"It will be raining in a moment."
Translation:Zaraz będzie padać.
It can't. The subject "it" (in Polish it is inferred) is neuter here so you can say either "będzie padało" or "będzie padać".
PS I highly discourage using Google Translate. There are a loooot of much better tools and dictionaries you can use for any specific purpose or generally as a substitute.
it is because "it will be raining" can be also translated as "będzie padać/padał deszcz". Default duolingo translation does not specify if it will be raining or snowing.
basically , there is no change in meaning between padać and padał/padało, but if you use past form, it has to match with subject.