The forecast called for 3 to 5 inches of rain in New Orleans in the coming days. That is dangerously close to the amount engineers said could send floodwaters pouring back into neighborhoods that have been dry for less than a week.
There is also the risk that could take a sharper-than-expected turn on its way toward Texas and hit much closer to New Orleans.
"Right now, it's a wait-and-see and hope-for-the-best," Army Corps of Engineers spokesman Mitch Frazier said.
He said the forecast brought renewed urgency to efforts to shore up levees with sandbags and bring in more portable pumps. The corps also installed 60-foot sections of metal across some of the city's canals to protect against storm surges.
The showers on Thursday were the first measurable rain in New Orleans since Katrina hit on Aug. 29, broke the levees in several places and flooded 80 percent of the city.
Engineers said the badly damaged levees in New Orleans can handle only up to 6 inches of rain and a storm surge of 10 to 12 feet. In neighboring St. Bernard Parish, a surge of 5 to 6 feet would be all that was needed to swamp the area again.
The lack of rain since Hurricane Katrina ripped through the city more than three weeks ago has been one of the few blessings for New Orleans. On Thursday, relief workers at Mardi Gras World, the city's largest builder of parade floats, handed out free tarpaulins to homeowners to keep Rita's rain from coming through their damaged roofs.
CBS News correspondent Sharyn Alfonsi reports that while forecasters do not think Rita will hit the city head on,
The entire Mississippi delta region was under a tropical storm warning. To the west of New Orleans, the storm was expected to bring 15 to 20 inches of rain to southwestern Louisiana. Gov. Kathleen Blanco urged the evacuation of some 500,000 people along the southern edge of the state.
"Rita has Louisiana in her sights," Blanco said. "Head north. You cannot go east, you cannot go west. If you know the local roads that go north, take those."
As for those who refuse to leave, she said: "Perhaps they should write their Social Security numbers on their arms with indelible ink."
National Guard and medical units were put on standby. Helicopters were being positioned, and search-and-rescue boats from the state wildlife department were staged on high ground on the edge of Rita's projected path. Blanco said she also asked for 15,000 more federal troops.
"Prepare your family and prepare your house," she warned. "I'm urging Louisiana citizens to take this storm very seriously."
Forecasters said the storm is expected to come ashore on the Texas coast late Friday or early Saturday and turn north along a path not far from the Louisiana state line. For the town of Lake Charles, La., not far from the border, it is a chilling prediction.
"At first, our evacuation orders were just for the low-lying areas, but now it's the entire Calcasieu Parish," said Cindy Murphy, a manager at the police bureau in Lake Charles. "We've got buses running continuously to get residents out. We're trying to learn from other areas, not to repeat their mistakes."