Lepidoptera at a single mid-elevation site in the central Cascade Range in Washington state were surveyed for six full seasons spanning 11 years (2009–2011, 2017–2019). Sampling was performed continuously with a blacklight trap, and was supplemented with pheromone traps, searches for immature stages, day collecting, observation, photography, and sugar bait traps. The habitat is forested transition zone, including old growth forest, slightly east of the Cascade divide in the southern part of the North Cascades ecoregion. The fauna of 822 species is categorized by the number of seasons each species was found over the span of the study. The higher-order distribution of the fauna within Lepidoptera, impact of sampling methods, annual and multi-year fluctuations in abundance, and local climate effects on flight period are presented. Taxonomic challenges and noteworthy records are discussed. Representative species and genitalia are illustrated.