The weather forecast doesn't look very promising that day.
この場合、次のような言い方ができますよ。
ーThe weather doesn't look good on that day.
「その日は天気が良くなさそうだ」
ーThe weather forecast doesn't look very promising that day.
「天気予報はその日あまり良くなりそうでない」
promising で「天気が良くなりそうな」
ご参考まで!