A tornado was spotted in Stillman Valley, east of Byron in Ogle County, shortly before 9:30 p.m. on Saturday, the National Weather Service in Chicago confirmed.
No word on damages is yet available.
Tornado warning sirens blared in Dixon and Oregon around 9:10 p.m., shortly before the Stillman Valley tornado was sighted.
An Ogle County Sheriff's squad blocked traffic on Route 64 east of Oregon, between Daysville Road and Hickory Lane, as ComEd crews worked to restore power to some households. Power was restored to some homes by 11 p.m.
In Whiteside County near Prophetstown, which was in the center of the storm system for a time, two large outbuildings were destroyed on a farm on Lomax Road west of Perkins Road, as was a small shed on a nearby property, Sheriff John Booker said from the scene.
A witness thought the damage might be the result of a tornado, Booker said.
No homes were damaged and no one was injured, he said, adding that power lines are down in the area, but no poles were snapped.
The severe spring thunderstorm that spawned the twister moved northeast through the Sauk Valley over the course of a couple of hours.
A tornado also materialized earlier, around 7:45 p.m. in the village of Cambridge, in Henry County, NWS Quad Cities reported.
Confirmation of other possible tornadoes will have to wait until the morning, a NWS spokeswoman said.
The National Weather Service had issued a tornado warning until 9 p.m. for southeastern Whiteside, northeastern Henry and northwestern Bureau counties, and until 9:30 p.m. for part of Lee and Ogle counties. Doppler radar tracked rotating winds swirling along the storm's path.
A calmer, but still breezy Sunday is in the forecast, with sustained west to northwest d winds of 25 to 35 mph likely, gusting up to 45 or 50 mph, and a 40 percent chance of rain, the NWS predicts. Expect highs only in the upper 40s.
Monday will be sunny and warmer, with highs in the mid 50s.