I am looking for annual precipitation maps for Minnesota and South Dakota. Preferably in a raster or polygon format. Basically, something I can spatially tie to data in a large database. I have been on NOAA, USGS, and several other sites that typically have this type of data, but I'm not having much luck. Any ideas?
I would start with
WCRP GCOS GPCC FDP version7 which is a global gridded land surface analysis of monthly precipitation, The data are monthly values from January 1901 to December 2013 at three spatial resolutions, 0.5°, 1.0°, and 2.5° lat/lon.
This link provides a meta list of rainfall datasets, http://fsg.afre.msu.edu/gis/rainfall.html.
Noaa daily data is at, ftp://ftp.cpc.ncep.noaa.gov/GIS/GRADS_GIS/GeoTIFF/GLB_DLY_PREC/
Another meta list is at, http://www.esrl.noaa.gov/psd/data/gridded/tables/precipitation.html
NOAA has these data. I apologize for a list of links but just so many sources exist.
WorldClim is a set of global climate layers (climate grids) with a spatial resolution of about 1 square kilometer. The data can be used for mapping and spatial modeling in a GIS.
Do not forget our cousin open data site at https://opendata.stackexchange.com/