These images come from the 0Z Tuesday NAM run.
As of 15Z, a moderate-though-not-spectacular 500mb vorticity maximum is located in central Iowa, with mid-level winds advecting vorticity into the Northern halves of Illinois and Indiana.
The same model run displayed an area of moderate temperature advection at 850mb (1-2 degrees celsius for every three-hour period) predicted for areas of Illinois and Indiana. Assuming that vorticity advection is not as strong at lower levels, we see that both "basic" (non-laplacian) terms for the traditional QG-omega equation are postive, suggesting an area of light-to-moderate precipitation. This ended up being the case.