"It may snow tomorrow."
It's not entirely necessary, but it's more grammatically correct. In real conversation it's common to leave out the 会 just for the sake of getting the point across faster, but including 会 is more specific and makes more sense and flows better. Duo probably wants you to inclhde the latter to teach you proper grammar. It's the difference between saying "It might snow tomorrow" and just "Might snow tomorrow". Both are understandable and neither are technically incorrect, but one is more proper whereas the other is more casual and conversational.
In this case, "可能“ is one word meaning "may" or "might" (indicating a possibility).
Additionally, 能, as its own word, can also mean "can" as in physical ability to do something, as well as "be allowed" or "possible due to circumstance". Differentiating between those three meanings is a matter of context.