Is “就快下雨了” correct Mandarin? I've been watching Peppa Pig to improve my Mandarin, but here at 1:59, the mama pig says  “就快下雨了”  meaning to say that it will rain soon. However I thought that the phrase &quot;就快&quot; was Cantonese only, and that you generally use something like &quot;很快&quot; in mandarin. Is the above sentence correct Mandarin?
Yes, 就快下雨了 is idiomatic in Mandarin.  The similar variations are: 快要下雨了，快下雨了，就要下雨了，天就快下雨了，天快下雨了，天要下雨了，etc.