The rain-to-snow line is the zone in a snowstorm where the precipitation is changing from rain to a winter mix of rain-sleet-snow and then to all snow. It usually occurs between Philadelphia and the shore, and often shifts and changes as the center of a winter storm passes through the region. Depending on the track of the storm, the rain-to-snow line can reach all the way up into the Lehigh Valley.
You can easily pick out this rain-to-snow line on Storm Tracker 6 Live HD by looking for the area between the green rain and the white or gray snow. It's usually a fairly thin zone highlighted in light blue, which denotes a wintry mix of rain, sleet and snow.