NEW ORLEANS (AP) - Tropical Storm Barry formed off the coast of Louisiana on Thursday and threatened to blow ashore as a hurricane with relatively weak winds but torrential rains that could test the flood-control improvements made in New Orleans since Hurricane Katrina 14 years ago.
Forecasters said the first hurricane of the Atlantic season could hit the state's swampy southern tip Friday night or early Saturday, with potentially ruinous downpours that could go on for hours as the storm passes through the metropolitan area of nearly 1.3 million people and pushes inland.
Plaquemines Parish, at Louisiana's low-lying southeastern tip, ordered the mandatory evacuation of as many as 10,000 people, and communities began handing out sandbags. Louisiana Gov. John Bel Edwards declared an emergency and said National Guard troops and high-water vehicles will be positioned all over the state.
"The entire coast of Louisiana is at play in this storm," he warned.
On Wednesday, with the gathering storm still out over the Gulf of Mexico, it dumped as much as 8 inches (20 centimeters) on metro New Orleans in just three hours. The deluge triggered flash flooding in the city's streets and raised fears about the even heavier rains on the way.
The National Hurricane Center said as much as 20 inches (50 centimeters) of rain could fall in parts of eastern Louisiana, including Baton Rouge, and the entire region could get as much as 10 inches (25 centimeters). The New Orleans area could get 10 to 15 inches (25 to 38 centimeters) through Sunday, forecasters said.
The storm's surge also could prevent water from emptying out of the already-swollen Mississippi River, possibly sending water over levees near New Orleans, forecasters said. The river has been running high for months.
Barry could have winds of about 75 mph (120 kph), just over the 74 mph threshold for a hurricane, when it comes ashore, making it a Category 1 storm, forecasters said.
National Weather Service meteorologist Benjamin Schott said the big concern is water, not wind: "Rainfall and flooding is going to be the No. 1 threat with this storm."
New Orleans Mayor LaToya Cantrell said that the pumping system that drains the city's streets is working as designed but that the slow-moving storm could dump water faster than the pumps can move it.
"We cannot pump our way out of the water levels ... that are expected to hit the city of New Orleans," she warned.
However, the city did not plan to order evacuations because the storm was so close and because it was not expected to grow into a major hurricane. Officials instead advised people to keep at least three days of supplies on hand and to keep their neighborhood storm drains clear so water can move quickly.
Katrina caused catastrophic flooding in New Orleans in 2005 and was blamed altogether for more than 1,800 deaths in Louisiana and other states.
In its aftermath, the Army Corps of Engineers began a multibillion-dollar hurricane-protection system that isn't yet complete. The work included repairs and improvements to some 350 miles (563.24 kilometers) of levees and more than 70 pump stations that are used to remove floodwaters.
The weather service said it expects the Mississippi to rise to 19 feet (5.8 meters) by Saturday morning at a key gauge in the New Orleans area, which is protected by levees 20 to 25 feet (6 to 7.6 meters) high.
"We're confident the levees themselves are in good shape. The big focus is height," Corps spokesman Ricky Boyett said.
In Plaquemines Parish, Justice of the Peace David McGaha waited with his mother, his wife and their 15-year-old son and 11-year-old daughter for a ferry so they could evacuate to his mother's house in Alabama.
"If the river wasn't so high, we'd probably stay. You have to worry about the water that'll be pushing against those levees," he said. "They made a lot of improvements to the levee, but they haven't completed all the projects."
Associated Press reporters Chevel Johnson and Janet McConnaughey contributed to this report from New Orleans.