Know of a source of historical 24-hour rainfall totals for US locations?

A great place to start is the PRISM Climate Group. There you can download data in gridded (i.e. text data) or image formats.

Other sources include:

• http://www.worldclim.org/current
• http://www.ncdc.noaa.gov/cdo-web/
• http://gis.ncdc.noaa.gov/geoportal/catalog/main/home.page (search for precipitation)

If you happen to have rain gauge data, here is a nice tutorial on generating isohyetal maps.

The Rainfall-Frequency Atlas doesn't give you the raw data, but it's a good reference.

The data is expensive to access through an API, but it might be worth looking at Wolfram Alpha sometimes for inspiration on things like this.

It's possible to get structured data out of it and also to look at the sources it uses to compile the information.