この場合、次のような言い方ができますよ。
ーWhat's the weather going to be like today?
「今日の天気は?」
ーWhat's the weather forecast today?
「今日の天気予報は?」
ーWhat's the temperature going to be today?
「今日の気温は?」
ーWhat's the expected high/low temperature for today?
「今日の予想最高/最低気温は?」
ご参考まで!