Coal inventories at power plants dropped below the monthly five-year average in April, the first time this has happened since December 2011. The decline in stockpiles occurred as coal burn increased across much of the nation during a winter that was colder than the previous winter. In addition, rising natural gas prices prompted some power plants to burn more coal and less gas to generate electricity.

via Today in Energy http://www.eia.gov/todayinenergy/detail.cfm?id=12151