The Monongahela National Forest encompasses most of the southern third of the Allegheny Mountains range (a section of the vast Appalachian Mountains range) and is entirely within the state of West Virginia. Elevations within the Monongahela National Forest range from about at Petersburg to at Spruce Knob. A rain shadow effect caused by slopes of the Allegheny Front results in of annual precipitation on the west side and about half that on the east side.