When the chorus hits almost two minutes into “King of Oklahoma,” the second track on Weathervanes, the new Jason Isbell and the 400 Unit album, it’s like an unexpected release of tension you didn’t know you needed. It proves once again that Isbell is not only a much-heralded lyricist, but also a sharp arranger, a side often overshadowed by searing personal observations that make him one of the great songwriters of his generation.