It's a bit ambiguous.
"Fair weather" is becoming an expression: a "fair weather friend" is only your friend in good times, and you might refer to something occurring in "fair weather or foul".
But "fair" is beginning to lose the general meaning of "nice/good/beautiful" in ordinary usage. We don't usually say, "She's a fair woman", or "It's a fair day" with that meaning. More often, now, fair means kind of so-so, not great, not terrible, and if you say, "The weather will be fair," people may well misunderstand which meaning you have in mind.
I am mindful that the above may be not universally applicable in the English-speaking world, but it's pretty widespread in North America, at least.