The word is parsed as a compound. It is a descriptive etymology, referring to short and heavy mediterranean rainfalls. The identification of the first element as βραχύς may have been featured by the fact the the Aeolic form of the latter is βροχύς with [o]. The obvious and correct etymology, which is that βροχή is derived from βρέχω, is never found explicitly in our sources, probably because it was obvious: there is an echo of it in Suda, alpha 1708 Ἀμφίβροχος: πανταχόθεν βεβρεγμένος, τουτέστι μεθύων