I use weather underground to look at seasonal averages:
http://www.wunderground.com/

Type in a location, then when you have the location page up, half way down the page on the left side you can click on seasonal weather averages. That gives you average and record lows and highs for that location.

Example, I am going thru Whitefish Montana this summer, so this is the info I get for that location:
http://www.wunderground.com/NORMS/Di...=none&IATA=FCA

I will bring a bag that is good for sub freezing temperatures for Montana and Canadian rockies in July, but I used to backpack in Wyoming and Colorado and have been snowed upon in July and August.