But even as the mayor defended his plan to allow residents to return ZIP code by ZIP code, the federal government's chief argument for slowing down - the possibility of another storm - was bolstered Sunday.

State and U.S. hurricane experts were keeping close watch on Tropical Storm Rita, which was situated south of the Bahamas. Though officials said storm movement can be difficult to predict with precision, computer models suggest the storm could pass between the Florida Keys, which are under a hurricane watch, and Cuba. By Tuesday afternoon it could be over the Gulf of Mexico, feeding on warm water and training its eye anywhere from central Mexico to the U.S. Gulf Coast, according to the National Hurricane Center.

A state official called the storm ''nebulous'' Sunday. But Ivor van Heerden, deputy director of the Louisiana State University Hurricane Center and a leading weather modeler, said even a peripheral brush with a major storm could flood the city again.

''We're working on it feverishly right now,'' he said. ''Our concern is that the levees in New Orleans are in a severely degraded state. Right now New Orleans is extremely vulnerable.''

The possibility of another storm was only one factor cited Sunday by Coast Guard Vice Adm. Thad Allen, the head of the federal government relief effort, who said he would encourage Nagin to rethink his program for bringing residents back.

Allen cautioned that the East Bank - the bulk of the city - has no potable water. Widespread problems remain with the 911 system, telephone lines and power, he added, and polluted floodwater also poses a threat to returning residents.

''Our collective counsel is for him to slow down and take this at a more moderate pace,'' Allen said on ''Fox News Sunday.'' '' ... The levees have been weakened to the point where if you're going to bring a significant amount of people into New Orleans, you need to have an evacuation plan on how you're going to do that.''

Still, there were indications that New Orleans will soon be ready to be a city again.

The causeway across Lake Pontchartrain, a critical thoroughfare connecting New Orleans to its northern suburbs, is expected to open this morning, Louisiana State Police Sgt. Cathy Flinchum said.

And a top Army Corps of Engineers official said that 87 percent of the floodwater that entered New Orleans has been pumped out.

Immediately after Katrina, officials said it might take as long as six months to pump out the city. The Army Corps then lowered that estimate to 80 days. If the current plan holds, the job will have been completed in 26 days.

In Orleans Parish, which includes the city of New Orleans, just 2,700 acres remain flooded, and part of that is an industrial pocket that has been kept wet deliberately for safety reasons. Immediately after the storm, 27,000 acres were flooded in the city.

Only three pools of water remain in the city, varying in depth from two to four feet. They will be pumped out in the next five days, according to Col. Duane Gapinski, the Army Corps' officer in charge of draining the city.

Late last week, Nagin proposed bringing back more than 180,000 residents - about a third of the city's pre-Katrina population - by the end of the month.

Business owners began returning to the city Saturday. Algiers, on the city's West Bank, across the Mississippi River from the French Quarter and the business district, is expected to open to some residents Monday. Algiers would be followed by the city's Uptown area, including the Garden District, and the French Quarter at the beginning of next week.

The areas targeted for returnees include those that best weathered Katrina and have not been affected by the standing water that has devastated other areas.

Nagin said his ''sensible'' plan was developed in cooperation with the federal government.

At a checkpoint in the Uptown area Sunday afternoon, a member of the Louisiana National Guard passed out leaflets warning about a citywide 6 p.m.-to-8 a.m. curfew and recommending that returnees bring gloves, masks and other protective gear to guard against contaminated water and soil. Item No. 1 on the list: ''You are entering at your own risk.''

''We believe our re-entry plan properly balances safety concerns and the needs of our citizens to begin rebuilding their lives,'' Nagin said in a statement.

Also Sunday, officials said Katrina's death toll had risen to 883 - 646 in Louisiana, 219 in Mississippi, 14 in Florida, two in Alabama and two in Georgia.

In Baton Rouge, Mark Smith, spokesman for the Louisiana Office of Homeland Security and Emergency Preparedness, said the state is struggling to create housing for roughly 1 million New Orleans-area residents who are scattered around the country.

''We're talking about an astronomical number of people that are without homes,'' he said.

In Washington, members of Congress geared up for a fight over which federal programs to cut in order to keep the massive cost of rebuilding from increasing the federal budget deficit. And as President Bush presses for budget cuts, lawmakers from both parties are stepping up their efforts to persuade the White House to name a strong manager to oversee the spending.

In Houston, where four main shelters once housed more than 27,000 evacuees, the shelter population declined by several hundred to 1,449 at the Reliant Arena and 362 at the downtown George R. Brown Convention Center. An estimated 2,000 evacuees have accepted an offer of a free one-way tickets out of Houston by Continental Airlines.

Gold and Vartabedian reported from New Orleans, Rosenblatt from Baton Rouge. Times staff writers Richard Simon in Washington, Nicholas Riccardi and Ann M. Simmons in New Orleans and Tony Perry in Houston contributed to this report.