Maybe there will be a heavy rain.
同义句：
Perhaps it is going to rain heavily.
不明白可继续提问,可要求是变为There_________a heavy rain.There__might be_______a heavy rain.两个空的话就这么填。可要求是变为There_________a heavy rain.几个空？至少应该有两个空吧。There__may be_______a heavy rain.或：There__might be_______a heavy rain.