CHICAGO (AP) — The National Weather Service planned to survey storm damage Tuesday in four northern Illinois counties after forecasters confirmed that at least six tornadoes touched down there Monday, a meteorologist said.

Based on video, photos and storm reports provided by storm spotters and storm chasers, the agency confirmed that tornadoes caused damage in Ogle, DeKalb, Kane Counties, and Lee counties, meteorologist Rafal Ogorek with the weather service’s Romeoville, Illinois, officials said Tuesday morning.

He said at least six tornadoes touched down in those four counties, but additional tornadoes likely moved through the region Monday afternoon and evening.

“There’s some uncertainty as to whether certain tornadoes tracked over a longer distance or whether they touched down in one spot, lifted and then touched down in another spot,” Ogorek said. “There were at least six tornadoes, but that number will more than likely grow.”

He said that the weather service staff would survey storm damage to determine how strong each tornado was and how long each was on the ground. Storm damage appears largely confined to trees and crops, with some damage to barns and other structures, including at least one residence which lost its roof.

More severe weather was possible late Tuesday afternoon or evening, Ogorek said. Meanwhile, the weather service said that heat advisories remain in effect until Tuesday night across north-central Illinois, northeast Illinois, and northwest Indiana.