It is raining cats and dogs outside.
句意:外面正下着倾盆大雨。
关于本句的句子成分,前面的老师已有分析。但我觉得也可以将 cats and dogs 分析为程度状语,在意义上相当于 heavily。
如果要表达:天正在下着雨,猫和狗在外面。可以考虑翻译为:
It is raining heavily, with the cats and dogs (being) outside.
It is raining heavily and the cats and dogs are still outside.