"Mae hi'n bwrw cesair."
Translation:It is hailing.
Bwrw is usually "to hit/strike". It has another meaning which is "to throw/cast/hurl" which isn't much used today, but that seems to be the idea with precipitation: Mae hi'n bwrw eira/glaw/eirlaw/cesair "It's casting snow/rain/sleet/hail". I seem to remember Manx has the same idiom: T'eh ceau sniaghtey/fliaghey/fliugh-niaghtee/sniaghtey garroo.