Now that we've passed the autumnal equinox, it has suddenly become a lot more like fall.
この場合、次のような言い方ができますよ。
ーNow that we've passed the autumnal equinox, it has suddenly become a lot more like fall.
「お彼岸を過ぎて、急に秋らしくなった」
autumnal equinox「彼岸」
to become like fall で「秋らしくなる」
ご参考まで!