Here’s why floods are inundating the Midwest right now

Flooding that spread across parts of Iowa, South Dakota and Minnesota this past weekend is forecast to worsen Monday and Tuesday as rivers continue to rise, and meteorologists warn that any additional rain from possible thunderstorms could extend or heighten the risks of inundation.

The floods are the product of torrential rain across parts of the upper Mississippi River basin since Thursday, dumping widespread totals of 10 inches to as much as 18 inches of rain, said Todd Heitkamp, meteorologist in charge at the National Weather Service’s Sioux Falls, South Dakota, forecast office. Soils were already saturated from months of wetter-than-average conditions before storms fueled by intense Gulf of Mexico moisture lingered over the region from Thursday into Saturday.

That sent runoff coursing into streams and rivers and overwhelming levees in some areas, Heitkamp said. Rivers are expected to crest Tuesday or Wednesday at moderate to record flood stages, forecasters said.

Streamflows are running “much above normal … suggesting that while soils are still wet, there is limited storage capacity to store additional rainfall,” forecasters wrote Monday. “Each additional round of rainfall will serve to further compound the flood potential.”

At the same time, intense heat that has been shifting around the country for the past week is forecast to bake the region as it manages the rising waters. Temperatures across the Upper Midwest were forecast to peak in the 90s, with high humidity expected to make it feel close to 110 degrees Monday. That will produce “unpleasant to dangerous” conditions for those spending time outdoors, the Weather Service warned.

Heat advisories stretched across the Mississippi River basin Monday, from Minnesota and South Dakota to the Gulf Coast.

In Minnesota on Monday, authorities were watching closely as rising waters on the Blue Earth River, not expected to crest until Tuesday, washed out a portion of the Rapidan Dam near the town of Mankato, 50 miles southwest of Minneapolis. Officials said the dam’s integrity was intact and being monitored, but the town of North Mankato was building a temporary levee to protect itself in case of the dam’s failure. Sen. Amy Klobuchar called it “a serious situation.”

Flood damage was already significant and included a collapsed rail bridge that connected South Dakota and Iowa.

Iowa Gov. Kim Reynolds said the waters damaged or destroyed more than 1,900 homes across at least 22 counties in that state. After surveying the toll by air on Saturday, Reynolds said that “the devastation is widespread.” She said rivers were rising above records set in the Great Flood of 1993, which spread across nine states, killed 50 people and caused $15 billion in damage.

At least one death was linked to the flooding in South Dakota, Gov. Kristi L. Noem said Sunday. She did not provide details.

A rapid surge of floodwaters within about 30 minutes destroyed an unknown number of homes in North Sioux City, South Dakota, on Sunday night, the Argus Leader reported.

Sioux Falls Mayor Paul TenHaken earlier urged residents to conserve water, as the floods had caused unprecedented stress to the city’s wastewater system, forcing authorities to release some untreated sewage into the Big Sioux River, the paper reported.

“Our wastewater collection and treatment systems remain strained, but they are back to a point where we believe it is manageable,” Mark Cotter, Sioux Falls’ director of public works, said in a statement.

Flooding caused widespread road closures across the region, including a brief shutdown of a portion of Interstate 29.

Thursday and Friday were the wettest two-day period on record for two South Dakota communities, Mitchell and Sioux Falls, with 7.7 inches and 6.49 inches of rainfall recorded within 48 hours, the Weather Service said.

As of Monday, rivers across the upper Mississippi Valley region were at major flood stage: the James, Vermillion and Big Sioux rivers in South Dakota and the Des Moines and Little Sioux rivers in southwestern Minnesota and northwestern Iowa, according to National Oceanic and Atmospheric Administration data.

Flood warnings were in effect in those areas “until further notice,” the Weather Service warned.

Isolated storms are possible Tuesday afternoon, meteorologists said, with the greatest chances for significant rain expected Thursday night into Friday.

“If you get any additional rainfall, it’s a punch in the gut,” Heitkamp said.