When the warning sounded shortly before midnight that a tornado could be approaching, the clock turned into a stopwatch for staff members working the night shift at a behavioral health center at a Sioux Falls hospital.

“We had 10 minutes to wake up 102 residents, get them to the center of the building,” said David Flicek, the president and CEO of Avera McKennan Hospital and University Health Center in South Dakota’s largest city. “And all are safe and sound.”

Although a significant tornado had not struck Sioux Falls for 25 years, the Avera Health System hospitals have kept up regular preparedness training. This work paid off when one of three EF-2 tornadoes pummeled the hospital campus.

The twister with wind speeds of up to 130 mph also roared over the system’s heart hospital after a man was brought in having a heart attack. Doctors and nurses continued operating on the man — and saved his life — as the storm blew on, according to the CEO of Avera Heart Hospital, Nick Gibbs.

“We talk at our hospital about doing drills. I’ve got to tell you our staff was courageous,” said Flicek.

Natasha Sundet, a 46-year-old nurse manager at the behavioral health center, arrived at the hospital shortly after the patients had been moved and said she hardly recognized the hospital and grounds.

“There are big chunks of metal hanging from the building; broken glass everywhere; tree limbs and trash; cars that have been picked up and moved with their windows blown out,” Sundet said. “When I walked into the building there was water pouring in through the ceiling. I have never seen anything like it.”

Of the 102 patients who were moved, 39 of them were adolescents, Sundet said.

“We had children ranging from 4 to 17 who were huddled with their blankets and really just frightened and in shock,” she said.

The National Weather Service has determined that three EF-2 tornadoes struck the city overnight, lead meteorologist Todd Heitkamp said Wednesday.

Dozens of buildings were damaged or destroyed, trees torn up and power lines downed. But no one has been reported killed or even seriously injured in the devastation of the storm. Flicek said seven people suffered minor injuries from falling debris at the hospital campus and another was hurt outside.

Noting that Sioux Falls hasn’t experienced a tornado since October 1996, Heitkamp said even the weather service staff ducked for cover as the storm rolled in.

On top of it all, there was a snafu with the city’s outdoor siren warning system. Most of the sirens sounded in southeastern Sioux Falls, where the most serious damage occurred, but they were not activated in the rest of the city. Mayor Paul TenHaken cited a “miscommunication” among staff and vowed it would never happen again.

“I’m owning it. It’s my team. It’s my administration,” he said.

At least 37 buildings collapsed or were damaged by the storm, and residents have been asked to stay away from the hardest-hit areas, Fire Chief Brad Goodroad said at a news conference early Wednesday.

Other damaged businesses included an Advanced Auto Parts store where a wall collapsed. Kohl’s and Best Buy lost part of their roofs and Pizza Ranch suffered heavy damage.

The Red Cross opened a shelter at the Sioux Empire Fairgrounds’ armory for people displaced by the storm. The city of about 190,000 people lies about 240 miles (390 kilometers) southwest of Minneapolis.

Xcel Energy says as many as 25,000 customers were without power at one point because of the damage, but that more than two-thirds of those had electricity Wednesday morning.

The storm system appears to have spared the rest of the state. The South Dakota Department of Public Safety received reports of flooding in Hutchinson and Brule counties, but no assistance was requested.

The weather service warned of possible severe thunderstorms Wednesday across the Plains and Upper Midwest, stretching from western Nebraska, Minnesota and Iowa to Wisconsin. The likeliest threat was in western Nebraska, and the weather service warned of possible flash flooding in the north-central part of the state.