LITTLE ROCK, Ark. (AP) — At least six people have been killed in a wide swath of violent storms and tornadoes that hit the South and Midwest, and officials are bracing for more severe weather and flooding in the coming days.

The destruction is part of a potent storm system that the National Weather Service said will bring “significant, life-threatening flash flooding” each day through Saturday.

What happened?

The first wave of storms killed at least four people in western Tennessee and one each in Missouri and Indiana on Wednesday and Thursday.

Also in Indiana, emergency crews spent several hours rescuing a woman from a collapsed warehouse.

There was massive destruction in Lake City in eastern Arkansas, where homes were flattened and cars were flipped and tossed into trees. State authorities reported damage in 22 counties due to tornadoes, wind gusts, hail and flash flooding. Seven injuries were reported, but no deaths.

More than 90 million people were at risk of severe weather across an area stretching from Texas to Minnesota and Maine, according to the Oklahoma-based Storm Prediction Center.

What’s causing this wave of storms?

Forecasters attributed the violent weather to warm temperatures, an unstable atmosphere, strong wind shear and abundant moisture streaming from the Gulf.

The prolonged deluge, which could dump more than a foot (30 centimeters) of rain over a four-day period, “is an event that happens once in a generation to once in a lifetime,” the National Weather Service said.

What’s next?

The national Weather Prediction Center in Maryland said satellite images early Thursday morning indicated that “catastrophic” flooding could soon occur in Arkansas, Tennessee and Kentucky.

“Communities in the region should prepare for possible long duration and severe disruptions to daily life,” it said.

Private forecasting company AccuWeather said it was concerned about “major disruptions” to business, the supply chain and shipping due to the flooding and severe weather. Shipping giant FedEx, for example, has a massive facility in the danger area, in Memphis Tennessee.

Forecasters said barge transportation on the lower Mississippi River could also be affected.

Water rescue teams and sandbags were being set up across the region in anticipation of flooding, and authorities warned people to take the threat of rising water seriously.

“Especially when we have this much rain, it’s the decisions about when to get out, about what to drive through, when to go stay with someone else, that can be the difference between life and death,” Kentucky Gov. Andy Beshear said.

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Associated Press writers George Walker IV in Selmer, Tennessee; John Seewer in Toledo, Ohio; Jeff Martin in Atlanta; Adrian Sainz in Memphis, Tennessee; Jonathan Mattise in Nashville, Tennessee, Seth Borenstein in Washington; Isabella O’Malley in Philadelphia; and Bruce Schreiner in Louisville, Kentucky, contributed.