The National Storm Prediction Center has the entire Chicago area from northeast Illinois into northwest Indiana under an Enhanced Risk of severe thunderstorms Saturday (tan-shaded area on the highlighted map depicting a 30% chance of severe storms within 25 miles of a given location). While widely scattered thunderstorms may occur at any time, the highest probability of strong to severe storms here appears to be later in the afternoon into the evening hours. The primary threat will be large hail and Damaging winds, but tornadoes may also be possible.
Ahead of a cold front approaching from the west out of Iowa, instability will increase as warm moist air with dewpoints in the upper 60s and lower 70s will ride southwesterly winds into northern Illinois and southern Wisconsin. The nose of a strong jet stream aloft will provide additional lift along with an upper-level disturbance providing an influx of cold air. With 90-degree temperatures boosting convective currents at the surface, all these ingredients will combine to further destabilize the atmosphere. Clusters of thunderstorms are likely to occur in Missouri and Iowa along and ahead of the cold front, moving east into Illinois and eventually the Chicago area later in the afternoon and early evening.