency and set up four shelters to accommodate residents driven from their homes.

Television news showed footage of several people on the roof of a building that had flooded with water levels topping out at about three feet (90 cm) on some streets.

Houston city and county officials were working to prepare emergency crews to fan out across the city ahead of a potential deluge in the coming hours.

"If the situation does take a turn for the worse, we have the assets ready to be deployed," said Gloria Roemer, spokeswoman for the County Judge Robert Eckels, head of the county's emergency management office.

Witnesses said state highway 225, the city's "refinery row," was closed earlier in the day because of high water, although disruptions to plants appeared to be minor.