Rule reminder
Final group 4 review: Imperfetto = background, ongoing, habitual past weather. Passato prossimo = specific, completed events, often with time markers (ieri, stamattina, la settimana scorsa) or results. Key: imperfetto describes the weather; passato prossimo reports what the weather did.
Quella sera, mentre cenavamo, … a tuonare e a lampeggiare in lontananza. (That evening, while we were having dinner, it started thundering and lightning in the distance.)