A strong upper level jet will spawn the chance for a couple tornadoes, one or two of which may be strong to violent later this afternoon and evening. The skies are clearing in central and southern Illinois at this hour, which will setting the stage for strong to severe thunderstorms in the next several hours. The more daytime heating, the more instability and stronger the thunderstorms can grow, as warm air parcels from the surfaces begin to rise as they are warmed. The threat should ramp up in the next 3-4 hours.
Attached are the winds at 500mb, which are key for supercell and tornado development. Notice a maximum in winds over Illinois, this overlayed with the clearing and instability are just some of the key ingredients coming together in this region. Below is a look at the EHI index, which is the energy helicity index, basically looking at the combination of instability and wind shear, values over 2 support tornado development. Be sure to monitor local weather warnings through the evening.