orig how tornadoes form_00000019.jpg
How tornadoes form
01:16 - Source: CNN
CNN  — 

A storm that tore across Sioux Falls overnight was so treacherous, even weather forecasters had to pause their work and hunker down.

Now city officials say human error prevented some of the city’s outdoor warning sirens from blaring like they should have.

The severe weather that struck South Dakota’s largest city spawned at least one tornado, the National Weather Service said.

“Obviously we had widespread damage throughout the entire city,” said Todd Heitkamp of the weather service’s Sioux Falls office.

“Most of that damage was a result of probably 100-mph-plus winds. And some of the damage was a little bit more intense, and we’re suspecting that was a result of some brief, spin-up tornadoes.”

Eight people at the Avera Behavioral Health Center were injured in the storm, Sioux Falls Mayor Paul TenHaken told CNN.

The hospital suffered storm damage and was evacuated.

Across the city, at least 37 structures either collapsed or had integrity issues, the fire department said.

01 sioux falls tornado

At 11:30 p.m. Tuesday night, employees at the local National Weather Service office paused and took shelter “due to the fact that our employees felt that their safety was being jeopardized,” Heitkamp said.

The office “shifted the responsibility of the warnings to our sister office up in Aberdeen. And then at about 11:35 we resumed those responsibilities.”

As residents sifted through the damage Wednesday, officials investigated why only some of the city’s outdoor warning sirens went off.

“There was actually a breach of our protocol, in that not the entirety of the siren system was sounded,” the mayor said.

“What happened with the system is we just had a human error,” TenHaken said. “We already have some plans in place to rectify that going forward. Our system is not broken, it works absolutely just fine.”

02 sioux falls tornado

But Heitkamp stressed that the sirens are outdoor warning sirens, not tornado sirens, and shouldn’t be used as the only method of getting tornado alerts.

“Those sirens are not meant to warn people that are inside. They’re meant to warn people that are outdoors,” he said. “That’s why it’s important to have multiple methods to receive weather information.”

03 sioux falls tornado

The mayor urged residents to stay off the roads, especially because of power outages throughout the city.

About 8,500 electricity customers were in the dark Wednesday, Xcel Energy said. That’s down from a peak of 25,000 customers without power immediately after the storm.

CNN’s Gianluca Mezzofiore contributed to this report.