One way to convey this is to say 明天会下雨(míngtiān huì xiàyǔ).

会(huì) is an auxiliary verb that we learn in lesson 23, to convey future actions that “will” happen, as well as predict future occurrences such as in the example sentence. 会(huì) is placed in front of the verb that “will” happen, and is negated with 不(bù).

Examples:
我夏天会回美国(Wǒ xiàtiān huì huí Měiguó).
I will return to the US during Summer.

这么晚了，她应该不会来了(Zhème wǎn le, tā yīnggāi bú huì lái le).
It so late already, she probably will not be coming.