A hot, sunny day will transition into a stormy evening across portions of Iowa, Missouri, and Illinois (perhaps into neighboring eastern Kansas or western Indiana) on Thursday, June 13.
A frontal boundary will sink southward across the Midwest this afternoon, running into an increasingly hot & humid air mass across southern Iowa, northern Missouri, and northern and central Illinois. Isolated to widely scattered thunderstorms should erupt late-afternoon along the front near the Iowa/Missouri border toward the Iowa/Illinois border. Initially, a couple of intense supercell thunderstorms are possible with the potential to produce hail over 2" in diameter and an isolated tornado or two. This initial risk for isolated, discrete supercells with hail, wind, and tornado risks will be greatest between 4 PM and 8 PM across southeast Iowa, northern Missouri, and northwest Illinois.
The potential for destructive downburst wind gusts over 80 MPH will increase through the evening as well as storms transition into high-precipitation supercells, and perhaps eventually into one or two bowing, linear storm segments.
A corridor or two of significant winds and embedded QLCS tornadic circulations is possible from southeast Iowa, northern Missouri, into west-central Illinois from 6 PM to 12 AM.
Less certain, but perhaps just as damaging is the potential for renewed supercell thunderstorm development on the cool, north side of the frontal boundary during the late evening and early overnight. The risk with these trailing storms would be large hail over 2" in diameter.
From a storm observation standpoint, I've been nervous about leaving my backyard with the potential for significant severe weather to approach during the late-evening, but I think the pull toward an intense supercell or two in southeast Iowa/northern Missouri, or perhaps far western Illinois is just too strong. I'm getting morning forecast products out the door and alerting folks to this evening's risk before I probably load up the car and head west.