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)

A flood event for the record books is taking shape across the Central and Southern United States this week, fed by a series of storms that have produced torrential rainfall over already-saturated ground. In southern Illinois, the Mississippi River is forecast to at least match — if not exceed — its highest flood crest on record. In eastern Oklahoma, the flood water has already risen above rooftops.

Heavy rain extended from central Illinois and Indiana to the Gulf Coast in front of a strong cold front on Monday. Over 18 million people were under a flash-flood watch as heavy rain fell on already-saturated soil. Over 1.5 million were under a flash-flood warning that covered nearly 20,000 square miles in southern Illinois, southeast Missouri and far western Kentucky. On the cold side of the storm, heavy snow and freezing rain fell from Oklahoma north into the Great Lakes and parts of the Northeast.

[This month is now the deadliest December for tornadoes in over 60 years]

Up to 10 inches of rain has fallen in the 24 hours from Sunday morning to Monday morning in eastern Oklahoma, northeast Texas and western Arkansas, but 24-hour rainfall totals exceed three inches from Indiana to Texas. Dozens of new rainfall records were set over the weekend, in some cases doubling or even tripling old records for Dec. 26 and 27.

The wet week has pushed St. Louis to its wettest year on record, breaking the old record of 57.96 inches in 2008. St. Louis will have accumulated over 5 feet of rain in 2015 after this storm is through.

Days of heavy and persistent rainfall pushed about 400 river gauges to flood stage in the Central United States by Monday afternoon. But in addition to its vast extent across the Midwest and South, what makes this late-December flooding so remarkable is its magnitude.

From Illinois to Texas, 30 gauges were in major flood stage, and many more were approaching major flood stage. The National Weather Service defines major flooding as extensive inundation of structures and roads. In this stage, significant evacuations are required to transfer people and property to higher elevations.

[On Post Nation: Midwest braces for more extreme weather after weekend of deadly tornadoes]

In some locations, forecasts are calling for record or near-record crests over the next few days as rainwater across a large area of the South and Midwest drains into tributaries that feed into the Mississippi River.

Near Chester, Ill., the Mississippi River 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.

(National Weather Service)

However, this week’s flooding isn’t truly put into perspective until you consider that nearly all of the historic crests along the Mississippi have occurred during the spring melting season or the summer rainy months. Wintertime flooding to this extent is typically not possible simply because there is usually not enough moisture in cold, winter air to support such incredible rainfall totals.

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.

In eastern Oklahoma, the Illinois River near Tahlequah has already crested at over 30 feet — well above its previous record of 27.9 feet set in May of 1950. To find the next-highest December crest on record, you have to go all the way down to number 32 on the list of historic flood events: 17.6 feet on Dec. 27, 1987.

The floodwater near Tahlequah, Okla., was rising above the rooftops on Monday afternoon.

Covering #okwx today and found these two horses trying to stay on higher ground, and that is next to the house.

A photo posted by Dominic Armstrong (@photogdominic) on