U.S. Army Corps of Engineers answers FAQs about 2011 flood

Flooding commenced with unseasonable heavy snows across the Great Plains. This was followed by three to six times the normal rainfall in May in eastern Montana, northern Wyoming, and the western Dakotas. Then the snow pack melt came perhaps two weeks later than usual with an accumulation that was around 23 to 40 percent greater than normal.

http://www.cass-news.com/articles/2011/07/25/news/local/doc4e2da4d018f56257550923.txt