MASON CITY – Much of northern and central Iowa will be under a flash flood watch until Wednesday afternoon, leaving areas that were already hit by heavy weekend storms vulnerable to more damage.

More than 6 inches of rain fell across north central Iowa on Sunday night and early Monday morning, causing widespread flooding and some road closures, according to the National Weather Service. More heavy rains were expected late Monday night into Tuesday morning.

“There are some areas in central Iowa, that could be pushed into flooding rather quickly [if thunderstorms return],” said Marc Russell, a forecaster with the National Weather Service.

“We might luck out. The farther south it goes the better. Some of those areas could use the rain.”

Thunderstorms passed through the region by Monday morning, prompting numerous sewage treatment systems to overflow, officials said. Rainfall as heavy as 10 inches was reported in Emmetsburg, forcing the city to bypass more than 2.7 million gallons of untreated sewage into a nearby creek by noon Monday.

Private well users in the area should have their water tested, said Ken Sharp, a spokesman for the Iowa Department of Public Health. Runoff due to excess rain can cause bacteria and other potential contaminants to be washed into the well, he said.

The rain also prompted several roads closures, including U.S. Highway 169 south of Fort Dodge and Iowa Highway 7 near Manson. A bridge near Dunkerton in Black Hawk County was washed out.

In Webster County near Moorland, the weather service reported a car was stranded in water in an underpass along U.S. Highway 20. The Webster County Sheriff’s Office said the road was open Monday morning, and had no information about the car.

Highway 7 remained closed Monday evening while damages were being repaired, according to the Iowa Department of Transportation.

In Pilot Mound, 3 inches of rain was measured in one hour.

In Mason City, strong winds tore a 20-yard swath of damage through a four-block area, ripping part of the roof off a Shopko store and knocking over construction trailers behind Home Lumber and Builders.

No injuries were reported.

The storm hit Mason City about 5:30 p.m. Sunday.

Under a Union Pacific-Highway 122 overpass, Kyle Harlan, 20, and Tim Hubbard, 18, found their vehicle stranded in high water. They got out and pushed three to four other cars out of the water.

“The rain came down so hard,” Hubbard said. “I rolled down my window a bit and rain just flew in.”

Sunday’s storms also produced a tornado, damaging wind and hail in Hardin County, but there were no immediate reports of any damage.