The storm that ran through Poplar Grove caused injuries, said Boone County Sheriff's Lt. Perry Gay. He did not know how many people were injured, or the severity, but said none of the injuries appeared to be life-threatening.
A sheriff's deputy spotted a tornado on the ground at about 3:30 p.m. about 1 1/2 miles north of the village of less than 1,500 residents, Gay said.
An unknown number of structures were damaged, including several that were destroyed, Gay said. Rescue crews were on the way to make sure no one was trapped inside.
Authorities also had to help people who were in trapped cars surrounded by live electrical wires, Gay said.
"There are a tremendous amount of live wires in the area, and that's a big problem for us right now," he said.
A spokesman for Union Pacific says a tank car at the scene of a train derailment in northern Illinois is leaking a hydraulic oil that is used as a shock fluid.
Police in McHenry County have already ordered the evacuation of 500 residents in the nearby unincorporated town of Lawrence because of a potential hazardous materials situation.
Railroad spokesman Mark Davis says seven total railcars have derailed, including five boxcars and two tank cars.
Davis says the other tank car was not leaking and contains ethylene oxide, a flammable material widely used to sterilize medical supplies. He says the boxcars are filled with auto parts.
Police say no injuries were reported.
Davis says the cause of the accident is under investigation. The train was traveling from North Lake, Illinois, to Janesville, Wisconsin.
Bill Lischka saw the tornado form as he sat drinking coffee at the Boone County Family Restaurant in Caledonia, near Poplar Grove.
"A tornado just popped right out of the clouds," he said. "Just a classic twister."
Lischka said the funnel looked like "a snake dangling" as it wove its way north and east. He didn't see the tornado touch down but said he has heard about damage to several businesses and homes.
The last time a tornado touched down in January in north-central or northeastern Illinois was Jan. 25, 1950, according to the National Weather Service.
That tornado hit Manteno, in Kankakee County, about 50 miles south of Chicago.
On that day in 1950, the air was warm and moist, and Chicago hit a high of 67 degrees, weather very similar to what Illinois experienced Monday, said NWS meteorologist Eric Lenning.
Northern Illinois is usually free of tornadoes in the winter because colder, more arctic air sits over the region, Lenning said. Tornadoes need warmer temperatures and moist air for support, conditions located much farther south in the wintertime.
But for the last couple of days, "we've had spring-like weather. That's when we'd normally expect this kind of (severe) weather," Lenning said.
Wheatland, Wisconsin hit by twister
Kenosha County Sheriff David Beth is calling it a miracle that no one died or even was seriously injured in the tornadoes that hit his county this afternoon.
A rare series of January tornadoes smashed houses and downed trees and power lines. But only about a dozen people were injured and none was seriously hurt.
Hardest hit was a subdivision in the town of Wheatland, about 50 miles southwest of Milwaukee.
Officials say about 20 homes there were damaged, and almost a dozen of them were flattened.
The collapse of one house was witnessed by sheriff's deputies who had responded to a rollover accident nearby.
The sheriff says they rescued the trapped motorist from beneath his car and then ran to the house and helped people get out.
Aurora Medical Group facilities in the area treated 13 people for storm-related injuries, but none of the injuries was major.
The Associated Press contributed to this report