Wind chills of near 20-below zero will occur and a Wind Chill Advisory will be in effect across northern Illinois, including the Chicago area from 8PM CST this Thursday evening until 10AM CST Friday morning. Very cold wind chills of 5 below to as cold as -15 (just shy of the Wind Chill Advisory criteria) will likely occur in northwest Indiana.
With Arctic-source air overhead, a fresh snow cover, clearing skies, and diminishing winds, temperatures will drop into the single digits this Thursday evening and continue to fall below zero in many areas by sunrise Friday. Winds will gradually diminish during the overnight hours, still running west-northwest 4 to 6 mph Friday morning when temperatures/wind chills will bottom-out.
Wind Chill Advisories will also be in effect in adjoining central and northwest Illinois and Wisconsin. Areas shaded in light blue on the map below will also be under Wind Chill Advisories tonight/early Friday.