City temperatures in the 20-below zero range are rare, with the last occurrence dating back to January 18, 1994. Arctic air sweeping into the region Tuesday brings the likelihood that Chicagoans will experience two days this week that will eclipse this frigid threshold. Late Monday, temperatures across portions of central Canada had plunged to near -40 degrees. Flin Flon, in northwest Manitoba sat at -35. Only slight moderation of this air mass is expected as it advances across an expansive snowpack on its way toward Chicago. The leading edge of this bitterly cold air will reach the area by midday Tuesday. Tuesday night, readings will become dangerously low. Wind chills are expected to plummet to near -50 degrees at times through Thursday morning. The core of this frigid air mass is transitory. By Friday, readings will begin to moderate, leading to thawing temperatures over the weekend.