An impersonal verb for “rain” attested only in its future form uluva “it is going to rain, it will rain” (PE22/167). Its aorist form is probably ✱ule “[it] rains”, its past form probably ✱úle “[it] rained”, and its perfect ✱úlie “[it] has rained”; as an impersonal verb, no explicit subject is required in Quenya. It is clearly derived from the root √UL “pour (out), flow” (WJ/400; PE17/168) and it seems that its primitive form originally meant “pour” (PE22/133), but elsewhere Tolkien gave the Quenya verb for “pour, flow” as ulya- (Ety/ULU).
Conceptual Development: In the Qenya Lexicon of the 1910s, the verb for “it rains” was (3rd-singular) ᴱQ. uqin from the early root ᴱ√UQU “wet” (QL/98). This verb reappeared as uqe or úqe “it rains” in the Early Qenya Grammar of the 1920s, having become an impersonal verb (PE14/56, 85). Another impersonal verb for “to rain” appeared in Quenya Verbal System of the 1940s: ᴹQ. kelya “(it) sends running down = it rains” from the root √KEL (PE22/114). The form uluva mentioned above appeared in Late Notes on Verbs from 1969 (PE22/167).
Neo-Eldarin: Based on attested forms I would limit ul- for “rain” as an impersonal verb only, and for “pour” would use ulya-. Interestingly, the Noldorin word for “rain” is eil [ᴺS. uil] from ᴹ✶ulyā-, so I think Quenya and Sindarin/Noldorin made different choices for which ancient verb became impersonal “rain”.
A noun for “rain” in Late Notes on Verbs from 1969 given as {ulla >>} ulo in the phrase ulo úva “rain (unwelcome) is coming”, clearly related to the impersonal verb ul- “to rain” appearing in its future form on the same page: uluva “it will rain” (PE22/167).
Conceptual Development: In the Qenya Lexicon of the 1910s, the noun for “rain” was ᴱQ. úqil, likewise related to the contemporaneous verb ᴱQ. uqin “it rains” (QL/98).