Scientists are warning that historic flooding could soon deluge parts of several Southern states along the lower Mississippi River, where floodwaters could persist for several weeks. The flood threat in the South will be discussed Thursday, when the National Oceanic and Atmospheric Administration releases its 2019 spring outlook.

Experts plan a briefing on their flood forecast at the National Water Center in Tuscaloosa, Alabama. Thursday's report is aimed at helping emergency managers and other safety officials to prepare for flooding.

Flooding in Southern states this spring will be "potentially historic," NOAA said in an advisory.

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Rapidly melting snow in the upper Midwest is contributing to flooding that will eventually make its way downstream to the Gulf Coast, forecasters have said.

The expected surge of water from the north is unwelcome news in parts of Mississippi. In the western part of that state, the Mississippi River is already swollen and has been flooding some communities unprotected by levees since last month.

One Mississippi region protected by levees is also flooding. That's because smaller rivers can't drain into the Mississippi River as normal because a floodgate that protects the region from even worse flooding by the big river has been closed since Feb. 15.

Around Rolling Fork, Mississippi, townspeople first noticed water rising from swamps near the Mississippi River in late February. The water eventually invaded some homes in that community, about 40 miles north of Vicksburg.

Major flooding is already occurring this week on the Mississippi River near several Southern cities including Arkansas City, Arkansas; Natchez, Mississippi; and Baton Rouge, Louisiana, according to river gauges and data from NOAA.

The specter of major flooding on the Mississippi River upstream from New Orleans is a more perilous situation now than in years past, some researchers believe. That's partly because the river floor has risen significantly higher over the years as sediment has collected in the river bottom, Louisiana State University hydrologist Yi-Jun Xu found.

The situation is so serious that Xu believes a "mega flood" could overpower a giant flood control structure north of New Orleans and send the Mississippi River rushing down another path entirely and creating a new route to the Gulf of Mexico. That would allow the Gulf to push saltwater upstream into the river, ruining the drinking water supply for metropolitan New Orleans, according to a summary of Xu's 2017 presentation to the American Geophysical Union.

Last month, a massive storm dropped an "incredible" amount of rain in the South, dropping as much as one foot to 1.5 feet of rain in some areas of northern Mississippi, Alabama and into Tennessee.

NOAA's forecast comes as rampaging rivers continue to rise in the upper Midwest, destroying bridges and roads and isolating farming communities. Experts say the cost of the flooding will likely top $1 billion.