The rain will fall over already saturated ground and into river basins that already are near flooding or are flooding. This includes many of the rivers across eastern MO, IL, IN and OH such as the Mississippi, Missouri, Illinois, Wabash and the Ohio. Model indications for the amount of precipitation that will fall through Sunday morning are in the 3"-6" range from MO to WV.
In addition, since this is a battle ground, embedded storms could reach severe levels with the main threat being large hail and damaging winds. A slight risk area has been issued by the Storm Prediction Center from TX through the MS Valley into the OH Valley for today and little change in that area over the next few days, too.