"Sut mae'r tywydd heddiw?"

Translation: How is the weather today?

[Robminchin]

"What's the weather today" doesn't seem to be accepted. It insists on having "like" in there, which means the same thing in English but seems a slightly odd idiom.

[GeekyGiraffe]

You should either say "What is the weather like today?" Or "How is the weather today?" Otherwise it is grammatically incorrect

[birkos]

How is the weather today means the same thing

[SLMeister]

Should this not be: How's the weather today?

[ibisc]

That is one of the accepted answers.