An unstable to very unstable airmass exists in the area with surface to MUCAPE nearing 3500-4000 J/kg. Meanwhile, up to 1200 J/kg of downdraft CAPE exists. This is a measure of storm energy & updraft strength & downdraft or wind gust strength.
We are just on the far southern edge of a bit stronger mid & upper flow aloft.
Although one outflow boundary is popping a few showers/storms in our northeastern areas, an MCV (old vortex from complex of storms yesterday/lastnight) is pivoting through northern Illinois with another outflow boundary. That, in combination of the factors listed above will bring more widespread storms this evening-tonight.
So, it appears multi-cells & perhaps a few supercells will form in northeastern/eastern Illinois with some wind/hail risk, then congeal into a line with some severe wind gust risk as they progress east & southeastward through the viewing area. Pretty much the entire viewing area is SLIGHT RISK criteria now, though the line may tend to gust out in our far southeastern areas as it outruns edge of better flow aloft & encounters capping.
Timing of severe risk here would tend to be 5-10 p.m. from far northwest to far east & southeast.
Behind the line of storms, lingering rainfall/storms may occur, followed by some additional scattered showers & storms tomorrow as actual surface cold front approaches & passes.