"Snowing heavily" is used to describe when it snows a lot.

This phrase comes from the verb "to snow", which means that snow is falpng from the sky, and the adverb "heavily", which means a lot or in large amounts.

Examples in Engpsh with Chinese translation:

"It's snowing heavily outside, I don't think I can make it to work on time." (外面下雪很大，我认为我不能准时上班)
"The forecast says that it will be snowing heavily all night." (天气预报说整晚都将下大雪)
"The roads are dangerous because it's snowing heavily." (因为下雪很大，道路很危险)