Found a great link with average temperatures and rainfall. Interesting that there is not a lot of rain or snow in November, December, or January.
http://countrystudies.us/united-states/weather/south-dakota/custer.htm
Another thing to note is the great temperatures during the summer months. Not too hot....80s are the rule and nice evenings. Really great weather for campfires and day trips to all the attractions in the Mount Rushmore and Black Hills area.
Of course if you really want some constant weather you can visit one of the many caves in the area. Jewel Cave National Park and Wind Cave are within 15 minutes of our campground. These caves have a nice 40-50 degree temperature all the time and it does not change. These caves have great tours and it is fun to ride the elevators down the 200 to 300 feet into the depths of the cave.
For information on the caves visit these sites:
Jewel Cave
Wind Cave
That's all for now. Stay warm.