Rivers are rising to dangerous and historic levels in the Midwest after crushing rainfall swept through the area. Residents in Ill., Mo. and Ark. documented their experiences in the flood. (Jenny Starrs/The Washington Post)

The scene is overwhelming in the Midwest this week, where river water is rising into homes and business. Forecasts are calling for record or near-record crests of the Mississippi River and its tributaries as a week of torrential rainfall drains into the basin. More than 20 people have died in the floods, and the worst has yet to come.

The disastrous river flooding is the result of a month’s worth of excessive rainfall, most of which has fallen over the past week. In the past four days, 6 to 12 inches of rain has fallen across a vast area of the Midwest from Illinois to northeast Texas. Some locations are running 8 inches above normal for the month of December. Dozens of new rainfall records were set over the weekend, in some cases doubling or even tripling old records for Dec. 26 and 27.

Now all of that surplus rain is draining through tributaries that are swollen well beyond their banks into the Mississippi. Around 400 river gauges across the eastern United States have been in flood stage since Monday.

Over 20 people have died from the flooding alone — to say nothing of the 20 people that lost their lives in deadly tornado outbreaks on Dec. 23 and 26. At a news conference Tuesday, Missouri Gov. Jay Nixon (D) cited 13 deaths at that point in his state, noting that 12 of them were totally preventable: They drove into the floodwater.

In St. Louis, the National Weather Service says to expect “major to historic” river flooding through the weekend. The mighty Mississippi, which runs straight through the heart of the city, is forecast to crest at 43 feet — a full 13 feet above flood stage, and one of the city’s top three river crests on record. This is what is considered a “major flood” for St. Louis, which means river water will inundate surrounding structures and roads. The National Weather Service notes that at this stage, significant evacuations are required to bring people to higher ground.

Downstream from St. Louis in Chester, Ill., the Mississippi is forecast to crest at 49.8 feet, which would set a new record for the location, surpassing the old record of 49.7 feet which was set during the Great Flood of 1993. That flood was the second-worst flooding disaster since the Great Flood of 1927, and in some locations became the worst flooding disaster on record. Damage totals for the Great Flood of 1993 topped $15 billion.

What is so incredible about this week’s flooding is not only the magnitude, but the timing. To put this flood into perspective, all of the historic crests along the Mississippi have occurred during the spring melting season or the summer rainy months.

In Chester, only a single wintertime flood has made it into the top 10 crests on record — 39.71 feet on Dec. 9, 1982. Not coincidentally, December of 1982 was also right in the middle of the strongest El Niño on record at the time. The same can be said for St. Louis, where of 133 record crests, only three have occurred in winter months, and only one in December — also in 1982.

Under normal circumstances, wintertime flooding to this extent is not possible simply because there is not enough moisture in cold winter air to support such incredible rainfall totals.

Although river levels will begin to drop over the weekend, the floodwaters will continue to move downstream on the Mississippi over the next two weeks, where it will meet runoff from excessive rain in the Southeast. Memphis; Vicksburg, Miss.; and Baton Rouge, La., are all expected to see significant flooding in the coming days.

The effects of a deadly storm system that triggered tornadoes and flooding in the Midwest and Southwest could be see in Union, Mo., where homes and businesses are nearly underwater. (Reuters)