I am interested in tracking the amounts of rain or overcast rainy days
this year versus previous years for the US, different regions and/or
different states.
Who collects and reports this data?

This is just a guess but you might want to check with NOAA The
National Oceantic Atmospheric Administration.  The website is:
(http://www.noaa.gov/) I didn't look very hard but I think it should
be there.
Hope that helps infopros-ga,

You can also take a look at this link:

http://precip.fsl.noaa.gov/hourly_precip.html

Thanks,
Kalyan.