If you're going to visit Germany, you might hear people say "Es ist am Regnen" instead of "Es regnet". In some dialects, "Es ist am Regnen" is typical and very widespread. So, when a German tells you "Es ist am Regnen", you'll know (and probably already see or feel ;-)) it's raining.
Grammatically and in written standard German, you won't find that regional "present progressive" form, because standard German doesn't have a present progressive.
As an addition to MoritzB7's comment: It's quite typical for Rhineland dialects as well. I even heard people who grew up in Lower-Saxony (in a specific region in Lower-Saxony, which is allegedly a "stronghold of standard German") using such "progressive" verb forms.
I'd use it for rain that is currently pouring down. I'd use it basically for instances when I'd use the English progressive in English. Works also in the past: Es war am Regnen (often written as: Es war am regnen). It's called am-Progressiv or Rheinische Verlaufsform. It's quite common in West-Germany, in particular Rhineland, Westphalia and areas bordering the Netherlands (Dutch has a similar feature). But it's certainly not standard and formal German. Cf. https://de.wikipedia.org/wiki/Am-Progressiv
Exactly! I mentioned it for people who travel through Germany and might stumble over it in colloquial German.