A verb in the Quendi and Eldar essay from 1959-60 glossed “be chill, cold (of weather)” (WJ/417). It also appeared in some etymological notes from around 1959 as a derivative of the root √NIK(W) and with the glosses “to snow, it is chill, it freezes” (PE17/168). In this 1959 note Tolkien gave several inflected forms making it clear nicu- was an impersonal verb: nīqua “it is freezing”, nicune “it snowed, froze”.
Neo-Quenya: For purposes of Neo-Quenya, I’d mostly use nicu- as an impersonal verb for cold weather: nique “it is cold, it is freezing”. For the freezing of water or similar substances, I’d use ᴺQ. hel-. For “to snow” I’d use ᴺQ. hris-, a modernization of archaic †hriz-.
nicu- ("k")vb. "be chill, cold (of weather); to snow, it is cold, it freezes" (WJ:417, PE17:168): 3rd sg. aorist niquë (q.v.) "it snows or freezes", present níqua "it is freezing", pa.t. nicunë "it snowed, froze" (PE17:168)