we 'd better cancel the picnic ,the weather forecast said it

答案是对的啊.译为：我们最好取消野餐,天气预报说明天会下雨（是雨天）.
rainy （adj.形容词） 下雨的；多雨的
这里rainy代指 明天的天气是下雨的 it is rainy tomorrow
例句：
Maybe it will be rainy tomorrow.明天可能有雨.
It will be rainy tomorrow but not cloudy.明天不是多云,明天有雨.
Sunny and mild today,cloudy and rainy tomorrow.今天晴朗温和,明天多云有雨.