“Side project” is the most convenient term to use when describing Black Prairie, but the group is much more than simply four-fifths of The Decemberists. On its third full-length, Fortune, the band lets its fiddle-playing frontwoman Annalisa Tornfelt shine warmly as she veers between homey folk-pop and propulsive indie-rock. All members of The Decemberists (save for Colin Meloy) join in—but Black Prairie’s roots-infused sound is catchier, punchier, and more sweetly spring-like.