"It is hailing."
Translation:Mae hi'n bwrw cesair.
None, really. It is usual to use hi for 'it' when referring to weather, but it is also common to omit the pronoun hi in this pattern.
So Mae'n is generally acceptable for "It is.." in other contexts too? I remember yn TGAU our teachers would tell us to use Mae'n for "It's" as a general rule, but this could've just been for simplicity