"It is too windy."

Translation: Mae hi'n rhy wyntog.

[ofred19]

Why does gwyntog still mutate to wyntog even though the yn is separated by rhy?

[rmcode]

Rhy is one of the few adjectives that come before the noun. They all cause a soft mutation.

[EllisVaughan]

Because "Rhy" also causes a soft mutation.