Update:

Strong to severe thunderstorms are moving ESE across central and southern Wisconsin this evening. The southern end of the line of storms is building, and  with an upper-level disturbance approaching from the west along with a low-level infusion of warm moist unstable SW flow, there appears to be favorable support for continued expansion of the storms southward into northern Illinois. Should this development occur, storms could move SE first into northernmost counties, track toward Chicago and then farther SE.

While there will be a threat of damaging winds and hail and even the possibility of a tornado with the strongest storms, likely heavy downpours on top of the 4 to 6-inch rains experienced earlier today in McHenry, Lake, Du Page and Cook Counties  leave those areas especially susceptible to flash flooding.

The Chicago National Weather Service Office has prepared a pictorial shown below that displays the potential development/movement of storms before Midnight CDT.