A cold-air outbreak is expected to impact a large part of the US east of the Rockies. Lows tonight expected to drop below zero tonight (first map) across Illinois and most of the Midwest. These extremely cold temperatures will come close to the record lows for February 19 in many locations in the state, but is not likely to break them.
Strong winds out of north are expected as well, causing NWS to issue a wind chill advisory for Illinois as windchill values fall into the minus teens and twenties. Values in this range can lead to frostbite of exposed skin in 30 minutes or less, as well as greatly increase the risks of hypothermia. Stay tuned to your local forecasts as this situation develops.
Even before this cold air outbreak, the statewide average temperature for February in Illinois was 4.4 degrees below average.
Note #1. You can see how windchill is calculated on this NWS web page.
Note #2. Three dog night is a folk expression of how many dogs you will need to stay warm at night [one dog night = cold, two dog night = colder, three dog night = coldest].