Conceptual Development: The earliest iteration of this word was G. lómin “shady, shadowy, gloomy; gloom(iness)” from the Gnomish Lexicon of the 1910s, the adjective form of G. lôm “gloom, shade” (GL/54). A similar word G. lumbrin or lumba “overcast” was an adjective based on G. lum “a cloud” (GL/55).
Neo-Sindarin: Since the root √LUM “shadow, darkness” survived in later writings (PE17/168), I think N. lhumren “shady” can be salvaged, though if adapted to Neo-Sindarin it would need to become ᴺS. lumren as suggested in HSD (HSD). Given the later use of Q. lumbo for “(dark) cloud”, I think the sense “overcast” from Gnomish lumbrin can be salvaged as well.