What is the warmest temperature it can snow?

You may have witnessed graupel, which is a soft white pellet of snow different from sleet, which is composed of ice. That can occur with surface temperatures well in the 40's, provided that the above-freezing layer of air at the surface is very shallow. I have seen snow flakes with temperatures in the mid 40's, again the relatively warm layer of air at the surface was very shallow, falling below freezing just a couple thousand feet above the surface.