New Orleans faces a never-before-seen problem with Tropical Storm Barry

The Mississippi River, which is usually at 6 to 8 feet in midsummer in the Big Easy, is now at 16 feet, owing to record flooding that's taken place this year all along the waterway.

Meantime, Barry is cranking in the Gulf of Mexico, threatening a storm surge of 2 to 3 feet at the mouth of the river, said Jeffrey Graschel, a hydrologist with the weather service's Lower Mississippi River Forecast Center in Slidell, Louisiana.

The unusual confluence of factors adds up to a forecast that has the river cresting Saturday at 19 feet, a level not seen since February 1950 and about 2.3 feet shy of the record set in April 1922, the weather service said Thursday.

"This is the first time we've had a tropical system with water levels on the river this high," he told CNN.

The prediction is rattling the nerves of residents also concerned about the 10 inches of rain Barry could dump before it moves out, CNN senior meteorologist Dave Hennen said. That deluge would follow the 9 inches that fell Wednesday in New Orleans, flooding parts of the city.

Mandatory evacuations in at least 2 parishes

Barry, the first tropical system to hit the US this year, is moving slowly, the weather service said. Residents of the coast and in the lower Mississippi Valley could see heavy rainfall through the weekend and into early next week, with flash flooding, river flooding and storm surge likely.

States of emergency have been declared in Orleans, Jefferson, St. Bernard, Plaquemines and St. Charles parishes. Jefferson Parish and Plaquemines Parish have instituted mandatory evacuations as a precaution in low-lying areas or those outside major levees.

Officials are expected to close dozens of floodgates to help mitigate the risk of flooding, according to Southeast Louisiana Flood Protection Authority East spokesman Antwan Harris, and FEMA has deployed personnel and supplies for staging in Louisiana and Texas, the agency said in a statement.

"Gulf Coast residents should prepare now for heavy rains, flooding and high wind impacts regardless of this storm's category," the FEMA statement said.

In New Orleans, 118 of city 120 pumps that drain neighborhoods are in working order, Sewerage & Water Board spokeswoman Courtney Barnes told CNN. The two that aren't are relatively small, she said, and are at stations with other functioning pumps in the Lakeview area and New Orleans East.

Still, the system of pumps, underground pipes and canals is only designed to remove 1 inch of rainwater in the first hour of a storm and a half inch in subsequent hours. It simply could not keep up with Wednesday's downpour, Barnes said, noting that any system in the country would have been outpaced.

"There's no system designed to pump that capacity of rain," she said.

'The real storm hasn't event hit'

Some residents aren't taking any chances.

Dannie Davis of New Orleans will evacuate, she told CNN on Thursday. She was struck by the flooding Wednesday, "and the real storm hasn't even hit," she said.

"I haven't seen this much rain and flooding before a hurricane in awhile," she said. "Who knows what's to come and whether the city will be able to handle it."

Another resident, Claire Grogan, was also planning to evacuate, she said.

For 40 years she's lived in the French Quarter, mere blocks from the Mississippi River, she said, and has never been scared. Now, that's changed.

"The river is so high that I am just scared to stay," Grogan said, adding that as a business owner, she also wants her employees to have the chance to leave if they want to.