Late summer heat in Iowa is not without historical precedent, but it is unusual. Perhaps more significant to growers is the possibility that extreme heat in late August or early September can lead to an early freeze in September.
Jim Lee with the National Weather Service in Des Moines reports that 1983 saw a temperature of 108 degrees on August 16th in Des Moines. That year, the first freeze took place, at least in central Iowa, on September 23rd.
The earliest fall freeze on record for Des Moines took place in 1913. The first week of September saw temperatures in the 90s or 100s every day. On the morning of September 22nd, temperatures in Des Moines dropped to 32 degrees.
Lee also reports that in 1881, a late-August heatwave preceded the earliest fall snow on record, which dropped several inches of snow across western Iowa. Lee also mentions broken trees limbs from the weight of the snow were reported.