"Is it snowing?"
I'll translate your sentence literally: "Is it snows?" That does not work. "snowing" the gerund is a concept that is not often used in German and works differently, more limited there. So what can you do? You use a different tense, here the simple present to fill the gap. In English: "Does it snow?" Or "Does it snow at the moment?" are synonymous to the sentence above. Translating those works better. "does" as the helping word for questions is not translated and you end up with: "Schneit es?" or "Schneit es gerade?".
I hope I could be of help.