سَحًّا وَتَسْكَابًا
In an abundant, pouring flow.
Both expressions intensify the idea of rain being poured out. Their accusative verbal-noun form serves an emphatic, manner-marking function. Instead of a few drops, the listener imagines a sustained downpour.
فَكُلُّ عَشِيَّةٍ
So every evening.
Kullu conveys recurrence. In lam yataṣarram, lam negates and makes the imperfect verb jussive; the verb means to stop or be cut off.