Hundreds of residents have evacuated the US Midwest as rain-swollen rivers threatened crops and livestock and left scores of buildings underwater after days of extreme weather in which 24 people died.
Several major rivers, including the Mississippi, were expected to crest at or above record levels on Wednesday as flood waters rushed towards the Gulf of Mexico, the National Weather Service said.
Flooding has closed many roads and parts of Interstate 44, a major artery running from west Texas to St Louis.
Water rose to the rooftops of homes and businesses in Missouri, where Governor Jay Nixon called the flooding "historic and dangerous".
About 300 people in Valley Park, Missouri, west of St Louis, were evacuated because a levee that protects the community might be breached by the Meramec River, said Chief Rick Wilken of the Valley Park Fire District.
Louisiana Governor Bobby Jindal declared a state of emergency as the waters moved south toward his state. Flash flood warnings were issued for parts of the Carolinas and Georgia.
At least 24 people have died in Missouri, Illinois, Arkansas and Oklahoma in flooding after days of downpours that brought as much as 30cm of rain to some areas.
Historic floods on the Mississippi in 1993, 1995 and 2011 occurred during warm weather, after snow melts in the north. AccuWeather senior meteorologist Alex Sosnowski called it highly unusual to have heavy flooding in winter and said it could presage trouble for the spring.
"The gun may be loaded again for another major flooding event," said Sosnowski, who cited the El Nino weather pattern as the source of recent heavy rains. "You're not supposed to get this kind of heavy rainfall during the wintertime."
Agriculture experts said that water standing more than a week could kill the soft red winter wheat crop. Export premiums for corn and soybeans were at their highest levels in weeks because of stalled barge traffic on swollen rivers.
Livestock also has been hard hit. About 2500 pigs drowned in an Illinois barn after a creek overflowed its banks, said Jennifer Tirey, a spokeswoman for the state's Pork Producers Association.
Several major rivers, including the Mississippi, and tributaries in Missouri and Illinois were poised to crest at record or above-record levels, the National Weather Service said, but parts of the region were already inundated.
Flood warnings were issued from eastern Oklahoma into southeastern Kansas, southern Missouri, central Illinois and parts of Arkansas, Kentucky, Tennessee, and the Florida panhandle.
While the rains have stopped for now, freezing weather is setting in, which will make the clean-up a miserable undertaking, Sosnowski said.
The severe weather has stranded tens of thousands of people during one of the busiest travel times of the year. More than 750 flights were cancelled and 4760 delayed as of mid-afternoon on Wednesday, according to FlightAware.com.
