This December may go down as one of the least snowy in history, the National Weather Service says. Only a trace of snow has been reported from Dec. 1 through Christmas in Boston, something that has happened only five times before in weather service records. The last time was 1973. In Worcester, 0.3 inches of snow has been recorded all month, tied with 1892 as the least snowiest on record. The outlook for the winter is still uncertain, said meteorologist Bill Simpson.