Prescribed pile burning is underway across the Mt. Hood National Forest as cooler temperatures and recent precipitation create favorable conditions for fire operations.  Forest officials say crews will burn piles of leftover branches, logs and other woody debris intermittently through the fall and winter, as weather conditions allow.  Burning locations include areas near The Dalles Watershed along Forest Road 44, Sportsmans Park and Rock Creek, as well as portions of the Grasshopper and Austin fire footprints.  Additional sites include several areas in Clackamas County and near the Wapinitia community along Highway 26.  Crews have already burned piles over the past week near Tilly Jane Campground, Clear Lake Lookout and Olallie Lake.  Officials say smoke may be visible during and shortly after burning, but significant impacts to air quality are not expected.  The Mt. Hood National Forest says crews will generally prioritize higher-elevation areas before snow accumulates.