In 1983, persistent late summer heat made August one of the hottest months in Iowa since the Dust Bowl of the 1930s, culminating on the 16th when Des Moines reached 108 degrees. The heat persisted well into September with Des Moines reporting highs of 96 and 97 on the 8th and 9th. Just a couple of weeks later, much cooler weather sent the temperature down to 32 degrees at Des Moines (only one day later than the earliest fall freeze on record, see 1913 below) and as low as 22 degrees at Mason City.
In 1976, a heat wave peaked on September 6-7 when temperatures reached as high as 103 degrees at Le Mars. Amazingly, a strong cold front swept across Iowa a couple of days later and on the morning of the 10th frost was reported in the lowlands with freezing temperatures at a few locations, ranging down to a low of 29 degrees at Elkader. At Sibley high temperatures of 102 degrees on the 6th and 99 degrees on the 7th were followed by a low of 33 degrees on the 10th.
In 1939, one of the hottest summers on record across Iowa persisted all the way into September, with a heat wave peaking on the 3rd-7th. Des Moines recorded three days of triple digit heat as listed above, and readings of 107 degrees at Logan on the 6th and 7th tied the Iowa September record set in 1913. An amazing cold blast then sent the temperature plummeting to 16 degrees at Sibley on the 30th, only one degree above the Iowa September record,with light snow and sleet reported at a few northern Iowa stations in the last few days of the month.
In 1913, the first week of September was very heat with temperatures in the 90s and 100s every day. On the 5th Bedford recorded a high of 107 degrees, setting the September record for Iowa that would be tied in 1939. An early statewide freeze and killing frost followed just a couple of weeks later, on the morning of September 22, when temperatures fell to as low as 19degrees at Rock Rapids and Des Moines posted its earliest freeze on record with a low of 32 degrees.
In 1895, late summer heat brought the temperature all the way up to 103 degrees at Sioux City, Glenwood and Rock Rapids on September 17. The month would end with a cold wave that resulted in scattered frost on September 25-29 and a widespread killing frost and freeze on the 30th, when temperatures fell to as low as 22 degrees at Atlantic.
In 1881, a heat wave at the end of August sent the temperature up to 100 degrees at Des Moines on the 26th and 97 and 99 degrees on the 27th and 28th. Less than three weeks later, on September 16, the earliest fall snow on record dropped several inches across parts of western Iowa with many tree branches broken by the weight of the snow and an estimated 4-6 inches reported between Stuart and Avoca.