The 0600 GMT GFS Operational (model) and 0000 GMT ECM OP agree on a severe thunderstorm scenario north of a warm front today and tonight focused on Wisconsin and leaving Illinois mostly dry. Disagreement between the 2 models begins after 7 AM CDT tomorrow morning. The GFS produces thunderstorms in Illinois tomorrow while the ECM is mostly dry. The ECM stays dry in Illinois tomorrow night while the GFS maintains a thundery pattern. The June soil moisture pattern across Illinois is much drier implying the memory of the current climate pattern is to deny rainfall in Illinois. The drier ECM is more correct. The ECM is also limiting a moist southerly fetch from the Gulf of Mexico States keeping most of the tropical rains well south (while the GFS entrains some of this moisture northward). The ECM has consistently defeated the GFS with a more limited northerly surge of Gulf moisture in recent weeks (explaining the dry soil trend) while rains have been more consolidated in the Gulf region.
Rains are expected later this week in Illinois. How this rain is produced is in complete disagreement when comparing the GFS to the ECM. The GFS indicates 2 conventional afternoon thunderstorm events Thursday and Friday while the ECM indicates heavy rain from mesoscale convective complex Thursday morning and then otherwise relatively dry. Although the ECM has just the one rainfall event the amount will be heavier due to the time of day.
Convection events occurring around dawn as the ECM indicates for Illinois Thursday morning can be excessively heavy as overnight cloud tops radiate heat into space and the lapse rate within the thunderstorm complex steepens and causes increased instability. The increased instability causes heavy rains. Expect a 1-2 in. rainfall event across Illinois the first half of Thursday (and amounts could be heavier).