Attached is a simple sample snowmelt model in SWMM 5 that has built in snowfall and temperature in a one subcatcment model with snowmelt.   You define the separation of precipitation into snowfall and rainfall by setting a base temperature in the Snow Pack Editor.   The precipitation that falls with when the air temperature is below the base temperature is stored in a snow pack where it eventually will melt when the temperature rises or is moved via plowing.  You can have an initial snow cover, final snow cover and runoff from the melting snow long after the snowfall occurs.