South Louisiana should brace for the effects of a storm expected to be named Hurricane Barry by the time it makes landfall with maximum winds of 85 mph along the state’s southwestern coastline Saturday, the National Hurricane Center warned Wednesday.

While the storm's winds are expected to reach only Category 1 status, forecasters believe it will pack a punch in the form of massive rainfall and formidable storm surge.

On Wednesday morning, forecasters increased to 20 feet their estimate of how high the Mississippi River will rise at the Carrollton Gauge in New Orleans, on Saturday. That higher crest would result from several feet of storm surge, compounding already-high river levels from Midwest flooding.

The official flood stage in New Orleans is 17 feet, but a combination of recently elevated earthen levees and floodwalls on both sides of the river in the New Orleans area protect most locations to water heights of between 20 and 22 feet.

However, some levee segments along both the east and west banks could be as low as 18 feet. Thus, a 20-foot river height could cause overtopping at some of those locations, something that has never happened in the city's modern history.

Based on the National Hurricane Center’s prediction of a Category 1 hurricane, Jeffrey Graschel, a hydrologist with the Lower Mississippi River Forecast Center in Slidell, said the river will rise between 3 and 5 feet above its present elevation. The higher levels are expected near the mouth of the river, in Plaquemines Parish.

But the surge will make itself felt as far away as Baton Rouge, where the water level is expected to rise about 10 inches by Saturday, he said.

The Southeast Louisiana Flood Protection Authorities – East and West were closing floodgates in walls along the river as well as in parts of the area’s hurricane protection system, a process that will continue through Thursday.

For now, the broad low pressure system is called Potential Tropical Cyclone 2, because the storminess just south of Pensacola has not yet formed into a tropical depression or storm. But thunderstorms spawned by the system caused sudden, widespread and in places severe street flooding in New Orleans and other parts of southeastern Louisiana on Wednesday morning.

The center of the storm was located about 250 miles east-southeast of New Orleans, with top winds of 30 mph, and was moving west-southwest at 8 mph.

Forecasters warned that the entire central Gulf Coast, including all of southern Louisiana, will see additional rainfall accumulations of between 6 and 12 inches near the coast and in inland areas through Saturday, with isolated maximum rainfall amounts of 18 inches.

"Moderate rainfall flooding may prompt several evacuations and rescues," said a storm message issued Wednesday morning by the Slidell office of the National Weather Service. "Rivers and tributaries may quickly become swollen with swifter currents and overspill their banks in a few places, especially in usually vulnerable spots."

Radar-indicated rainfall in the New Orleans area on Wednesday morning already approached 8 inches in some locations, sparking the issuance of a flash flood emergency warning for street flooding.

Much of coastal Louisiana wetland areas from Morgan City to the Pearl River are also under a storm surge watch. Forecasters warned that timing of surge flooding will depend on the movement of the storm and on the tidal cycle, and said it could vary greatly over short distances.

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Storm surge watches were in place for St. Bernard, St.Charles, Jefferson, Plaquemines, Assumption, Lafourche, and Terrebonne parishes.

Forecasters warned that surge could cut off near-shore escape roads. The surge also is likely to result in beach erosion, damage to marinas, docks and piers, and could break small craft from their moorings.

Graschel said forecasters also are concerned that heavy rainfall over the next few days will swell other rivers and bayous along the coast that capture rainfall runoff, causing localized flooding.

That includes many of the bayous along the North Shore, he said.

Forecasters also warned that tornadoes may spawn during thunderstorms before arrival of the main tropical system, and during the system's passage. Several waterspouts were reported along the New Orleans lakefront on Wednesday morning.

The National Hurricane Center said storm surge watches may be needed farther west in Louisiana and Texas later Wednesday. A tropical storm watch also has been issued from the mouth of the Mississippi to Morgan City, meaning tropical storm conditions, with winds of more than 39 mph, could occur within the next 48 hours. Additional tropical storm or hurricane watches could be needed later Wednesday or Wednesday night for the remainder of the Louisiana coast and the upper Texas coast.

Forecasters said the thunderstorms forming within the low off the Florida coast were clearly getting better organized on Wednesday morning.

“Although the system is currently experiencing some northerly vertical wind shear, the shear is expected to gradually subside over the next day or so, and the low has a high chance of becoming a tropical depression or tropical storm by Thursday,” said Senior Hurricane Specialist Stacy Stewart in a 10 a.m. discussion message.

“Some erratic motion will be possible during the 24 hours or until a well-defined center develops,” he said. But the weather models indicate it will move to the west-southwest or southwest today and tomorrow, and by Friday turn toward the west-northwest and turn northwestward by Saturday.

Stewart said the system is expected to strengthen slowly over the next 24 to 36 hours because it still doesn’t have a well-defined center or an inner core wind field, and because there is a bit of northerly wind shear that can disrupt cloud formation.

“By 48 hours and beyond, however, the combination of atmospheric and oceanic conditions become ideal for intensification,” he said, spurred along by low wind shear and sea surface temperatures of 86 to 88 degrees.

+14 Flooding, severe storms hit New Orleans: See photos, videos of street flooding, waterspouts Severe thunderstorms dropped several inches of rain across a wide swath New Orleans early Wednesday morning, causing street flooding and promp…

Forecasters urged coastal residents to follow directions for possible evacuations, and to check personal and business emergency plans and supplies.

Editor's note: An earlier version of this story misspelled National Weather Service hydrologist Jeffrey Graschel's name.

Staff writer Jeff Adelson contributed to this report.