Weather Underground makes nearly all climatological data available in easy-to-consume formats (csv mostly). Using your rainfall example, here's how I'd get a listing of rainfall by day for the month of September, from my local National Weather Service monitoring site:

  $ curl -s "http://www.wunderground.com/history/airport/KMSP/2015/09/31/MonthlyHistory.html?format=1" | grep -v Precip | cut -d "," -f 20