Days of persistent heavy rain puts millions in flash flood threat throughout the Midwest

This persistent round of storms will begin around the North Dakota-Minnesota border late Monday evening, then move into Northern Wisconsin and Michigan by Thursday.

An early-week storm threat is expected to affect millions of Americans across the Midwest, as forecasters monitor a growing flash flood threat.

Starting on Monday and lasting through the week, above-average heat and humidity are fueling a flash flood threat.

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This is partly because the Upper Midwest has already experienced so much rain that saturated soil can no longer handle the buildup of water.

The FOX Forecast Center is monitoring a slow-moving storm system that will pass through the Upper Midwest beginning Monday afternoon.

The front traveling with this storm will bring high levels of combined moisture, allowing days of persistent thunderstorms throughout the region.

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These storms will create a "train" effect, with each system following the next.

"Much like boxcars on a train track, individual thunderstorm cells will pass over the exact same piece of land, one after the other," the FOX Forecast Center said.

This persistent round of storms will begin around the North Dakota-Minnesota border late Monday evening, then move into Northern Wisconsin and Michigan by Thursday.

As a result of this impending storm setup, a Level 2 out of 4 flash flood threat is active starting Tuesday through Wednesday for major cities such as Minneapolis, Duluth and Green Bay, Wisconsin. 

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These locations could see moderate to heavy rainfall through the week, with some areas seeing up to 3 inches.

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As the week progresses, flood alerts will remain in place as localized rain totals begin to add up, raising concern as the wet weather shows no signs of slowing.

Stay with FOX Weather for the latest.