Most Wednesday nights at Snug Harbor, the live music starts before any musicians walk onstage. Clustered at the top of the club’s mezzanine stairs, trombone slides and trumpet bells vying for space in the near-darkness, the dozen-plus-member Uptown Jazz Orchestra discharges a bright brass phrase, finds a parade beat and processes, single file, down into the main room as curious audience members turn their bodies toward the source of the music.

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