In other weather sentences Sut becomes what in English but not in this one?
(This sentence is a zombie - make sure that you have latest version of the Duo app if that is what you are using. The sentence should read Sut bydd y tywydd ddydd Mawrth?)
Sut followed by a verb (bydd, here) means 'How...?', not 'What?'
It can also be used in a slightly different pattern to ask 'What sort of person/thing is xxx?' or 'What is xxx like?