CHICAGO (AP) — Wind chills have dipped to 30 degrees below zero in northern Illinois with the bitter cold weather prompting schools to close and warming centers to open.

The National Weather Service had a wind child warning in effect until 12 p.m. Friday for counties in northwestern and north central Illinois. Forecasters warned of frostbite on exposed skin in as little as 15 minutes. Temperatures Friday morning were negative 11 in Rochelle and Sterling. Chicago's O'Hare International Airport was negative 6 with a wind chill of 20 below.

Metra warned commuters in the Chicago area that trains would be traveling more slowly to reduce stress on tracks, which can become brittle in subzero temperatures.

The weather service says Friday's deep freeze is a preview of next week, which could bring more than a half-foot of snow and the potential for the "coldest stretch in years."