"Era nevando" cannot be used because it doens't exist. There is no way "era nevando" would be grammatically correct. Because "era" is the past of "ser" and "ser" is only used for actions that will remain the same for ever or for a very long time: like your nationality or being someone's relative, or beign a student. (They are things that cannot change, or that at least will remain being that way for a long time- you never stop being norwegian, you never stop being you're brother's brother, and you can't be a student for a day or so)
So, era nevando is wrong. You must use "estar", which is for things that change quite rapidly: like like being in a certain place, or baing in a certain mood. You can change your location easily, and you can change your mood rapidly.
Knowing that you can use any tense you need to use:) Estaba nevando, estuvo nevando, ha estado nevando, estará nevando, nevará, está nevando, nieva... and so on:)
In this case you're asking about, "it was snowing when I entered the café" would be "estaba nevando cuando entré a la cafetería":)
"Was snowing" is literal translation for "estaba nevando":) Think the "-ando" at the end of the verb "nevar" as the "-ing" in english verbs:)
I hope I've been clear:)