悪い

How it works

が (subject) marks what the description is about (天気てんき (weather)), and the い-adjective 悪いわるい (bad) states it directly — no です (is) needed, since い-adjectives can end a plain sentence on their own. から (because) attaches to that whole clause to give the reason, so the order is reason-first, then result. に (to) points to where you'll stay (家いえ (home)), and いよう (let's stay) is the casual volitional of いる (to be, stay) — a relaxed “let's…” to oneself or a friend.