Madison, Wis., and Cedar Rapids, Iowa, lie in the heart of the threat, where the potential is high for more tornadoes, Accuweather reported Sunday.
At least five people are dead in Oklahoma after more than 100 reported tornadoes ravaged Oklahoma, Kansas, Nebraska and Iowa late Saturday and early Sunday, the National Weather Service said.
Five people died in Woodward, Okla. -- two children at a trailer park, two people near the Tangier area and one who died at the hospital, The Oklahoman reported. City Manager Alan Riffel said about 10 people are in critical condition.
The Woodward News said some 3,900 homes in the city of 12,000 people were without power early Sunday.
Keli Cain, a spokeswoman for the Oklahoma Department of Emergency Management, said early reports from the area northeast of Oklahoma City were grim.
"We're hearing of what sounds to be significant damage in the area," she said.
In Kansas, Gov. Sam Brownback declared a state of disaster emergency as twisters tore through the state late Saturday.
In Iowa, emergency officials told The New York Times 75 percent of the town of Thurman was obliterated by a tornado Saturday.
By early Sunday morning, about 7,000 homes were without power throughout Iowa, the Des Moines Register reported. The town of Creston was struck Saturday night, with Greater Regional Medical Center and Southwestern Community College both sustaining damage.