这里是虚拟语气,如果明天下雨,与将来的事实（将来不下雨）相反,从句,应用动词过去式/should+ 动词原形/were to + 动词原形；而主句则应该这样,would/should/could/might+动词原形
而Should it rain tomorrow在句中是从句,若要用be动词,应该是were it to rain.