Gnomish
nictha-
verb. to rain, hail, snow
ubra-
verb. to rain
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
An impersonal verb in the Gnomish Lexicon of the 1910s appearing as G. nictha “it is raining, hailing, is snowing” (GL/60), clearly based on the early root ᴱ√NIQI “white” (QL/66) with qt [kʷt] > cth [xθ].
Neo-Sindarin: If updated to Neo-Sindarin, this verb would be ᴺS. nítha- since since k spirantalized and then vocalized before th. However, I would limit its use to “snow, hail”, and for the verb “rain” I’d instead use ᴺS. uil.