Wally, you are right...I typed in badlands, not Black Hills. Here is the info on May in Black Hills, specifically Mount Rushmore:
"May precipitation mostly comes in the form of showers and late spring accounts for over a third of the Black Hills annual moisture. Daytime highs now reach the 50’s, with 60 degree temperatures not uncommon, however overnight temperatures are still chilly, with morning frosts not uncommon." Here is the link:
http://www.mountrushmoreinfo.com/weather.htm