When I left my house this morning, the weather looked really threatening. I was a bit worried.
この場合、次のような言い方ができますよ。
ーWhen I left my house this morning, the weather looked really threatening. I was a bit worried.
「今朝家を出た時、天気が荒れ模様で少し不安だった」
the weather looked really threatening で「天気が荒れ模様だった」と言えます。
ご参考まで!