Cloud Peak Energy Annnounces Proven and Probable Reserves as of December 31, 2011

Cloud Peak Energy Inc.

announced a substantial increase in proven and probable reserves for the year ended December 31, 2011. Following the successful bids for two large coal tracts at the Antelope mine in May and June of last year, Cloud Peak Energy had approximately 1.37 billion tons of reserves as of December 31, 2011, up from approximately 970 million tons as of December 31, 2010.