There will definitely be an intrusion of Arctic air next week, but just how far the mercury will drop during that time depends on where the heaviest snow falls over the weekend.
If low pressure tracks closer to Northern Illinois Saturday and Saturday night, heavy snow would most likely occur from Minnesota into Wisconsin. If low pressure tracks a little further south then the heaviest snow would shift south as well. Having a fresh blanket of snow on the ground will only help enhance the cold that is projected to move south next week. Highs Tuesday through Thursday might not make it out of the teens and we could feel our first sub-zero temperatures of the season. If a deeper snow pack occurs to the north, the coldest temperatures will be felt up through Wisconsin and Minnesota with highs possibly not making it out of the single digits. We would feel the cold, too, but temperatures might be able to modify a little. Either way, it will definitely be colder next week! As for the snow, keep the shovels handy. After all, it is winter (well, Meteorological Winter).