At least five tornados swept through central Iowa, injuring at least 17 people and demolishing homes and businesses.
"This is the widest area and the most destruction", she said in Marshalltown.
UnityPoint Health said a tornado struck the hospital in Marshalltown about 5 p.m. Thursday.
More severe storms are likely in the Midwest on Friday.
Roy Schweinebart, of Marshalltown, Iowa, shovels bricks from a tornado-damaged building near Main Street. He says forecasters hope to make a determination by Friday evening.
The Iowa tornado battered homes tearing roofs quickly off the buildings in a matter of seconds. Brick walls collapsed in the streets, roofs were blown off buildings and the cupola of the historic courthousetumbled 175 feet (53 meters) to the ground.
While there was heavy damage to structures and over a dozen injuries reported throughout the area, so far there have been no fatalities reported.
The storm knocked out power and telephone service at city hall, officials said. But she said she's relieved.
"It didn't look like tornadic supercells were possible", Krull said.
'If anything, we were expecting we could get some large hail, if strong storms developed'.
The tornado hit in the town of Pella, about 40 miles (64 kilometers) southeast of Des Moines.
40 patients had to be evacuated from a hospital in Marshalltown after it was badly damaged by a tornado.
Crystal Jensen, tears running down her face, said the storm passed in just three minutes, but it left her house in ruins and blew away photos and a bracelet from her father.
Marshalltown resident Stephanie Moz told AP she, her husband and 2-month-old baby were in the downtown clothing store she owns when tornado sirens went off.
They could hear booms and crashes, and a gust of wind blew through the basement, but they emerged safely.
"It was bad. I've seen a lot of tornado stuff (in the past)", she said.
It's not often the National Weather Service is caught off guard by tornadoes, Krull said, but it does happen.
The town of Bondurant, just northeast of Des Moines, saw multiple funnels caught on camera, including one incredible image from a passenger on an incoming flight taken through their airplane window.
In Pella, some workers at Vermeer Manufacturing were taken to a hospital, according to police. People were still working at the plant at the time.
Linda Parker, a spokeswoman for the Iowa Department of Homeland Security and Emergency Management, said seven people were treated at the scene with six taken to hospital. He said there were minor injuries but no deaths, and officials were going door-to-door to check on residents.