If you are looking for actual NWS COOP observations with no estimating and very little quality control done, check out this page. If you are looking to download the entire dataset, please check out this directory!
This page allows you to download observed, sometimes quality controlled, and sometimes estimated once daily reports from the NWS COOP network. More detailed documentation exists here and should be consulted before using the data.
The quality controlled data for Iowa is currently updated to 01 May 2017. While data is available for dates up until yesterday, the values are estimated by the IEM. The quality controlled data is provided to us by the Iowa State Climatologist. News items are posted to the homepage when a month's worth of data is sent to us.
Data is also available from the following states: Illinois , Indiana , Iowa , Kansas, Kentucky , Michigan , Minnesota , Missouri , Nebraska , Ohio , North Dakota , South Dakota , Wisconsin. This information is mostly estimated until NCDC provides updated GHCN datafiles, which the IEM then merges into this dataset. The unfortunate side effect is that data changes as new information becomes available.
If you really need calendar day data and other variables like wind speed and relative humidity, check out the IEM Reanalysis effort.