Since they always film in Hawaii, it's probably pretty hard to get fake snow to stay frozen on the street. Especially in the city, snow can melt. Also, the "Christmas season" starts as early as the beginning of November in a lot of places. Just because it's in Manhattan doesn't mean there has to be snow. It gets really cold in NY in the winter so the question to ask is whether or not you could see their breath. Snow comes and goes, but the breath should show the temperature.