"Occasionally we have a death but we have warning system. Seven deaths. It's been a long time since we've had those kinds of injuries and deaths reported," Miller said.
Witnesses said parts of Parkersburg were reduced to rubble, including most of the town's high school and homes.
A tornado also struck just to the east in the nearby town of Dunkerton and elsewhere in Black Hawk County, where there were reports of homes damaged or destroyed.
Dunkerton has fewer than 800 residents and New Hartford has at least 700 people.
At least 20 people were unaccounted for in Minnesota after a swift storm blew through the St Paul suburb of Hugo, damaging about four dozen homes, Mike Ericson, a city administrator said.
Many of them could be out of town for the holiday weekend, he said.
Residents reported a tornado touching down in the area, but that had not been confirmed by the National Weather Service. Emergency crews descended on the town to look for those who hadn't been accounted for and to assess the damage.
About 300 or 400 homes were evacuated in the storm-damaged area because of concerns over hazards including downed power lines and leaky gas lines, Ericson said.
The city set up a shelter at an elementary school for those displaced.
Tornado season typically peaks in the spring and early summer, then again in the late fall.