"Louriest" is not a commonly used word but it is typically used in the context for describing the most gloomy, dark and overcast of situations. Other synonyms that could be used in place of "louriest" include "dismal", "depressing", "gloomy", "dreary", "somber", "funereal", "grim", "sorrowful" and "melancholy". These words all connote a sense of gloom and darkness and are often used to describe weather, places, or emotions. It is always good to have a variety of synonyms on hand to help add depth and variety to writing and to accurately convey the emotions and tone of a piece.