よさ

あしたも、天気が、よさそうですね。

"The weather should be good tomorrow too."

I think I understand all of this, except for よさ. Is this related to よい / いい? Somewhere in the back of my mind I'm half-remembering that さ might be some kind of inflectional ending, but now I can't seem to locate any information about it.

Right. よさそう is from よい/いい. そう（だ） is a na-adjective to mean "to seem", "to look like", "to seem to be about to", etc., and basically used with the stem of adjectives or the -masu form stem of verbs.
e.g.
高そう: to seem expensive
気に入りそう: to seem to be pleased

However, one-mora-stem adjectives like よい, ない are one of exceptions. さ is inserted for the case, as in よさそう, なさそう and not よそう, なそう.

Thanks!
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