ST. LOUIS (AP) -- Warm and wet weather over the last several weeks followed by storms that brought a deluge of rain in recent days have produced a severe threat of flooding along the Mississippi River, where water could reach record high levels soon in some places. The winter flooding is unusual and could portend even worse problems in the spring depending on weather the rest of the season. Here's a look at what is prompting the concern:
The sudden flooding along the Mississippi River is both rare and historic.
At a time of the year when both precipitation and the river level typically are well below normal, there could be record crests in some places along the Mississippi.
The river already was high due to an unseasonably warm, wet late fall and early winter. Torrential rains -- 6 to 10 inches in parts of eastern Missouri and western Illinois -- since Saturday pushed it to the unprecedented levels.
The Mississippi is expected to reach 49.9 feet by Friday at Chester, Illinois, south of St. Louis. That would top by two-tenths of a foot the all-time high reached on Aug. 7, 1993. It also is expected to match the highest level ever on Saturday at Cape Girardeau in southeast Missouri. St. Louis Mayor Francis Slay declared a flood emergency. Major flooding is predicted at several other points from just north of St. Louis through the South.
Flood plain buyouts along the Mississippi in recent years may mitigate the damage, but some homes are endangered. The mayor of West Alton, Missouri, 20 miles north of St. Louis, ordered the 520 residents to evacuate because the flood levee isn't high enough for the expected crest.
WHY SO HIGH?
Much of nation has enjoyed an unusually warm late fall and early winter, but it has come with a price.
"It has been a lot warmer than normal, and the warmer air can hold more moisture in the atmosphere," said Scott Watson, a hydrologist for the National Weather Service office near Kansas City, Missouri.
The result has been higher than normal rainfall, causing the Mississippi to be unusually high. St. Louis has received nearly 10 inches of rain in December, according to the National Weather Service -- four times the normal amount. Around 7 inches of rain have fallen since Saturday.
The warm spell seems to be coming to an end. Highs along the river basin in Missouri and Illinois are expected to be in the 40s for most of this week, with overnight lows in the 20s. But Watson said that's probably not cold enough to cause ice to form on the fast-moving river.
National Weather Service hydrologist Marty Pope said that flood crests on the Lower Mississippi below the mouth of the Arkansas River won't set overall records, but will probably be the highest ever recorded in winter.
"We're looking at water stretching from levee to levee," Pope told reporters Monday in Pearl, Mississippi.
That means that inhabited areas on the river side of levees, as well as ports and casinos, could be affected.
The river is predicted to crest in Memphis on Jan. 9, with the peak water traveling downstream to Natchez, Mississippi, by Jan. 18.
"It's still raining in parts of the basin," Pope said. There could be a lot of change between now and then."
Heavy rainfall has already inundated a levee in northeast Louisiana. Part of the Mississippi River levee near Alsatia, in East Carroll Parish, collapsed. Greg Raimondo, a spokesman for the Vicksburg District of the U.S. Army Corps of Engineers, said the district plans to activate its flood-fighting plan on Friday, increasing patrols along levees, looking for sand boils, seepage and landslides.
Raimondo said Corps officials are concerned that a levee collapsed before the river really began rising.
While extensive flooding this time of year is unusual, it's not unheard of, according to Bob Holmes, a Rolla, Missouri-based national flood hazard coordinator for the U.S. Geological Survey.
He points to January and February of 1937, when Ohio River flooding was blamed for hundreds of deaths and, when adjusted to today's dollars, billions in damage from Pittsburgh to southern Illinois' tip near Cairo.
Floodwaters in Illinois reached nearly 30 miles inland and nearly destroyed the town of Harrisburg. In southeastern Illinois' Shawneetown, residents were forced to flee before the Ohio wiped out the town, reaching a crest more than 32 feet above flood stage -- still a record there. The town ultimately was rebuilt on higher ground, three miles from the river.
The 1937 crest at Cairo still is second only to one reached there in May 2011.
Extensive wintertime rains pose a possible agricultural risk, saturating soil with moisture that could become trapped when the season's deep freeze finally sets in. Come spring, when the ground thaws, that moisture -- and the likelihood of spring rains -- could leave farm fields water-logged, slowing plantings of corn and soybeans and perhaps crimping yields.
Under that scenario, Holmes said, "it will be mushy" for farmers and their heavy machinery, prone to getting mired in the mud.
Seeds planted in wet soil don't develop deep roots and they run the risk of rotting if they're subjected to prolonged wet soil.
But Holmes cautions that it's still early, with winter still yet to play out.
"It will all depend on what the weather will do in February and March," he said. "It's so far out, we're all talking about the possibilities."
Suhr reported from Kansas City, Missouri. AP reporter Jeff Amy in Pearl, Mississippi, contributed to this report.