(Chiming in) The individual conductors in type NM may be rated as THWN, but the overall cable assembly is not rated for wet locations. The paper filler will wick water like crazy. THHN/THWN is not rated as wet location because of the nylon coating N. It is rated by the W. Type TW and type THW are both rated for wet locations, and don't have the nylon jacket.
