ᴺS. [N.] ^uil- v. “to rain” (Category: Rain)
An impersonal verb appearing as N. eil “it is raining” in The Etymologies of the 1930s derived from primitive ᴹ✶ulyā- (> œil > eil) under the root ᴹ√ULU “pour, flow” (Ety/ULU; EtyAC/ULU). This verb is abnormal in that its final a disappeared rather than surviving as it usually did for derived verbs, giving eil rather than *elia-, *eilia- or *eila-. However, as an impersonal verb there would never be any pronominal suffix to help preserve the final a by analogy, which probably explains the vowel loss.
Neo-Sindarin: In Sindarin, the likely developments of primitive ✶ulyā would be to *oly(a) > ᴺS. uil “it rains”; I believe this form was first suggested by Helge Fauskanger in his Parviphith Edhellen wordlist. Compare ᴺS. uil to: S. ruin “fiery red” < (perhaps) *runyā and S. fuir “north” < (perhaps) *phoryā, and see the entry on how [œi] became [ui] or [y] for further discussion. Any inflected forms would probably restore the stem, such as (hypothetical) intransitive past and future forms *eilias “it rained” and *eiliatha “it will rain”).
References ✧ Ety/ULU; EtyAC/ULU
|ᴹ✶ulyā > œil > eil||[uljā] > [ulja] > [olja] > [olia] > [œlia] > [œli] > [œil] > [eil]||✧ Ety/ULU|