Strong southerly winds Sunday gave temperatures almost a ten degree boost over Saturday's high. Incoming cloud cover and a south wind will keep overnight lows in the 40's Sunday with highs warming back into the 50's Monday.
An approaching warm front from the south will fall short of reaching northern Illinois and southern Wisconsin. Already this front had been the focus for thunderstorms over central Missouri Sunday afternoon. As the front begins to shift north, instability and moisture will increase and lift north with it. Strong winds within the lower level of the atmosphere will increase and interact with the increase in moisture. This will likely cause thunderstorms to develop over central or northern Iowa late Sunday evening, likely after 10pm. Those storms are then expected to move east into Illinois after midnight. These storms will be elevated and could produce marginally severe hail.
The image on the left is a simulated radar image around 2am. Most short term forecast models suggest the bulk of storm activity staying south of Rockford. Most of the thunderstorms will be southeast of the Stateline by Monday morning, but an approaching cold front could help redevelop a few showers/storms mid-morning Monday to the southeast of Rockford.