Climatology provides information on what you may expect for various variables and durations. The IEM Climatology section is organized by different periods of time (see second row of links in the banner).
The Climodat application provides pre-generated reports of Iowa's climate based on the most popular requests we get from the general public. You should check out that page first, it may have what you need.
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Iowa's Earliest Snowfall on Record
There is no one exact or official answer to this question. In the modern era, September 25th 1942 could be considered as the earliest date in the fall of accumulating snowfall. Up to 4 inches fell that day with nearly all of the state seeing something.
However, there was an earlier event back on September 16, 1881 when snow accumulated over about the western one-half of Iowa and 4 to 6 inches was reported between Avoca and Stuart in west central Iowa.
There were reports of a few flakes on September 6, 1929 near Alton about 9 a.m.
The Iowa State Climatologist's page has a number of products from the official source of climate data in the state.
The National Climatic Data Center is the online archive for climate information from Iowa and the world!
The High Plains Climate Center and Midwestern Regional Cimate Center have a number of quality products for Iowa.