Big rainstorms are not uncommon in the the midwest. The Chicago area got hit with similar rains some years ago. We had a similar set up a couple of years ago north of Champaign,Il. Gibson City got hit with 9+ inches in less than a day. It was a devastating flood but the insurance payouts will be chump change compared to the St. Louis area claims..
Missouri holds the one hour world record rainfall. That strangled a few frogs.
12” in 42 minutes at Holt, Missouri on June 22, 1947