Since the Spring wheat is way ahead on harvesting, the heavy rains coming up are not going to be a big problem. 

However, if they extend beyond a week in places like Nebraska, they will delay (ideal) planting for the Winter wheat crop:

https://cropwatch.unl.edu/winter-wheat-seeding-date-can-have-major-effect-yields

 Figure 1. Recommended planting dates for Nebraska winter wheat.

My confidence in a major flooding event has only increased the past 3 days because the guidance continues to support it.
Still am fairly confident in a large area of 6+ inches in the next week, probably over Iowa.  What makes this potentially very significant is the possibility of the excessive rain signal to continue  into week 2...........and over much of the same area.Should that occur, rain amounts of 10+ inches are possible and historical flooding for September which is seasonally the driest month of the year.

This may not hurt the corn that much but could be devastating to some of the beans. September is the ideal month to plant Winter Wheat in Nebraska and this wet weather is may effect planting there possibly points north and south.

Last 12z GFS operational, total rains.

mage URL: http://mag.ncep.noaa.gov/data/gfs/12/namer/precip_ptot/gfs_namer_384_precip_ptot.gif