Historic flooding lashes Mississippi Valley; Threat ongoing

A dangerous flooding situation has been developing through parts of the nation's Heartland, thanks to an intense, slow-moving weather system.

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By Saturday morning, up to 4 inches of rain had fallen over parts of Missouri, Oklahoma, and Illinois, with more than 6 inches between Evansville, IN, and Louisville, KY between Friday and Saturday.

By Sunday morning, as much as 11 inches of rain had fallen on parts of southern Missouri and northern Arkansas, with amounts up to 8 inches recorded from Oklahoma to Indiana.

Missouri Governor Eric Greitens declared a state of emergency on Sunday (April 30) due to flooding that he described as "historic" for some parts of the state. Greitens said thousands of National Guard troops might be called upon to assist in flood fighting efforts over the next few days. 

The cities of Joplin, Cassville and Reeds Spring were among those hard hit by floodwaters, according to a local NBC news affiliate via Reuters.

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Slow-moving features in the atmosphere were to blame for the extensive rainfall, including a deep trough in the jet stream which allowed cold air and snow to seep down over the Rockies and dump snow on Denver. The flip side of this feature is a broad ridge in the jet stream over the east, drawing up a plume of Caribbean moisture to inundate the Gulf coast and Mississippi Valley.

This jet setup supported a nearly-stationary boundary draped from Texas to Ohio, which was the focal point for clusters of nearly stationary thunderstorms, and staggering rainfall totals.

In addition, the diving jet stream was fueling a severe thunderstorms through the Deep South over the weekend, with downpours intensifying already high rainfall totals, along with the risk for large hail and damaging winds. Dozens of tornadoes were reported Saturday and Sunday, with at least four dead and many injured in one long-lived tornado near Canton, TX.

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45 stations along the Mississippi, Illinois, and Ohio rivers are expected to hit the major flooding category over the next week.