It was another hot afternoon Wednesday as high temperatures warmed into the upper 80s and low 90s, officially reaching 90 degrees in Rockford. While not as humid as Tuesday, heat index values did reach the mid and upper 90s, with Rochelle tagging 100 degrees once again.
Unlike Tuesday night, much of Wednesday night has been dry. Thunderstorms were quick to develop over west-central Illinois, racing southeast towards central Illinois during the evening. The remnants of shower activity Wednesday morning left behind what is known as an MCV, mesoscale convective vortex, or mini-low pressure system. As the MCV moved into the unstable environment in west-central Illinois, thunderstorms rapidly developed. Those storms are now moving towards Chicago with a few isolated storms in eastern DeKalb County. No severe weather, locally, is expected Wednesday night, but patchy dense fog will be possible Thursday morning.
Believe it or not, a cold front is moving towards southern Wisconsin and will be entering into northern Illinois overnight. This front will briefly stall before pulling back north as a warm front Thursday afternoon. Behind the front temperatures will once again warm into the upper 80s and low 90s. Isolated thunderstorms are possible during the afternoon Thursday but the lack of forcing in the atmosphere will limit how much storm activity we see during the day.
Across the upper Midwest thunderstorms are expected to rapidly develop late morning in Minnesota, racing southeast through Iowa and eventually towards northwest Illinois by Thursday evening. The greatest risk for severe weather will be closer to Iowa and Minnesota as storms move through during the day, but western Illinois is under a slight risk for storms capable of damaging wind gusts and heavy rain. The arrival of the storms into northern Illinois will likely hold off until after 10pm/11pm. By that time some weakening may occur, but gusty winds are still possible. Storms will then lingering into Friday morning before lifting north into Wisconsin by Friday evening.