"Tomorrow is supposed to get snow" is actually correct and sounds like very good English to me. You could also say "It's supposed to snow tomorrow", which is a bit more common. As 一 mentioned, always use "supposed to". When a verb follows "is/are/am", it usually must be "-ing" or "-ed". "Suppose to" isn't used in this expression. Just remember always use "supposed" in this case. If you want to know the grammatical reason, literally "to suppose" means "to think" or "to assume." So, when you say "Tomorrow is supposed to snow", you're literally saying "People think tomorrow will have snow" or "Tomorrow is rumored to snow." However, the expression doesn't really mean this anymore; it means "Tomorrow should have snow." If knowing the grammatical reason helps you remember and understand, that's good; but if this is confusing you more, just remember to use "supposed to". :)

Tomorrow we are suppose to get snow...that is what you HEAR Tomorrow we are supposeD to get snow...that is what is SAID The two sound the same to the untrained ear.

by adding the "ed" making it past tense. otherwise don't.

I have never heard that:-??

Be suppose to is incorrect, it's a common mistake people make because that "d" causes very slight difference in pronunciation by contrast to the only one correct structure: Be supposed to. "tomorrow is supposed to get snow" <<< Unless you are making up a fairy tale for your kids. Otherwise, Chinglish. But people will still be able to understand you.