Braf has a variety of translations in English including:- agreeable, bonny, delightful, fine, jolly, lovely, nice
Different English dialects would transpose 'mae hi'n braf iawn' using different adjectives.
My local English dialect (South Wales) would most likely say 'It is really lovely' in reference to the weather.
Other dialects would use a different phrase.
Thank you, that makes a lot more sense. Being South Walian as well, it sounds a lot better to say "it is really lovely"