そうだ

降ふりそうだ reads how things look right now, and predicts what's about to happen. ようだ concludes a present state of affairs (どうやら雨あめのようだ).

Both allow third-person feeling talk bare 怖こわい forbids (怖こわがっている reports behavior over time). 怖こわそうだ reads a momentary appearance instead, がる needs fixed emotion words, そう fits any adjective.

そう is the everyday spoken choice, for a snap read right now (痛いたそう). げ leans written, sticking to emotion words, a lasting air (寂さびしげな人ひと).

そうだ reads real signs in front of you and guesses what happens next (雨あめが降ふりそうだ). かのようだ instead describes a likeness already known to be false, guessing nothing.

そう guesses from the outside, so nothing has started yet (死しにそう). かける reports real progress already made, not a guess (死しにかけた).

そうだ reads a state straight off the surface (元気げんきそうだ), and refuses a noun (×学生がくせいそうだ). く見みえる instead compares looks with what you already know is true.

降ふりそうじゃない？ mostly asks someone to agree, not a flat no. The flat negative instead needs the fixed ending そうもない or そうにない.

そう works on almost any adjective (新あたらしそう). み only works on a short, fixed list (甘あまみ).

さ makes a noun you can measure, with no guessing (高たかさは三さんメートル). そう makes a guess about how something looks (高たかそうな時計とけい).

Plain-form 降ふるそうだ relays what someone said ("I hear"). The ます-stem form 降ふりそうだ instead reads the sky, one kana apart.