原句应该是:Beijingwillhaveaheavyraintomorrow.
=Beijingisgoingtohaveaheavyraintomorrow.
=Itwillrainheavily/hardinBeijingtomorrow.
=Itisgoingtorainheavily/hardinBeijingtomorrow.
=TherewillbeaheavyraininBeijingtomorrow.
=ThereisgoingtobeaheavyraininBeijingtomorrow.