According to agency officials, parts of Illinois, Indiana, southern Michigan and western Ohio were at the greatest risk of seeing tornadoes, large hail and damaging winds throughout the day Sunday. Strong winds and atmospheric instability were expected to sweep across the central Plains during the day before pushing into the mid-Atlantic states and northeast by evening. Many of the storms were expected to become supercells, with the potential to produce tornadoes, large hail and destructive winds.
In McHenry County, northwest of Chicago, funnel clouds were spotted late Sunday morning, dropping out of the clouds and then retreating again, said Bob Ellsworth, the assistant director of the county's emergency management agency. Ellsworth added that none had touched the ground or caused any damage.
Around the same time, the weather service issued a tornado warning for parts of Kenosha, Racine and Walworth counties in Wisconsin.
Friedlein said that such strong storms are rare this late in the year because there usually isn't enough heat from the sun to sustain the thunderstorms. But he said temperatures Sunday are expected to reach into the 60s and 70s, which he said is warm enough to help produce severe weather when it is coupled with winds, which are typically stronger this time of year than in the summer.
"You don't need temperatures in the 80s and 90s to produce severe weather (because) the strong winds compensate for the lack of heating," he said. "That sets the stage for what we call wind shear, which may produce tornadoes."
He also said that the tornadoes this time a year happen more often than people might realize, pointing to a twister that hit the Rockford, Ill., area in November 2010.
Friedlein said that the storm will move across northern Illinois from 10 a.m. to 4 p.m., meaning Chicago could see the brunt of it about the time the Bears-Ravens gets underway.
NFL games in Cincinnati and Pittsburgh also could be affected by the rough weather.
Associated Press writer Sophia Tareen in Chicago contributed to this report.