From the National Weather Service:
Rain moves in tonight, cooling the air falls through and evaporates in the dry air. Temperatures will fall below freezing in most areas along and north of the Mass Pike, as well as parts of northern CT, northern RI, and interior SE MA.
The result will be sleet and freezing rain in these areas, while locations farther south and along the coast will be mild enough for rain.
Graphic provides a look at how much ice is forecast in Southern New England tonight through Monday night.