Multiple extremely strong tornadoes will most likely tear through parts of the central United States on Wednesday ahead of days of potentially once-in-a-lifetime flooding hitting the same region of the Midwest and South, forecasters said.

National Weather Service offices warned of the potential for a “tornado outbreak” across a region that stretches from Texas to Michigan, with special risk alerts in place for the Memphis area, home to 1.3 million people. The storm system generating these tornadoes is expected to stall by Thursday, prompting the government forecasters to issue their highest alert for flooding over this same area as more than a foot of rain may fall over the next five days.

• The threat for tornadoes will intensify Wednesday afternoon into the evening.
• Memphis is the epicenter for where the worst of both the tornado and excessive rainfall threat is expected this week.
• A broad risk of tornadoes will exist from North Texas northeastward to the southern Great Lakes, with large populations of people at risk of tornadoes in Detroit; Milwaukee; Cleveland; Chicago; Dallas; Indianapolis; St. Louis; Little Rock, Ark.; Louisville, Ky.; Columbus, Ohio; and Evansville, Ind.
• The rain threat shouldn’t be ignored. “This is not your average flood risk,” forecasters with the Weather Service in Memphis warned Wednesday morning.

Precipitation intensity

Source: National Oceanic and Atmospheric Administration via Iowa State University.  Note: All times are Central. By William B. Davis

Meteorologists said the system would not move quickly, allowing heavy rains to persist from Wednesday through the weekend. The relentless rain may lead to “significant and potentially historic” rainfall totals of 10 to 15 inches. This could create what forecasters called a “generational flooding” event, particularly in a region stretching from northeast Arkansas through western Tennessee, western Kentucky and into southern Indiana.

“One of the problems is that this storm system isn’t going to be allowed to continue moving east,” said Richard Bann, a meteorologist with the Weather Prediction Center. An area of high pressure off the Atlantic coast will block it, he said, keeping the storm stalled while warm, moisture-laden air from the Gulf fuels heavy downpours. “That’s the crux of our concern,” Mr. Bann said.

The Weather Prediction Center has placed areas from northeast Arkansas through the mid-Mississippi Valley and into southern Indiana under a threat of “high risk,” what it calls a Level 4 out of 4, for excessive rainfall on Thursday.

A higher risk of excessive rainfall shouldn’t be taken lightly. Over the past decade, some of the deadliest and most destructive floods have occurred in areas that forecasters said were at this level of risk — a level they rarely use. Areas that don’t usually experience flooding might in this case.

Source: National Oceanic and Atmospheric Administration Notes: Values are shown only for the contiguous United States and are in inches of water or the equivalent amount of melted snow and ice. By Zach Levitt, Bea Malsky and Martín González Gómez

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